Sound activated flash trigger

I have been asked to share my technique with regard to triggering a flash for 'special' pictures .

Here's the

youtube video

the basics :

To trigger a flash you simply need to join the centre pin to the side 'earth' contact which is basically what the 'test' button does on a flash . The 'pc' plug on the side of the flash has these two contacts . if your flash doesn't have a 'pc' plug you can always use a cheap hotshoe adapter to make the contact for you

plug a 'pc' cord into it and take the two wires to the relay on the switching kit , connect them to pin 'com' and 'no' contacts of the relay [ 'common' and 'normally open' ] .

The picture is taken in a very dark room or at night outside , especially if you are shooting water balloons with an air rifle ! Set up the camera in manual mode, lowest iso , around F9 and two seconds exposure which gives a pitch black image if there is no additional light .

Take a few test images and adjust the distance of the flash to get correct exposure before you start destroying anything .

Then you need to trigger the flash , there are various ways to do this but in this case I bought a voice activated switch kit and modified it .

I also added a two metre length of wire to the mic so I could move it around .

It would have still worked with the original relay but there would have been a slight delay when it pulls in - I added a solid state relay for instant switching and the first images were of a balloon with a hole in it that hadn't had time to split yet !

What you need to do in a case like this is keep moving the mic further away so the sound has to travel further , until you catch the explosion at the right moment in time .

I set the camera up on a tripod [ D90 , 70-200VR lens , but any camera would do the job if it has a manual mode - even a compact ! ] , fill the balloon and place it on the table , trigger the camera with the remote and you have two seconds to fire !

The sound of the air rifle triggers the 'voice activated switch ' and the relay joins the two 'pc' wires together which fires the flash . the lowest power on an SB24 is 1/16th which fires at 1/11000th sec and for some shots I used the Sb800 at 1/128th power which gave me 1/41600th sec .

That's the pellet behind the balloon , at 1/11000th sec .

Someone sugested I try safety goggles so I said "ok"

You could add food colouring

And of course not every country allows the use of air rifles and you don't all live on a farm like I do so you could just put the mic on a table and drop a glass of water or something to trigger the flash ...

These are just a few of the things I tried , you can photograph anything that makes a sound that will trigger the flash - how about setting your SB800 to 100hz repeating flash and dropping a golf ball .... it needs some work still but shows one of the many other possibilites for sound activated flash pictures !

Friday 2 /10/2009 update .....................

Also see

"Stop motion photography"

High-fp flash

Nikon's manuals don't explain auto 'high'- fp very well - or the limitations involved .
Most cameras have a focal plane shutter

{ Except the D1 , D40 , D50 and D70[S] , they have electronically switched sensor/shutters and can do what I call 'realsynch flash' and effectively have 6X as much power with flash at maximum speed , compared to a pro body in high-fp mode } , but other cameras need high-fp mode .

At your camera's maximum synch speed [ 1/200th sec for the D90 ] the shutter opens , the flash fires , and the shutter closes , which means that the entire sensor gets the full flash .
Here's an older camera with a 1/90th sec synch speed ...




As the shutter speed increases the " first curtain " of the shutter opens and the " second curtain " starts chasing it across the screen right after it starts moving which means that a slit slides across the screen . If the flash were to fire at a higher shutter speed than the maximum synch speed you will get a partially-lit photo .....

As you can see in the above image the flash fired , it lasted about 1/10 000th sec and turned off - the slit was still in the beginning stages of its travels though and so only a slit has been lit , as the slit continues its journey with no light source the top of the image will be black .
The solution is for the flash to act as a ' continuous ' light , it fires rapidly in succession and may fire tens of thousands of times per second , and so is effectively a 'continuous ' light and behaves the same as the ambient with regard to the fact that shutter speed now controls the flash output as well as the ambient .
It would look something like this in slow motion - as the slit slides across the frame the flash keeps firing .... bzzzzzzzzzzzzzz




This is an old Canon camera so it is not exactly what the Nikon shutter would look like but it conveys the general idea . The main trade-off being that a lot of light is being wasted on the back of the partially open shutter .
To do the maths you simply need to put your SB800 on the camera and assume 'sunny 16' rule - for the brightest clearest day you could have - at iso 200 and 1/200th you would be at F16 - set it to that , it's a " worst case scenario ' for outdoor flash .
Now have a look at the working distance shown on the back of the flash with the flash head facing forward , we have :
1.) 18mm 8.8 feet .
2.) 35mm 11 feet .
3.) 105mm 17 feet .
Now when we keep the ambient exposure where it is we can assume that changing to F8 will let in 4 X as much light so we would have to go to 1/800th sec shutter speed which is high-fp mode .
This gives us :
1.) 18mm 4.6 feet
2.) 35mm 5.8 feet
3.) 105mm 8.8 feet .
The calculations :
1.) 4.6/8.8 = 0.522 X the distance .
2.) 5.8/11 = 0.527 X the distance .
3.) 8.8/17 = 0.517 X the distance .


So that means that we have just over half the working distance in each situation when we keep the ambient exposure where it is . To confirm this we can go to F4 , let in 4 X as much light , and to keep the ambient where it is we will have to go to 1/3200th sec effectively cutting that 4 X as much light in 1/4 again and negating the " advantage ' of opening the aperture .
So now the flash is behaving the same as continuous light and whatever we do , keeping the ambient correctly exposed , we will have that same working distance .
Because of the 'inverse square law' [ 2 X the distance needs 4 X the power , while 1/2 the distance only needs 1/4 the power ] .


When the subject is twice as far the lengths of the sides doubles , 2 X 2 = 4 X the area .
And of course halving the distance means you will only need 1/4 the power .


Once again it's not all that simple . The distances shown are with regard to the flash having to light the entire scene , something we don't need when we have the ambient correctly exposed .
A common setting for those who use 'fill-flash' outdoors is TTL-1.7 compensation , just to add some light to the subject without overpowering it with flash .
TTL flash doesn't know how well you have the ambient exposed and negative compensation will need to be dialed in . TTL/BL on the other hand takes the ambient exposure into account and if it detects correct exposure it will automatically dial itself back to the equivalent of TTL-1.7 in situations where they meter equally .
Many people will dial in even more negative compensation so if you had to assume an 'average' of -2 EV for the flash compensation that is two stops , or 1/4 the power that you are 'demanding' of the flash anyway so it will be able to do that at the first distance suggested by the flash head before you changed to high-fp mode .
If you were in TTL mode and went to high-fp , at105mm , and shot a subject 17 feet away [ when the flash head tells you that your limit is 8.8 feet ] the flash head will probably register -2 after firing telling you it never had enough power .
But if you have dialed in '-2' on the flash head it will be happy to light a subject 17 feet away because it can do that at '-2' :) .

Wireless flash tests

I'm in the early stages of doing tests with Nikon's wireless flash system . [ I am making deductions from tests , I have no other knowledge besides the results I see , if I make a mistake feel free to point it out and we can work it out together ]
Much of what I know is derived from my tests with TTL/BL and the realization that Nikon wireless meters the same as TTL/BL mode . The basic concept I have gathered so far is this : While TTL flash simply fires what it thinks is necessary to light the subject at a given aperture and iso REGARDLESS of the ambient lighting , TTL/BL [ and wireless ] takes the ambient into account and has the ability to add and subtract flash readings taken from individual focus points .


After reading varying opinions on whether using separate channels adds to the exposure or not I decided to do some tests . These are the first three pictures I have taken and they tell me a lot already .
I used a black background so my subject was the lightest object in the frame and exposure would be calculated according to the brightest focus point reading as derived from my previous tests with TTL/BL .




First a picture with one flash at left ....





Then we add another flash on channel 3a as well .....
[EDIT: it may look brighter because more of the face is lit up due to the second flash being closer to the camera]





You can see the other flash was closer to the camera as the area above Stinky's left eye is lit up now .... but only half as much as the rest of the face lit by flash - because both flashes fired at half the power of the original exposure .


Then we change one flash to channel 3b - the camera still calculates exposure by adding ambient plus the reading from the other channel[s] and gives similar [ slightly brighter ?] exposure .






I'll do some more tests but I'm already pretty sure that the exposure will stay consistent regardless of what channel the flashes are on .
The only reason I could think of for putting flashes on different channels is that you could then set different exposure compensation for the various channels without walking up to individual flash heads to do this .
Then I made the background white - knowing that Nikon's wireless flash defaults to TTL/BL mode I knew that it would meter for the brightest object under a focus point which would be white and would underexpose . The second shot was at +1.3 compensation to counteract this effect .








QUOTE from another member :
I've been following some of the posts which basically seem to say, "Group A is set by Nikon to under expose so that if and when you add a flash in group B, the additional light won't be too over exposed since the AWL/CLS system is too stupid to take into account lighting from two (or three) groups."


My reply :
Whoever says that is making a false assumption because they don't understand how TTL/BL meters . As I've shown in my "TTL/BL consistency theory"it meters for the brightest object under a focus point , which is why it underexposes when there is white in the frame . As shown by my recent tests the exposure comes out the same no matter how many channels [ groups ] you have . The fact that TTL/BL has the ability to take ambient into account makes it easy enough to understand that if it can say "x = f-a " [ where "x" = final output , "f " = pre-flash reading and "a" = ambient , in my made up equation ] , it can go a step further and calculate "x = [f1+f2]/2 -a " where "f1" and "f2" are group 3a and 3b pre-flash readings .
It's probably not that simple but basically what I am saying is that if it can take ambient into account , it can take pre-flash readings from different groups into account as well and do the maths .




Tonight I separated the subjects to ensure no interference between pre-flash readings .
First though , I did a few 'normal' shots ..... adding a white object under a focus point causes underexposure again .






Here we have two flashes on the same channel/group metering off the brightest focus point and adjusting accordingly . The controller doesn't know how many flashes are in each group - it only knows to make sure that the brightest focus point does not overexpose which means underexposure when white is in the frame - as it aims for a neutral gray .





If I move both flashes to the left the exposure is correct for that side - the white paper hasn't interfered with the exposure and the combined output from both flashes is the same as for one flash on that side with no white in the frame .





Now if I put the flashes on separate groups and their pre-flash readings can't interfere with each other then the left channel puts out the right amount of flash for Stinky while the right channel underexposes due to the white brochure in the frame . They have each metered for the brightest focus point that they registered on their individual pre-flashes and adjusted accordingly .





I can see a lot of confusion resulting from using different groups without knowing exactly how they will meter . I can also see advantages in using different groups in situations that would call for separate metering in a complicated composition .
For the moment I can see the simplest approach being to use one group no matter how many flashes you are using , and setting compensation on the camera body for the brightest object under a focus point . Any other adjustments for varying degrees of output from different sides could be set on the flash heads themselves .

Also have a look at these interesting tests .

Consistency theory

The main ''weakness'' with TTL/BL is that it tends to under-expose when using straight up bounce flash . But as mentioned before there is a 'consistency' to this weakness that can be exploited . It under-exposes because it is exposing for the brightest object within the focus point diamond for some reason . This is not a problem when there is a black background but causes severe under-exposure with white in the frame . When spot metering off a white subject you would normally add +1.3 EV compensation to make it 'white' without blowing any details .
I've used this same compensation in my examples .
One instance where this 'weakness' can be used to advantage is when there is a consistent light coloured background - for example at a craft fair or art fair where everything is mounted on the same white wall or in white cubicles . If the ceiling is low enough to bounce off you only need to make one compensation adjustment for that background and be sure that at least one focus point is on the background in each picture .


In this case +1.3 dialed in to the flash head did the job .



Then each picture with the same colour wall in the background shows a consistent histogram on the right , the position of that right portion is in almost exactly the same place regardless of a bright subject or a dark subject .






Now compare this to TTL flash - look how the right hand side of the histogram is all over the place . Not that you couldn't set TTL to +1.3 and use FV-lock off the wall to get the same consistency - just make sure you understand how each system works and know beforehand what results you will get and why- then you can choose for yourself what you want to use .







Since TTL uses the centre of the frame and the 'matrix' pattern which I selected [ Centre weighted would have given similar results ] I suppose you could say it could be 'predicted' and adjusted accordingly but it would involve a lot of skill to know what settings to use in such a variety of situations - with an equally large variety of compensation adjustments .
It is good to know that the 'weakness' of TTL/BL can be used to advantage because of its predictability but you would think Nikon could have done better .
I need to do more tests with the diffuser dome - I am getting some interesting results with i

Sb400

On the main page I've discussed how TTL/BL works differently to TTL by also using the focus points to meter from . It is not foolproof though , it has it's weaknesses . The main one being bounce flash in the presence of white subjects - or white in the frame .
With a fairly neutral subject it is similar to TTL .





Even when we move further back things look ok .....



Then we introduce something white in line with one of the focus points .....

and it almsot behaves like spot metering looking for the brightest part of the frame and under exposes . +1 flash compensation recovers this loss of exposure but it is a nuisance to have to do this . Actually it should probably be +1.3 stops to get the white's ''white'' .


There is good news though , it is consistent in its underexposure unlike TTL metering . We can attain TTL metering by having the camera in manual mode and changing to spot metering , then the SB400 reverts to TTL metering if that is what you are familiar with and prefer to use .



I would say you need to make a decision between using the SB400 in manual and spot metering to get TTL flash along with FV-lock OR use it in normal matrix or centre weighted metering and set one compensation value for any picture with at least one focus point on a white subject .
To show you what I mean the next three pictures are what TTL flash does with a subject that is not always central , and with varying colour backgrounds- or subjects for that matter .
The right side of the histogram is all over the place ....





Now compare the erratic metering of the above pictures to the consistency of TTL/BL+1 .





See how all the histograms in TTL/BL+1 are in the same line ! As long as at least one focus point was on that white background the exposure stayed in the same place !
The D90 only allows +1 flash compensation but I can get more by dialing it in to normal EV compensation - it is cumulative , if I dial in +1 flash compensation and +0.7 EV compensation I will actually have +1.7 flash compensation .
I find this to be the biggest weakness with TTL/BL . It means either resorting to spot metering for normal TTL flash or watching what is in the frame carefully .
The consolation is that if you know there will be a light coloured object somewhere in the frame all the time that one exposure compensation will fix it with consistent results as long as at least one focus point is on that light coloured object .
No flash mode is perfect which is why it is good to learn how they behave - especially if you have an SB400 which only does TTL/BL unless you resort to spot metering !

Focus point summary

More on the 'focus point' ' concept :
The D40 with its three focus points makes it easier to illustrate . These pictures were taken without flash and the focus point was selected manually in " Dynamic Area " focus mode where the user moves the focus point to the subject . This can give confusing results the first time until you learn what is happening .



With the centre focus point selected it behaves like the 'old' matrix metering and normal TTL flash by over-reacting to the black in the centre of the frame .
Then I moved the focus point onto the subject for completely different results ......



All of a sudden it is behaving like TTL/BL flash [ remember there was no flash used here , this is just to illustrate the 'focus point' concept ] , it is doing a much better job of exposing for the subject regardless of the centre of the frame .

Again , here is a comparison between ''normal'' matrix metering and TTL flash .....




And then again with the focus point moved to the subject compared to TTL/BL flash .





The similarity is that TTL/BL automatically metered off the brightest part of the frame even though I had the centre focus point selected in that last shot , but you can still get varied results without flash depending on which is the active focus point which is what annoys many people about the metering on the newer Nikons .
Basically this means that when you are not using flash results can vary quite greatly depending on where your focus point is . When you are using flash TTL will meter strongly for the centre of the frame while TTL/BL will automatically meter off the brightest object under the focus point diamond regardless of which focus point you have selected .

Today I did some outdoor tests in bright sunlight to test the focus point theory in a real-life situation . When the subject is mostly front lit the focus points are picking up a lot of light and backing off the flash . here is an extreme example of what could happen .
We introduce a bright white object to the bottom focus point and the flash backs right off ....




I suppose this is a real-life possibility , we have a subject with a strip of white under three focus points and the flash backs off in response . We replace that with a darker strip which is no longer the brightest object under the focus points and it takes the next brightest object - the face in this example , and gives what it considers the right amount of flash .....




As I've learned more , and of course considering the D40 only has the three focus points in the centre , but reacts to objects near the top of the frame , it has become more obvious that TTL/BL uses the entire 'focus point diamond' and has the ability to select an area based on colour to meter off . This sound very much like Nikons " 3D colour matrix metering " concept in that it can decide , based on colour , what to meter off within the important area that you normally find the focus points in .

.

Direct flash and TTL/BL

Direct flash :TTL/BL vs TTL

---

Recently I was involved in a discussion regarding the advantage of distance info when using TTL/BL and direct flash .
Anyway I decided to try to explain this in an uninterrupted logical manner for those who are interested in the subject .
Ideally we avoid direct flash but there are times when we have no choice . Outdoors in daylight is a good example with nothing to bounce off and when we only want subtle fill-flash as well .
First we need to understand how they meter :
TTL doesn't care how well you have the ambient exposed , it simply tries to get an average grey in the centre of the frame ......

TTL/BL on the other hand is more complicated , when the flash head is pointing forward it meters quite heavily toward the distance info from the lens as well as using a different pattern to meter from , it uses the " focus point diamond '' and can select a smaller area from anywhere inside that pattern ......

With direct flash though it only uses a small portion of the pre-flash reading from this pattern .
On to the tests . First we have TTL/BL , I've underexposed the background in these pictures so we only have the flash lighting the scene .





With the advantage of distance info TTL/BL does a pretty good job of maintaining a consistent exposure for the subject regardless of its size in the frame - now TTL flash .





As you can see TTL doesn't know the distance , all it knows is that it wants an average grey in the centre of the frame and to illustrate this I took both of these images and applied an 'average blur' in photoshop in the centre of the frame .....

When the subject is central and fills a decent percentage of the centre of the frame it's not bad ....





But when the subject is smaller and far from the background TTL has to make that small subject really bright to get and average grey in the centre of the frame , TTL/BL does not suffer from this weakness since it knows how far away the subject is [ with direct flash only ! ]





Now we take a darker subject with a lighter background , some may say TTL flash looks better here ....





In reality TTL has fired stronger because the subject does not fill the frame and is black so it's trying to make it an average grey . The result may be more pleasing to the eyes for some but essentially the problem is not with TTL/BL flash in this case . When you are using direct flash and want to light the subject correctly you can't expect the flash to light the background correctly as well as the subject and be accurate in both cases ,though , with TTL flash , there will be the odd occasion where the results will look good with the right combination of subject colour , size , and distance from the background which is pure luck .

So the problem is not the 'car' but the 'nut that holds the steering wheel ' .
If you are using direct flash you have to make sure you expose the ambient correctly - so that is what I have done , I adjusted the shutter speed to expose the background the way I wanted it and let the flash handle the subject - in this case TTL/BL senses that the ambient is having a greater effect on the subject and backs off accordingly .



TTL on the other hand still thinks it is the only light source and fires too strong causing more blown highlights .



The conclusion of the matter ? If you are using direct flash with TTL/BL you are concentrating on the subject at a set distance and have to expose for the background with your normal camera settings , with direct flash and TTL on the other hand it all depends on how big your subject is in the centre of the frame , the subject colour , the background colour behind the subject in the centre of the frame that it meters from , the distance form the background that is having an effect on the metering .......
I choose TTL/BL and the advantage of the distance info from the lens with direct flash .Choose whichever you want - just understand how it works . [ and shoot manual flash when possible :smile: ] .




On page 1.) we established that TTL/BL uses the distance info from the lens "when the flash is facing forward" . Of course in most situations we want to avoid direct flash but outdoors where there is nothing to bounce off , and in bright sunlight where the flash isn't the main light source , we still use it sometimes .

Just a quick review of TTL/BL and the distance factor - first a properly focused picture ....



Then we manually go to close focus and the flash dims down automatically



and as we change the focus distance to 20 metres it brightens up ....



proving that the focus distance directly affects the flash .
Now for the proof in real life . I set up my model ''Stinky'' indoors to only show the flash with little ambient to detract from the results . I left a reflector in the same position and then moved the subject under the focus point further away . This is what TTL did with the scene - it was totally fooled by that white reflector and exposed for it the whole time .





Now on to TTL/BL . Even though the reflector was in the same position reflecting back a lot of light TTL/BL continued to meter for the active focus point on the subject and was not fooled by the white object in the frame .





This proves that TTL/BL is superior with direct flash photography - it's a pity it doesn't have that reliability with bounce flash and wireless CLS .
Of course it isn't a simple program and there may still be some surprises when we use it but I am slowly coming to grips with its 'logic' .
Using direct flash outdoors is not the problem , predicting results indoors where the flash is the main source of light is more of a challenge because there is little ambient light to hide our mistakes as with pictures in bright sunlight .

Thoughts on Nikon TTL/BL 'evolution' .

There has been much confusion on the subject of TTL/BL . I don't know the full history but this is my 'theory' based on what I do know and the tests I have done .
Nikon have changed how TTL/BL works without explaining much to us .
If anyone tells you that you need a bright background for TTL/BL to work ask them why Nikon made the SB400 to only work in TTL/BL mode if they knew beginners will be using it in all conditions !?! Ask them why the built in speedlight and Nikon's wireless CLS meters in TTL/BL mode ?!!
The concept is not so hard to accept is it ?

Early cameras : TTL/"BackLit" , camera sends info about ambient on subject/[centre of frame] and background/[ the rest of the pattern , matrix or Centre Weighted ] , flash fires at the correct power to make the subject equal to the background light . - needs FV-lock if your subject is not central , not so with the "new" TTL/BL because now it uses all the focus points to meter .

Newer cameras : TTL/"BaLanced fill flash " - it was too much trouble to change the name on the flash heads so they threw the word 'balanced' into the equation . Nikon decide to use this superior technology with all situations along with the metering off the focal points . Now you can spot meter off any focus point , so they are metering off all the focus points for TTL/BL ! Todays cameras seem to meter strongly toward the focus points so it seems to be part of their 'evolution' .
With the focus point metering now available why not just ignore the background and use it to concentrate on superior exposure on the subject in any situation !
So they change what the camera tells the flash , the flash still thinks it's balancing the 'subject' with the 'background' but let's say for example they send 'fake' information about the background telling the flash it is perfectly exposed and then send the 'subject' info as the focus point readings- the flash tries to 'balance' the 'subject' with what it thinks is a perfectly exposed ' background ' and basically now it works in any situation - the flash tries to get perfect exposure of the subject regardless of the background !
Try it for yourself if you don't believe me . Use an off-centre subject to really test it because a totally central subject will give almost exactly the same results as TTL flash . Try a black background indoors or go outside at night [ because the 'experts' say it needs a bright background to work or it will fire at its weakest and underexpose ] . take a test shot in TTL and TTL/BL and you will see that TTL/BL flash does work in low ambient - better than TTL !
If you introduce a white object under one of the focus points it will underexpose but it's consistent so one exposure compensation should do for any further pictures with that white object under a focus point [ It's spot metering off the focus points ! ] . plain TTL will give varying results .
So : TTL/BL does NOT need a bright background to work and it does NOT need the subject to be central ..... anymore .

Update :

I just did some quick tests to confirm the fact that when using direct flash indoors [ which we probably never will do ] , no compensation is needed because of the superior distance info . That is , with the built in speedlight , and the SB400/600/800 when facing forward - the flash is constant .For a while I wondered then , why the output varies with direct flash outdoors in sunlight with different reflectivity if it is constant indoors ?
I think the reason it is accurate indoors with direct flash is that it must use the focus points to determine the ambient as well . It detects no ambient and adds the flash needed for correct eposure at that distance . I am beginning to doubt that the metering pattern is used at all which may be how they 'bypassed' the original program that required ''background'' lighting to balance with .The old program used the brightest part of the outside of the metering pattern as the 'background' to balance with , calculated how much ambient was on the centre of the frame , and added the correct amount of flash to equalize them . The new program , perhaps , uses the brightest focus point that detects the pre-flash as the subject , and adds what it deems to be necessary to get an average grey exposure . It still puts out the right amount of flash it would need with direct flash , less what it deems to be the ambient on the subject - which does get fooled by the reflectivity of the subject . Even though it could give 'perfect' exposure when there is no ambient due to the distance info it gets from the lens - when it detects ambient it is also at the mercy of the focus points metering off varying subject colours .By using the brightest focus point for its metering it would [ theoretically ] ensure that there are no blown highlights as long as the object is under a focus point . So perhaps that is what Nikon changed in the cameras to fool the old "back lit" system . A simple theory is that instead of sending the outer metering reading to the flash , they send a fake reading that says it is perfectly exposed there , and then instead of sending the centre of the pattern as the subject info they send the info from " the brightest focus point detecting the pre-flash " as the subject reading .Either way , that's how it appears to work .

Nikons new TTL/BL flash and CLS wireless

{ "New" since the D100 that is , but the wrong information is still circulating so I thought I would provide an update :) The official site for TTL/BL is inaccurate regarding newer cameras  }
Update December 2013: Please read my blog regarding the fact that TTL-BL is linked to matrix metering performance. Newer bodies are behaving slightly differently to my D90 test results so results will vary depending on the model Nikon you have.

Short version :- " The 'new' TTL/BL selectively meters off the focus point diamond and exposes for the brightest object in that area , taking ambient into account regardless of the background . The subject does not need to be central , it does not need a bright background to work ".


Long version : -
First of all why should you care about how TTL/BL works if you only use TTL flash ?
For one thing your built in speedlight works and meters in TTL/BL mode .
The SB400 defaults to TTL/BL mode . Note : When using forward flash with the SB400 , SB800 and the built in speedlight no compensation should be needed as the distance info is being used .
And as soon as you start using Nikon's wireless flash features in "TTL" mode it is actually using TTL/BL whether you like it or not and that is a good thing as I'll explain later [ basically because it is a superior metering system to TTL ]


The confusing part is the manual that suggests that TTL/BL tries to balance the subject with the background - that's only true for cameras older than the D200 . This means it measures the back-lighting and determines how much flash to add to the subject to make it as bright as the background . The manual's description is so ambiguous that it has even caught out top photographers . In a popular book on flash the author , who uses Nikon flash extensively , makes the incorrect statement "It means the camera is now in balance mode , trying with all its might to make the foreground and background roughly equivalent exposure zones " ... his description of the results shows that he thinks the flash will try to light the background as well as the subject  . This statement is totally wrong and what he doesn't realize is that when he is using wireless CLS he is in fact using TTL/BL mode so his advice not to use TTL/BL is contradictory  !


The correct term for the newer cameras is "balanced fill flash" which doesn't care about the background , it concentrates on correct exposure of the subject taking ambient "on the subject" into account and compensating accordingly - REGARDLESS OF THE BACKGROUND ! .
I did my tests with a Nikon D40 and 18-105VR lens and a Nikon D90 with a Tamron 17-50 F2.8 . This model lens [Tamron 17-50 ] has been known to give incorrect distance info - my copy gave equal results to the 18-105 lens so it must be good !
My tests were duplicated by Mike who is an ex-pro photographer and software engineer using his D3 and D300 . His two cameras varied quite a bit in TTL flash mode but gave consistent results in TTL/BL+1/3 according to his tests .


First we need to get get some myths out of the way . There are many sites out there stating that


1.) the subject needs to be central , and
2.) you need some ambient for the flash to balance with .
Those statements are as obsolete as a D100 because that is generally the model camera/time-period that it applies to . So those statements are correct for anything older than a D200 but after that they are wrong !


I've done a series of tests where I break down the different 'elements' of TTL/BL flash compared to TTL flash and dismiss these myths that are so widespread .
In summary before you view the results this is my conclusion :
1.) TTL flash only uses the matrix or Centre Weighted pattern , whichever you select , for its metering so the subject needs to be central or you need to use '' flash value " lock when recomposing .
TTL/BL uses the FOCUS POINT DIAMOND as well which makes it superior . I have not seen it overexpose unless all the focus points are on a black subject . Even if one focus point is on a white subject it will make the blacks 'black" as they should be while TTL would try to brighten them .
2.) TTL flash is similar to normal camera metering in that if there is a dark background or a lot of black it overexposes by concentrating on brightening the shadow areas .
TTL/BL concentrates on the lighter areas and tries to preserve highlights mainly with 'pure bounce and no forward flash . This often leads to under-exposure when there is a lot of white in the background and good exposure if there is a lot of black in the background - setting the flash to TTL/BL + 1/3 fixes a lot of this and produces a much more consistent batch of pictures that might only need a slight tweak of the levels to be good compared to TTL which often clips highlights beyond recovery .
3.) When you use Nikon's CLS "creative lighting system" wireless flash the camera/flash defaults to TTL/BL which is another nail in the coffin of the theory that you need ambient for TTL/BL to work [ on newer cameras that is ] .
4.) TTL thinks it is the only light source which is why you need varying degrees of compensation in daylight and cloudy weather .
TTL/BL also takes the camera's metering for the ambient into account and then adds the amount of flash it deems necessary for a correct exposure .


I'll start with these pictures ...




They prove that TTL/BL does not use the centre of the frame , it does not need ambient to balance with [ it was taken at night ] and as long as the subject is under the focus point diamond , not necessarily the active focus point , TTL/BL will do its best to preserve highlights - and it also means that you don't need to use Flash value lock " FV lock " . It also shows that when you use wireless 'TTL' the camera meters the same as TTL/BL !
TTL , on the other hand , is a simple exposure based on the centre of the frame , it does not use the focus points in its calculations and doesn't care how much ambient already exists , so you need to be able to use negative compensation more often when using TTL . TTL/BL does this for you .


An interesting aspect of TTL/BL is that it uses the focus distance from your lens to calculate direct flash exposure , as well as the TTL reading . Try this experiment , take a flash picture with direct flash [ We avoid direct flash when possible but it is useful in bright sunlight ] .
Then manually move the focus distance and watch the flash exposure change !











Then try this , take a 'normal ' picture in TTL/BL mode [ or TTl for that matter ]





Now try again but block the pre-flashes using second curtain and 2 seconds exposure [ as in the first picture ] then move your hand away before the flash fires .....





This proves that TTL/BL uses focus distance with direct flash but also takes the TTL reading into account .


When the subject is central there is not much difference between TTL and TTL/BL but when a tricky situation arises , like a highly reflective subject , TTL/BL shines above TTL because it also uses the focus point diamond for its metering even though tilting the flash head cancels out the distance info it receives , the extra info from the focus point diamond gives it that edge . [ It ''maps out'' the subject and rejects bright highlights 'recognizing them as reflections ' ]





As you can see I have found that TT/BL + 1/3 gives me a much more consistent exposure than plain TTL flash . When there is a lot of white in the frame TTL/BL +1.3 has a certain 'consistency' as you can see in the link at the top right in my "TTL/BL consistency theory "







It would appear from my tests that TTL flash [ and TTL metering ] looks more at the shadows and darker areas and tries to light them sometimes resulting in blown highlights while TTL/BL uses the focus points and tries to control highlights often resulting in slight underexposure which in fact results in more 'keepers' because it is easier to tweak the levels up than recover blown highlights ! [ As mentioned simply using TTL/BL+1/3 gives a better average than plain TTL ]


I'll be doing more tests with wireless flash but so far it is evident from my results that when using wireless flash and "TTL" mode the flash is in fact resorting to TTL/BL metering using the superior focus point info as well . [ compare this to the first two pictures ] .





The focus point theory ? Have a look at these pictures - [my D40 only has three focus points .]
All focus points on the black surface ....





Then I slowly moved until just one focus point was touching the white surface - it's not the in-camera metering ! I have tried this in many situations .





As long as the focus point 'diamond' finds contrast the meter does its best to preserve highlights resulting in slight underexposure when there is white in the scene but it remains consistent so one flash exposure compensation covers you .


Here's an extreme example of what I am talking about - since the white area was small and in shadow it wasn't as if it spot metered that area but it shows how well the system 'manages' differences in contrast .





I eventually worked out a test to illustrate the concept of TTL/BL compensating for the ambient . Basically TTL/BL doesn't really know what the ambient is , it relies on the meter reading from the exposure mode you have selected which is important to remember - I'll show you why ...


Since many people shoot in manual mode when using flash I thought I would illustrate the concept by having an off centre subject to exaggerate the issue a bit . First I set the meter to zero in matrix metering and took a picture using TTL/BL and bounce flash . It fired rather weakly because the meter told it the subject was already correctly exposed .

Then I changed to centre weighted metering and the meter said "+1" with regard to exposure but the flash output stayed the same ....[ note : I never changed any settings besides metering mode ]



It doesn't seem to care if you want to over-expose the subject with the ambient - it fires the same strength by the look of the results .


Now I increase the shutter speed , still in centre weighted metering mode so that the meter goes back to zero - the background changes but the flash output stays the same ....





Now see what happens when I simply change the metering mode - all the settings were the same as the previous shot in this picture except for the fact that I changed to matrix metering and suddenly the meter showed "-1" because matrix metering is strongly weighted toward the active focus point - which was on the doll .... TTL/BL responded by firing one[?] stop brighter !





So ...... it would appear from this test that TTL/BL uses the focus points for its pre-flash reading and watches the built in meter to see what your shutter speed setting is doing with the ambient .
That would mean that in a rather round-about way its output is affected by the shutter speed you select but that reading will also be affected by the metering mode you have chosen and its characteristics in different situations . If you are in an auto mode outdoors TTL/BL will always fire weakly - once you are in manual mode , the metering mode you have selected will affect how strongly TTL/BL fires as it watches your meter to determine the ambient exposure .
Once you are indoors , F5.6 iso 200 1/30th sec for example , the ambient will show up as so badly under-exposed that TTL/BL will realize it is the only light source and push out the required amount of flash .


Now on to wireless CLS which is easier to understand once you know how TTL/BL meters - because wireless CLS uses TTL/BL by default !


I set up my model " Stinky " with a black background and had one SB800 to my left on channel 3a and the other on my right on channel 3b





As with TTL/BL bounce flash it doesn't matter where the subject is in the frame as long as it is under the focus point diamond .











When we introduce a white object on the right channel 3a fires normal while 3b sees the white and "underexposes" that side [You could dial in compensation , depending what you want it to do of course ]





and exactly the opposite if we move the reflector to the other side ...





When we look at an image taken with a single on-camera flash bounced off the ceiling it looks ok until we introduce the reflector .






So the pattern we see is underexposure when there is white anywhere in the frame . This is still more consistent than TTL flash which meters only off the matrix pattern . With TTL/BL also using the focus points we have a more consistent exposure as long as at least one focus point is on the white object - easily fixed with exposure compensation .
I have found that TTL/BL + 1.3 [ dialed in to the flash head ] has much more consistency than plain TTL flash . Just remember that when you use wireless flash with Nikon's CLS you are using TTL/BL metering so in the examples above , as soon as there is white in the frame you need to add around +1.3 to the exposure !
When the background is black TTL/BL meters of the subject as the brightest object in the frame .
Tonight I walked around the house with this concept in mind - I have been trying to work out the concept of TTL/BL with bounce flash indoors for a few weeks now , taking lots of pictures and trying to solve the riddle . Well I think that's basically it now : With TTL/BL and bounce flash , as with the SB400 in bounce mode - and when using wireless CLS :- "TTL/BL meters off the focus points and adjusts exposure for the brightest part of a scene under any one of the focus points ." And these were my test pictures based on that theory :
I looked at this kettle and decided " That's white enough to be a +1.3'' [ To be able to keep the camera to my eye I dialed it in with the Ev [+-] compensation button ] .





I looked at "Stinky" and decided his face was a +0.3 - and made sure a focus point wasn't on that white bit on his shirt .





I decided the phone was a little dull to be called 'white' so went for +1 only .





This bulb looked a little grey so I went for +1 , but by the look of it I could have made it a +1.3 considering all the light objects in the frame .





Regardless of the rest of the scene I knew my shiny new manual warranted a "+1.3" ...





and just about anything that has some white in the frame seems to do well with +1.3 ....





Once again , I'm not saying TTL/BL is perfect - I wouldn't need to dial in compensation if it was - but I am saying I have discovered the 'predictable' aspect of TTL/BL bounce flash - that it meters off the lightest part of the picture - as long as it is under a focus point .....


I worked hard to make sure there were no focus points on the white manual and my 'default' setting for 'average grey' scenes of +0.3 did the trick without blowing any highlights while +1.3 totally blew the highlights in this shot .





So maybe that is the problem solved for now with regard to TTL/BL and underexposure - time will tell . Just remember that when you use forward flash with the SB400 , SB800/600 or built in speedlight indoors you don't need this compensation because the flash is using the distance info from the lens .
26/10/2009 I did some more tests to see exactly how much difference there is in the amount that TTL/BL meters compared to TTL .
I borrowed a flash meter and took slow exposures and used rear curtain flash mode so I could meter the main flash without interference from the pre-flash .


The first image was in a situation where I know TTL and TTL/BL give the same results - a neutral tone centralized in the frame - they both gave the same result according to the flash meter .





Then I increased the time the shutter was open to expose the ambient correctly - TTL/BL responded by decreasing its power by 1.7 stops .





This test proves that TTL/BL does in fact adjust according to where the meter is which is affected by the shutter speed and how much of the ambient you capture . When the ambient is badly under-exposed TTL/BL takes over and provides as much lighting as it deems necessary . As you drag the shutter and let in more ambient TTL/BL responds by backing off its output all the way to -1.7 when the ambient is correctly exposed - which is a commonly accepted setting for for fill-flash in TTL mode . Of course since TTL and TTL/BL meter differently the results will not be exactly the same in varying situations .


In my recent blog on tests with the diffuser dome a comment was added by Russ McDonald who helped design the original system in the 80's which I find very interesting .

" In TTL-BL mode, When the preflash return is strong much more emphasis is placed on the preflash and much less is placed on the distance.

In TTL-BL mode, when the preflash return is weak, much more emphasis is placed on the distance.

This may be part of the differences you are seeing between using the diffuser and not using the diffuser. The diffuser can cause a big difference in the preflash return energy. "

A while ago he mentioned how the original program was designed to use distance info in case the pre-flash reading was messed up by the bright background for which TTL/BL was originally designed [ before they changed it ] .
So it appears that in an obscure way the diffuser dome gives greater accuracy because the weaker pre-flash reading encourages the program to use the distance info more in its calculations taking advantage of the direct light coming from the dome .

An interesting read :)









I've added a 'donate button' while I wait for adsense to be approved so perhaps I can spend more time doing further research .