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

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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).