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Does anyone know how TTL calculates the amount of power to put in after the preflash is done? I understand that the value is somewhat brand specific, but what is the general logic? Because if you think about it, before flash the camera calculates the exposure to the 18% rule. But this makes no sense for flash.

For example, if I'm already at the perfect exposure (18%) before flash, the flash will still fire. If it followed that logic, it should not fire at all.

Also, if the logic is to bring the whole scene to 18%, doesn't that mean the flash power is inversely proportional to my continuous light exposure? i.e., the more I underexpose the background, the more power it will flash?

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For reference do also have a look at this question/answer. –  Peng Tuck Kwok Apr 8 '13 at 3:45
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Trying not to be overly pedantic but TTL just means, "through the lens". I believe proper usage should be "TTL metering". :) –  Regmi Apr 8 '13 at 17:39
    
In this case it would mean "TTL flash metering" –  Michael Clark Apr 9 '13 at 1:03
    
Thanks Michael. –  Regmi Apr 9 '13 at 2:55
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3 Answers 3

The camera will calculate the amount of flash to use in one of two ways, depending on the conditions.

The first case is when the flash is the primary light source. In this case, the shutter speed is irrelevant. The target exposure is still the standard 18% gray, but flash power effectively replaces shutter duration in the calculation. That's because the ambient light is low enough that any moderate shutter speed just wont let in enough to make a difference: only the brief powerful flash impulse will matter. The camera's program calculates the ISO, aperture, and flash power needed. (In some modes, you may supply ISO or aperture and the camera will compute the rest.) In modern TTL systems, the amount of flash power needed is measured with a very brief, lower-power "pre-flash" (possibly combined with the focus distance info from the lens). If you're calculating it yourself, you use the guide number for the flash (adjusted for ISO) divided by distance to the subject to find the aperture to use.

The second case is when the flash is used for fill. Here, the ambient light is bright enough, sensitivity is high enough, and aperture wide enough that the shutter speed actually does have an impact on the exposure. The way to set the exact balance between shutter speed (ambient light) and flash power (your additional fill) will vary between camera models and camera modes, but that basic principle is that there's some balance between light sources. The scene without flash would be slightly underexposed (but not completely dark), but the flash burst uses a power level intended to bring it back up to "correct" middle exposure. If you're making the calculation yourself, basically you calculate as above but then subtract a stop (or half, or two, as desired), compensating with the ambient exposure. (More on that under How do I manually calculate fill flash?)

When exactly the camera uses one approach over the other is also brand/model specific; usually putting the camera into aperture- or shutter-priority mode makes the camera expect you want fill flash. It may also make that decision automatically in other automatic modes when the ambient light is high.

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In the second mode you described, I'm trying to understand what some balance "means". Because I'm already correctly exposing everything, how much light is added via fill is considered balance? And doesn't that by definition means it will overexpose my scene as a whole? –  erotsppa Apr 8 '13 at 19:04
    
You (or the program mode) select a shutter speed, aperture, and ISO so that without a flash the scene would be underexposed by a stop or so; the fill flash provides the rest. Alternatively, the background of the scene may be correctly exposed but a foreground shadowed subject may need the flash. –  mattdm Apr 8 '13 at 20:08
    
If I correctly exposes the entire photo, what would happen? The camera would calculate the exposure specific to the foreground and then apply fill as necessary? –  erotsppa Apr 8 '13 at 21:52
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@erotsppa: Have you tried this? Compare the Tv and Av displayed in the viewfinder after metering (but before the preflash) to the EXIF info for the Tv and Av actually used by the camera when you take the shot following the preflash. Try it in bright as well as darker environments and note the differences. –  Michael Clark Apr 9 '13 at 1:02
    
@erotsppa: Also try this: Meter with the TTL flash turned on and again with it turned off. How, if at all, do the Tv and Av displayed before the preflash is fired change in bright conditions? In darker conditions? –  Michael Clark Apr 9 '13 at 1:10
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TTL systems in DSLRs use a preflash just before the mirror begins to move out of the way to measure the reflectivity of the subject and calculate the amount of flash needed for a proper exposure. Some systems even use distance data from the lens to determine what will be exposed properly based on the distance to the in-focus subject.

How much flash the camera applies is influenced by several variables. Which mode you are shooting in (Auto, P, Tv, Av, M, etc), the overall EV and dynamic range of the scene, and which metering and focus modes you have selected can all factor into the calculation. If you are shooting in Av mode with a Canon in low light environments, for example, the camera will assume you want to use slow sync to allow proper exposure of the background and will power the flash to provide fill light based on the distance and reflectivity of your subject. The maximum (longest) shutter speed allowed can be modified using the camera's custom function menu.

From Wikipedia:

In most film and digital SLRs, the light sensor(s) for exposure metering are incorporated into the pentaprism or pentamirror, the mechanism by which a SLR allows the viewfinder to see directly through the lens. As the mirror is flipped up, no light can reach there during exposure, therefore, these light sensors can be used for ambient light TTL metering only. In newer SLRs as well as in almost all DSLRs, they can also be utilized for preflash TTL metering, where the metering is carried out before the mirror flips up using a preflash and the necessary amount of flash light is precalculated and then applied during the exposure without any real-time feedback.

Some film bodies that were TTL capable metered using light reflected off of the film in real time during the exposure, but this proved to be unreliable with the different reflectivity of digital sensors. Some even earlier "automatic" strobes had sensors built into the flash itself to measure the amount of light, but the variables of lens shape and size, filters over the lens but not the flash, as well as angle of view, made this less accurate than the more current TTL systems.

When used with first curtain sync, the time between the preflash and the main flash during exposure is so short that most people don't even notice that the flash has fired more than once. Consider that the fastest camera bodies are capable of focusing, metering, moving the mirror, stopping down the aperture, opening and closing the shutter, moving the mirror back down, and opening the aperture back up at 10+ frames per second.

Here is a diagram of how the preflash system works with Canon's E-TTL when using multiple off camera flashes.

enter image description here

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Quick note: "auto" mode is still there in most of the higher-end flashes (like the Nikon SB 9xx and Canon 580EX — don't know about the 600; it doesn't really need it) though you may have to dig through menus to get to it. If you know how to use it (and it takes some experimenting, which luckily no longer costs film) it can be very useful with fire-only remote triggers in some situations where manual is impractical. –  user2719 Apr 8 '13 at 3:39
    
Re " ... moving the mirror ... at 10+ frames per second." Or 12 fps in the case of my A77 thanks to the lack of need to move the mirror :-) - while allowing continual metering including during image capture. I know you know that, but the implications for better results are significant. The system does actually start to "run out of steam" in other areas in the highest speed mode, but that's a function of the implementation and not of the system per se. Disappointingly, the full frame A99 provides only 6 frames per second, which is slower than some mechanical mirror marvels, suggesting that ... –  Russell McMahon Apr 8 '13 at 5:57
    
The full frame Canon 1D X goes 14 fps with the mirror locked up. :-). It matches the 12 fps of your A77 with the mirror cycling up and down. –  Michael Clark Apr 8 '13 at 8:33
    
... .. they have not provided much different processing power than in the A77 and/or that they are leaving a performance step for the new Sony yet to be announced 36 MP FF to fill. It's hard to guess what the 36 MP will be named, as the A99 has already taken "top" 2 digit spot. Maybe an A99. Or A90. Or A9? Starting to sound familiar, somehow :-). –  Russell McMahon Apr 8 '13 at 9:39
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The basic idea of TTL flash is to automatically light the subject for 18% exposure - but it does have additional smarts for taking care of the situations you describe.

When you don't under-expose ambient light:

For Canon cameras (I assume other brands have a similar system), when the ambient exposure is above some EV value (don't remember the exact number) the camera will use the flash in fill flash mode - that means it will not set the flash power to light the scene but only to fill in the shadows.

I have no idea how the camera decides just how much shadows there are and the flash power that is required to fill them but it works pretty well.

When you do under-expose ambient light:

The system has to identify the subject and calculate exposure only for the subject, otherwise in nay night photo where the background is far away you will get an over-exposed subject on black background (because the camera would blast full power flash at the background that is way out of range).

Even if the background is in flash range you can easily get to a situation that if the camera will expose for the background it will over expose the subject.

So, just by the observed fact that TTL flash works we can assume it has some smart subject/background detection magic and that it only exposes for the subject.

By the way, Canon ETTL (but, as far as I know not other brands) in Av or Tv mode will always expose for the background (never use the flash to light the scene) and will use the flash in slow-sync mode if you under-expose the ambient or fill-flash mode if you don't (flash can be used as the primary light source in Auto, P and M modes).

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