Friday, February 29, 2008

8. How the Built-In Flash Metering Works

Here is how the metering works when using the Nikon Built-In Flash.

When you select camera Matrix metering the built-in flash automatically switches to TTL-BL mode.

The question I will try to answer is: Why does it do this?

Well, when you use Matrix metering, the camera first meters the brightness of the ambient light of the entire frame fairly evenly from edge to edge. Then, when a subject is placed in a small portion of the center of the frame, you can see that it does not cause a significant change to the matrix measurement made by the camera. In other words, in matrix mode, the measurement that the camera makes is a very good approximation of the background ambient light BEHIND the subject.

This approximation concept is the fundamental basis for the TTL-BL mode!

Next, the built-in flash fires its preflashes, and it measures the strength of the reflection only in the center portion of the frame, because it is trying to measure the reflection from only the subject. In other words, the flash metering system always assumes there is a subject centered in the frame when it fires its preflashes.

Then, the flash computer compares the brightness of the subject (based on the reflected preflash from the center of the frame) to the brightness of the background (based on the matrix measurement of the entire frame from the camera), and it computes a flash power that will make the subject brightness equal to the overall scene brightness. Then the shutter is opened and the main flash is fired and the picture is taken.

Now, for the pop-up flash Nikon wanted to simplify things, so they forced the flash to always operate in TTL-BL whenever the camera is in Matrix mode. This relieves a novice photographer from ever having to make the fairly complex decision of when to use TTL mode and matrix together, which is allowed when using an external flash in the hotshoe.

Also, it's important to understand that the choice of the camera metering mode has absolutely no effect on the way the flash meters the preflash reflection. It always looks at the center of the frame regardless of what camera metering mode is chosen.

Now, if you switch the camera to spot metering, the camera now only measures the brightness of the subject due to ambient light; ie there is no background metering data, so obviously there is no way for the flash to work in TTL-BL mode when the background information is missing.

So the flash automatically switches to TTL mode, and no additional metering information is sent from the camera to the flash.

In TTL mode the flash uses only its own reflected preflash information, to set a flash power to make the brightness of the center of the frame (the subject) be a 'standard' brightness. This 'standard' brightness is adjusted in the factory to properly expose a subject of normal reflectivity placed in the exact center of the frame.

The size of the flash metered area is exactly the same whether in TTL or in TTL-BL modes. The size of the camera metered area changes depending on which camera metering mode is chosen.

Now, the pop-up flash can also work in TTL-BL when the camera is in Center Weighted mode, but this will only work correctly under certain circumstances. One is when the subject occupies only a very very small portion of the center of the frame, so that the camera metering information that is sent to the flash contains at least a little background information. Then the flash computer runs the TTL-BL calculations assuming the data from the camera is background information. It doesn't work very well most of the time, because the subject usually occupies too much space in the center of the frame and affects the background information substantially.

In summary, you may be able to see that there are a few situations that won't meter very well when using the built-in flash coupled with matrix metering, because you are forced to use TTL-BL. In fact, indoors the built-in flash will often work much better if you select Spot metering which forces the flash to function in TTL mode. Matrix mode will usually work better outdoors than indoors, because the background outdoors is generally brighter than the subject, and with the flash in TTL-BL the flash will add brightness to the subject to make it equal the background. Matrix and TTL-BL will not work well if the subject is brighter than or equal to the the background. That's when you need to switch to Spot and TTL.

This last situation highlights one of the big advantages of an an external flash, where you can leave the camera in Matrix and switch the flash manually to TTL.


Anonymous said...

How do you know what cameras have TTL-BL? I thought it was quite a new tech.?

Russ MacDonald said...

To Anonymous,

No, the concept of TTL-BL is not new. To my knowledge, it was first used on the F5 and F100 which were introduced in the early 1990's (I actually worked on it as an electrical engineer). The breakthrough technology was the new 'multi-sensor' metering on the camera. The initials BL originally stood for BackLight, which is still a good way to decide when to use this feature (ie, if the subject is backlighted).

However, the original system had shortcomings and didn't work too well. Since then, it has been refined with each new camera and flash.

The key to really good TTL-BL operation is the modern matrix metering. TTL-BL has to accurately know the intensity of the ambient light, and the better the matrix metering system, the better the BL function works.

The matrix metering on the latest cameras is truly phenominal, and this makes TTL-BL work extremely well.

The one caveat for BL to work right, is that the subject must be darker than the background. The flash can only brighten (obviously) and if the subject starts out brighter than the background, all the flash can do is set minimum power, which will often underexpose the subject, because the camera shutter speed is restricted by the 'Flash Shutter Speed' which is 1/60th at the slowest. This often makes the whole image come out dark.

BL works best outdoors during the daytime, when the subject is almost always darker than the background, and this is why BL is mostly used for fill flash outdoors.

Indoors under artificial light, the subject is nearly always brighter than the background, so BL doesn't work well for most indoor shots. It is best to use regular TTL for most indoor shots.

Long answer to a short question! Hope that helps.


Anonymous said...

I think it is quite unfortunate that such an explanation is not found in the Nikon manuals.
The FV lock is another useful workaround for avoiding TTL-BL underexposure issues in the dark with the built-in flash, without switching to spot metering.
But this is not just because of what one would expect FV lock to do, but rather because a different variant of the flash exposure program is used - not sure what exactly it is but seems to be closer to standard TTL.
Kind regards, Sem

Russ MacDonald said...

To Sem,

I think you are right!

I have also noticed a difference between FV Lock and regular shots when using the built-in flash.


- Steve P. said...

Thanks for a very interesting article. I was surprised to read that matrix metering requires the centering of the subject to get accurate exposure with iTTL BL.

I'm working on an instructional lesson on the Nikon CLS system as a component of my master's thesis. I will be sure to credit you and your site for this really helpful information.

Keep up the good work!

My best,
Steve P.

Russ MacDonald said...

Hi Steve,

I'm glad you find my articles useful!

Just to be clear, the subject needs to be centered in both TTL and TTL-BL modes. The flash metering is always center weighted. That's why FV Lock was invented, and I wrote another article on that.



- Steve P. said...
This comment has been removed by the author.
Russ MacDonald said...

To Steve,

Also, in TTL-BL mode, the centering requirement isn't as strong, because distance is used in conjunction with the monitoring preflash.

Often an off-center subject will come out pretty well in TTL-BL, where flash is only being used for fill.


- Steve P. said...

One more note. I did compare a static shot taken using flash lock and one without. Both images were taken in aperture priority, front curtain, iTTL BL. Although the camera aperture and shutter speed where the same, the flash output looks to be a full stop brighter using flash lock (shooting a centered dark subject on a light background).

My best,
Steve P.

Russ MacDonald said...

Steve P,

Be careful when using TTL-BL with FV Lock. I'm not convinced it works very well. FV Lock is really intended for regular TTL, which gets all of its data from the monitor preflashes.

In TTL-BL, most of the information for the flash power calculation comes from the distance. I do not believe that the info locked in when using FV Lock includes the distance.


- Steve P. said...

I just tested in iTTL mode and found similar results; using flash lock looks to be 1/2 to 3/4 stop brighter than without.

Thanks Again.

- Steve P. said...
This comment has been removed by the author.
Russ MacDonald said...

Hi again Steve P,

Yes, in TTL-BL when the head of the flash is not pointed straight forward, then the monitor preflashes are used to determine the flash power. The distanc eis not used unless the head is point forward.

I really recommend that you read my other blogs. I have discussed all this at length in these articles:



John Meyer said...

Great blog!

Rich Smith said...

Nice description.

I've felt for some time that Nikon's decision to use TTL-BL by default with the popup flash is completely wrong. You don't come right out and say that but you do say these things:

"Now, for the pop-up flash Nikon wanted to simplify things, so they forced the flash to always operate in TTL-BL whenever the camera is in Matrix mode. This relieves a novice photographer from ever having to make the fairly complex decision of when to use TTL mode and matrix together, which is allowed when using an external flash in the hotshoe."


"In fact, indoors the built-in flash will often work much better if you select Spot metering which forces the flash to function in TTL mode. Matrix mode will usually work better outdoors than indoors, because the background outdoors is generally brighter than the subject, and with the flash in TTL-BL the flash will add brightness to the subject to make it equal the background."

Lets assume a novice user in Auto mode, especially someone graduating from a P&S. What behavior does this person expect from the built in flash and when are they most likely to use it? The flash will popup automatically indoors and use TTL-BL. This is often not the best mode. Outdoors where TTL-BL can really help with fill, the flash will not automatically pop up. This behavior is exactly the opposite of what it should be.

Russ MacDonald said...

Hi Rich,

You are right - but - with every new generation of camera (and its pop-up flash), they are improving TTL-BL and getting closer and closer to the point where TTL-BL will be able to handle all lighting conditons.

In the D90 and D300, the pop-up flash in TTL-BL mode recognizes that the background is darker than the subject, so it switches itself to a pseudo regular TTL mode and does pretty well.

In older cameras, like the D80 and D200 and older, it is much better nsiderably to switch to Spot metering to force the flash into regular TTL indoors, but in the newer cameras it doesn't make as big a difference and TTL-BL should be fine for novice photographers.

The pop-up will still do better indoors in dim ambient if you force it into regular TTL, but the difference has become slight. I suspect that within the next couple of generations, you will only need regular TTL mode when you want more positive control of the flash.

Also, in camera Auto Mode, the newer generations of cameras try to recognize when the subject is darker than the background and automatically pop-up the flash in TTL-BL mode to add fill. The newer cameras get it right most of the time, and they are getting better with each generation of camera.


Rich Smith said...


Thanks for the response. I recently upgraded from a D70 to a D5000. I usually use an external flash, but I'll experiment with the built-in flash for times when I don't have the external flash with me. I'd like to be able to hand my camera to a novice and get good results.

Russ MacDonald said...

Hi Rich,

I haven't got one to test, but I believe that the D-5000 uses the newest TTL-BL algorythms, so it should work very well under most conditions using Matrix metering.

You should be able to put your camer in P mode, Matrix metering, and pop up the flash and hand the camera to your frind. This should make a very good picture.


Anonymous said...

HI Russ, I admire your website!!!
Question: how does the flash meter for the center??? my guess is that just as the matrix metering system breaks down the light meter into different regions of metering, and gives each zone a certain level of priority over another, the flash metering system does the same. It gives priority to the center of the light meter for it’s metering. Question: is the camera’s flash meter shape similar to the shape of the camera's sensor behind the shutter, so that it knows when the light is striking it in the center??? Is it very similar to when light hits the sensor behind the shutter...if the center of the scene is dark and the edges are light, then there will be less light hitting the sensor in the middle than at the edges? Is the shape of the camera's light meter shaped rectangular like this too?

I hope I don't have things all confused. Thanks Russ for all your generosity with this website!

flybywire said...

Hi Russ,

Your information is great.
This may be a double post.

If I choose exposure to manual dial and activate the pop up flash and use spot metering to meter the background exposure say 1/125s f8 and place a subject in shade and in the center of the view finder. Then I switch to Matrix metering and take the picture. Will the camera (D7000) preflash in TTL-BL or will it preflash in TTL?



Russ MacDonald said...

Hi Vincent,

In Camera Manual mode with Spot Metering, the flash will be in Regular TTL mode. Then, if you select Matrix Metering, the flash will switch to TTL-BL mode.

For TTL-BL to work correctly, the flash computer assumes the camera controls are set for proper exposure of the background (ambient), so what you propose will make TTL-BL work right, since that's exactly what the system is attempting to do automatically in Matrix Metering mode.

In fact, the method you described is how I shoot people with sunsets behind them. Except, I usually don't bother to switch to Spot mode. I just step beside the person and aim at the sky with the person out of the picture, and matrix mode gets it right.

Good thinking!


flybywire said...

Hi Russ,

Thanks for the fast reply. I will assign Fn button for spot metering and that should ease my fiddling with so many buttons.

Anonymous said...

Hi Russ,

Awesome blog, as a novice I really learn a lot here...

The thing you mention here is the big advantage of using matrix metering in combination with an external flash.

I can't seem to use this though with my SB-700. It only switches to TTL (not TTL-BL) in spot metering. I can't seem to find any way arround this.

Do you have any explanation why Nikon might have removed this feature on their (and mine) new SB-700?

Kind regards.

Russ MacDonald said...

To anonymous (wish you would sign your name),

Your SB-700 is working correctly.

I actually have explained this in my blogs, but let me review the key points:

If you select Spot metering on the camera, that will block TTL-BL mode on the flash, because in Spot metering mode the only metering data that is sent from the camera to the flash metering algorythm is reflected from the subject. TTL-BL requires a measurement of the background ambient to wiork properly.

If you select Matrix or CW metering, then, the metering data is considered by the flash a proxy for the brightnerss of the ambient. The flash then sets its power to make the subject brightness equal the ambient.

One big caviat:

For TTL-BL to work correctly, the meter on the camera must be zeroed. That's why I recommend you use camera P mode whenever you use the flash in TTL-BL mode. It will zero the meter automatically.

A second big caviat:

For TTL-BL to work correctly, the ambient light must be quite bright, or the ISO turned way up (which simulates bright ambient). Otherwise, the camera will not be able to zero the meter due to the Flash Shutter Speed that is 1/60th minimum by default.

I don't recommend using TTL-BL indoors.

Use regular TTL indoors with the camera in Manual.

Hope this helps,


MerretKroam said...

Hi Russ,

Thank you for the quick response to my question.

I now see that i did not quite state my issue very to the point.

The things you mention were indeed in the blog, but thank you for the quick summary anyway.

The thing is that i can't seem to use or select TTL in combination with CW or Matrix metering on my SB-700. Only when selecting spot metering it forces the flash to TTL (logical!).

Is this a caviat for the SB-700? This because you state that a big advantage of an external flash is the use of it in combination with CW or Matrix metering and TTL.

Kind regards, (and signed)

Russ MacDonald said...

Hi Luc (thanks for your name, btw:)

I understand.

It is true that you can select matrix or CW with regular TTL on some of the other speedlights. The question is, why would you want to?

When using regular TTL, the flash metering totally ignores the camera metering, so switching modes has no effect on the flash power.

Those combinations probably should have been disallowed on all CLS speedlights. I cannot think of any legitimate use for them.

When you are in regular TTL, the metering mode on the camera only affects the ambient portion of the exposure. The flash portion is metered by the flash metering circuits which does not use the camera metering data. The flash power remains the same no matter what metering mode you choose.

When in TTL-BL, then the camera metering data is used to help set flash power, and it makes a big differece.

I don't know why they just now decided to disallow those combinations on the SB700.


jonathan said...


You describe two different metering systems, camera and flash. Are both contained in camera? In your discussions, which are terrific, it is unclear if the external flash unit has a computer and is using pre-flash data to determine output, or whether all of the information gathered and calculated in the camera then transmitted out to the flash. If flash is doing some of its own computing, where is the sensor located?

Thank you!


Russ MacDonald said...

Hi Jonathan,

The physical location of the components of each system is not really important, and I normally do not dwell on it for sake of simplicity and understanding.

However, since you asked, both meterings systems are contained entirely within the camera. Both systems use the same sensor in the camera, but they use it at different times and they measure using different algorythms.

Both systems also use the same main computer that is in the camera, but at different times and running different algorythms.

The flash also has a computer, but it is used only as a controller for data communication. It receives information from the camera.

The flash has no TTL sensor and does no processing of the monitor preflash. The flash sends no data to the camera.

The flash fires the preflash when told to do so by the camera and the strength of the reflected energy is measured inside the camera (TTL - through the lens) by the flash metering system.

It's best to think of the two metering systems as completely seperate, even though they share some components. They work independently and they measure information at different times and under different algorythms.

jrc said...

Russ, Thank you SO much for this post. I'm convinced you need to write a book; I'll buy it! In fact, I'll buy one for my brother! I've never read a more informative (and caring) post.

Question: In the last post regarding using matrix/CW with TTL, you ask why would anyone want to use this combination....? Did you mean the combination of Spot metering and TTL-BL? I thought one of the benefits of using an external flash was the ability to use matrix for camera/ambient exposure and TTL. Please review the comments and help!

Thanks, JR

Russ MacDonald said...

Hi JR,

Thanks for the compliments, but a book is too much like work. This is for fun!

I reread my comment, and it says what I entended to say. The answer to your question is in the next paragraph. I will copy it here to make it easier:

I wrote:
"When using regular TTL, the flash metering totally ignores the camera metering, so switching modes has no effect on the flash power."

So, in Regular TTL, even though it can be selected on some flashes, there would be no purpose for using CW or Matrix metering.

However, Spot metering is used in regular TTL mode, to meter the subject alone, which is what you want to use to determine the settings required to underexpose the subject by two to three stops.

Hope that helps,


Nicu Mihai said...

Hi Russ,

Great article and is so well explained, good work.

I have a little problem. Case study: I have an SU-800 on camera and I fireup two flashes thru CLS with TTL. One in left and one in right of the subject (for exemple - same channels and same group). How the TTL measure is made? The L and R flashes will pre-flash on subject and then the camera reads the light on the subject and adjust the correct amount of light? Even is my body flash (the focus assist lamp) it's not directed to the subject and to the wall for example. So, practically, doesn't matter where is directed the body of external flashes because the camera reads the amount of light reflected on subject (center of frame) and after that it will transmit the shutter to be open to make the exposure. So, the TTL is measured on every flash independently? Thank you for your time!