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How does the Nikon popup flash metering work?

Russ MacDonald (Arkayem)

Keywords: nikon, speedlights, lighting, flash

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?


Nikon - Built-In Flash

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 pre-flashes, 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 pre-flashes.

Then, the flash computer compares the brightness of the subject (based on the reflected pre-flash 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 pre-flash 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; i.e. 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 pre-flash 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 background. That's when you need to switch to Spot and TTL.

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


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(17 Votes )

Originally written on July 29, 2013

Last updated on January 24, 2021

Russ MacDonald Russ MacDonald (Arkayem)

Awarded for his high level skills in flash photography

Richmond Hill, GA (Savannah), USA
Gold, 6156 posts


David Powell (DaveP142) on February 20, 2020

2020: Just stumbled across this article and the timing couldn't be better as I am just beginning to use flash occasionally. This is a very clear account of some basic choices. This is a great background from which to approach themanuals for whatever kit actually gets used. Thank you.

Kenneth Swann (prophetic2) on August 11, 2013

Thanks also Russ...adding to Will Welch's question, my problem with the TTl mode has always been if the focus point is off center and you have more than one subject in the picture at different distances from the camera- like seated at a round table- I have not found the TTL to be much good at that kind of shot which I do a lot of. Mind you, it is most often with an SB-700, but with either flash the different subject distances don't get metered correctly. Any thoughts?

User on August 11, 2013

thanks for your time....I always try and get one new learning from each article i read.....i accomplished my goal !!! dc

Will Welch (wSq) on August 4, 2013

Thanks, Russ, for this easy to understand explanation. What happens if I put the subject off-center? And does focusing on the off-center subject make any difference to how the flash is metered?

J.P.M. Driessen (johnnie5) on August 3, 2013

as always: great article, thank you. question on your last note: when and why use camera in matrix mode and flash in ttl mode? John

User on August 2, 2013

Mikey, Looks like a Lastolite Box... I have one...they are great !!! dc

Mike Banks (unclemikey) on July 31, 2013

Victor, what is that you have on your flash, I want one.

Mike Banks (unclemikey) on July 31, 2013

I was wondering why I was getting screwy exposures with the built in flash. Now I know. I don't use it often but sometimes its the only game in town. Thanks Russ a great tutorial and very well explained.

Victor Rakmil (VR8) on July 31, 2013

Thanks. Very helpful. Any chance you will address the R1C1?

User on July 30, 2013

Thanks Russ, Your comprehensive explanation is greatly appreciated. I now understand much better how to maximize the use of the built in flash.

Hugh Robinson (Love2Photo) on July 30, 2013

Thank you for sharing your knowledge about the built-in flash. This information comes at a perfect time for me. Thanks again.

Richard Luse (DaddySS) on July 30, 2013

Ribbon awarded for his generous support to the Fundraising Campaign 2014 Ribbon awarded for his generous support to the Fundraising Campaign 2015 Ribbon awarded for  his generous support to the Fundraising Campaign 2017 Ribbon awarded for his generous contribution to the 2019 Fundraising campaign

Very helpful and well explained as usual Russ, thanks!

Peter Teuben (pteuben) on July 29, 2013

My SB-600 (i know, not the builtin) flash says TTL-FP but doesn't seem to bring up the always discussed TTL-BL mode. Or is FP synonymous to FP?

User on July 29, 2013

Very helpful explanation of how the pop-up flash works. Thanks. Is there a way to adjust the intensity of the pop-up flash when it's used for fill-in flash purposes? Saul...

User on July 29, 2013

Excellent instruction..many thanks Robert S Lappin