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So I'm only beginning to learn TTL but from what I understand the preflash determines how much flash power to use. But the camera already exposes everything correctly for the entire scene (assuming not in M mode and no compensation dialed in) even before the preflash. So come time when the flash is added, it's not going to change the original exposure setting and hence will by definition overexpose it. Is this assumption correct?

And I guess the amount of overexposure will be implementation specific.

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I notice that your earlier question is essentially the same. Is this meant to be a follow up to that? Can you clarify what needs further explaining? –  mattdm May 30 '13 at 18:03
Did you ever compare the camera's pre-exposure metering setting to the EXIF info from the image file after the shot? It was in the comments section of @mattdm's answer to the same question you asked on April 8. photo.stackexchange.com/questions/37727/… –  Michael Clark May 31 '13 at 4:07
You also asked the same question on April 9. photo.stackexchange.com/questions/37775/… –  Michael Clark May 31 '13 at 4:08
Your assumption of what correct exposure is, and thus your assumption of what TTL does, is what is incorrect. –  Michael Clark May 31 '13 at 4:59
Like a few others, this question feels like you've asked it before. Can you please clarify how it's really different or I'll close and merge the questions. –  John Cavan May 31 '13 at 10:23
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marked as duplicate by mattdm, Michael Clark, AJ Henderson, John Cavan May 31 '13 at 14:29

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

5 Answers

Not exactly, but I understand why you would think that. The goal of TTL flash-metering is actually to calculate the correct amount of light to output in order to expose a scene properly.

There are several flash modes which are independent of TTL metering. Which ones are available will depend on your camera. The one you describe is called Fill flash. In this case, the scene is correctly exposed without the flash and the flash is added to fill out shadows and reduce contrast. Therefore the flash output is set to be generally weak compared to the light in the scene. On dark parts, they would become brighter but should not over-expose. On the brightest part of the image, one could conceivably add just enough light to push someone to over-expose but that should be rare given the flash power is weak relative to the brightest parts of a scene.

There are other modes where the scene is exposed less than needed to be properly exposed and then the flash is counted upon to add enough illumination. Results depend on the scene because flash affects things differently based on distance and reflection. Depending on how your camera meters, the resulting exposure can bring one part of the image to look good while another part may overexpose.

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That makes sense, but even for Fill Flash, you are adding light to the shadow area without darkening other areas of the frame. So even by just a little, you are overexposing the frame no? –  erotsppa May 30 '13 at 15:49
@erotsppa - That is what I meant in the end of paragraph 2. However this is minimal because influence is relative and it is difficult to brighten significantly a bright part of the image. –  Itai May 30 '13 at 16:02
Sorry maybe the terminology is confusing. When I say "Overexpose" I don't mean blownout highlights. I just mean "overexposed" according to what the camera would usually consider as "right" exposure (according to whatever metering mode you were using, in this case I assume matrix). –  erotsppa May 30 '13 at 18:27
@erotsppa -- consider a shadow that is, say, five stops down from the highlights (which is typical in a hard-lit scene). An additional light strong enough to raise that shadow by two stops, meaning that you're quadrupling the light on that part of the image, will still only raise the illumination of the highlights by 1/8 part, or about 1/6 of a stop -- half of the minimum change you can ordinarily make manually to an exposure. It really is negligible. –  user2719 May 30 '13 at 19:59
@erotsppa - You are making a false assumption: That there is only one correct exposure value for an entire scene, and that changing the value of the shadows relative to the same highlights will change the correct exposure value for the scene. This is not the case. Normally, correct exposure is based on the brightest parts of a scene. How dark the shadows are compared to the highlights doesn't change the exposure needed to insure the highlights are not blown out. –  Michael Clark May 31 '13 at 4:14
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TTL is factored in to the metering of the camera. It doesn't work independent of the normal metering. It works with it. When using E-TTL, the camera will use the information from the pre-flash as well as the information about the available non-flash lighting to formulate correct exposure and flash settings. You can further adjust it's calculations by using both flash exposure compensation (which will increase or decrease the flash power in relation to what it calculates) or exposure compensation (which will increase or decrease the amount of exposure that the camera performs in relation to what it calculates.)

Your assumption that E-TTL has no impact on the standard metering of the camera may be the error. On some, but apparently not all, cameras. It also only applies when in automatic modes on cameras that support it. If the exposure isn't linked or the exposure is in a manual mode, then the TTL will attempt to configure the flash for a standard exposure and thus would put out a minimum amount of light if standard exposure is already achieved.

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You are correct, that is my missing piece. But can you confirm that is the case? I don't have a camera with me this second, but from my memory that was not the case. In other words, when the flash was off the exposure setting is the same as when the flash was fire (aperture, shutter, iso). P.S I was using a Sony dslr system. –  erotsppa May 30 '13 at 15:54
@erotsppa - E-TTL is Canon specific. Noone else works quite the same way and so it is camera dependent. Plus it depends on the flash mode, therefore what is described here could be the case sometimes but it is definitely not the case all the time. –  Itai May 30 '13 at 16:05
Just a quick check with some cameras and I can confirm you that neither Nikon, Pentax, Sony or Fuji adjust values in Fill/Forced mode. Unfortunately, I do not have a Canon camera with flash or any Olympus or Panasonic ones today. –  Itai May 30 '13 at 16:12
It won't necessarily overexpose the frame. Consider a typical case: foreground subject with some shadow where the background is bright. The exposure program balances the whole scene, which without fill-flash would leave the foreground subject in shadow — "underexposed". The fill flash brings it up to a level closer to the rest of the exposure. –  mattdm May 30 '13 at 18:07
Let's try the simple model with averaging instead of adding. For 2+2+2+1, the matrix metering may choose "2" as the correct exposure, leaving "1" underexposed. If the darker sector is in the foreground, with the addition of flash, it would become 2+2+2+2, and the value of 2 gives a nice even exposure. Since this is actually a common case, there you go. Now, it may be that the matrix metering would choose instead to expose for the "1" without the flash, but it probably won't with flash. Here it does depend some on the system. –  mattdm May 30 '13 at 19:05
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Your assumption is incomplete more than anything. "Assuming" TTL is providing you with correct exposure (there are many reasons and situations in which TTL must be corrected) and "assuming" the flash TTL input is accurate, then the flash will only change quality and direction of light.

Let's call TTL 0. At 0 all things are good. When the flash is calibrated with the preflash the cpu is discovering what 0 looks like. If your TTL settings for your flash output are 0, then the cpu will mix accordingly. The end exposure will still be 0. The only thing that has changed is the softening of shadows under eyes (assuming you're photographing a person).

Let's say you want to underexpose your scene by 1/3 stop and overexpose your subject (lit by flash) by 1/3 (to create a nice 2/3 stop difference between the 2) then you would use your exposure compensation feature to control the two. In this case, because you told the cpu to do so, the flash will indeed overexpose while ambient has gone under (as directed).

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No. Generally, if not in M mode, the flash exposure will replace shutter speed as a factor, and the flash brightness will be automatically selected to create a correct neutral exposure for the ISO and aperture.

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So you are saying that when the flash is fired, the camera will increase shutter speed and arrive at a neutral exposure? –  erotsppa May 30 '13 at 17:44
No. Unless the combination of ambient light, open aperture, and ISO is sufficient, the amount of surrounding light admitted in a brief shutter opening is negligible compared to the brief burst of light from the flash. In the simple case, the shutter speed is completely irrelevant. –  mattdm May 30 '13 at 18:00
Ah! I knew I was having deja vu here -- see photo.stackexchange.com/q/37727/1943 –  mattdm May 30 '13 at 18:01
Since the flash duration is normally much shorter than the shutter duration at x-sync, changing the shutter speed doesn't affect the amount of exposure due to the flash illuminating the scene. –  Michael Clark May 31 '13 at 4:32
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No it does not.

As has been answered here to the same question you asked 3/28, here when you asked the same question on 4/8, here on 4/9, and almost the same question here on 5/7.

It seems that all of your questions are based on the incorrect assumption that correct exposure for a scene is additive. That is, you appear to incorrectly assume that proper exposure is calculated by averaging all of the luminance values in the frame. This is not the case! Any given exposure value, which is a combination of ISO, aperture, and shutter speed is only correct for a single luminance value.

Look at it this way: If you have four gray objects in a scene and each one has a different luminance value, only one at a time can be properly exposed to appear to be at 18%. If you expose correctly for the darkest object to appear to be 18% grey, the other three will be varying shades brighter than 18%. If you expose correctly for the brightest object, the other three will be varying shades darker than 18%. What most TTL systems attempt to do is adjust the ISO+Tv+Av correctly for the brightest object (just as your camera would normally try to do without a flash), and then add enough flash to raise the other three as close to 18% as it can without also causing the brightest object to be pushed into being overexposed.

What exactly does TTL flash sets its power to?

How do TTL flash metering systems calculate how much power is needed?

Will the camera compensate exposure anticipating for flash?

Does a flash's TTL automatically adjust as I change its location?

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You are right, I really did thought the proper exposure is by calculating by averaging all the luminance values in the frame. How else would it do it? Is this a separate question? –  erotsppa May 31 '13 at 15:29
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