TTL and subject distance

Since I have always used Manual settings for my flashes I have never thought much about TTL.

My thought process is if I use a single focus point on my subject and the meter uses the entire scene or portion (as set in metering),

Is there any compensation in the resulting flash power (duration) for distance to subject.

A focus point should be able to determine distance with at least some accuracy.

So if my subject is 5 feet or 15 feet or 25 feet away, realizing flash fall off, does the TTL factor this in?

Thank you

(I think...) It first determines the exposure at ambient. Next, it does a very low-powered pre-flash, and determines how much effect that had on the exposure, compared to the ambient only exposure. Then, based on how much effect that low-powered pre-flash had, it can figure out the power needed for the main flash burst. So, not only will it be correct for the distance, but for the way a particular material reflects back the light...

As Dave said the preflash does help achieve the same thing, but Canon incorporated distance data into the process with eTTL II. This increases the effectivness of the preflash by also using distance information as long as the lens is capable of communicating that information (not something a manual focus lens will do).

Found this from Bob Adkins

The Canon EOS E-TTL II Flash System

With the announcement of "E-TTL II".

There is a new flash algorithm, similar to the existing E-TTL, but designed to give better results under difficult situations, such as when there is a very bright reflection from a small object in the scene.

Just like E-TTL there is a preflash which is used, along with ambient light metering, to determine the flash exposure using the center 17 metering zones. Zones which differ greatly in brightness are given less weight since they are likely to be reflections from small, highly reflective, objects.

With lenses which return distance information, this may also be taken into account by the flash algorithm, but the point to note here is that the E-TTL II system works with ALL Canon lenses, it's just that those which return distance information provide an additional parameter for flash calculation. Even without distance information ETTL II should provide better results in difficult situations. So all your lenses will work just fine with E-TTL II, it's just that under some circumstances, E-TTL II compatible lenses might give slightly better exposure when using shoe mounted on-camera direct flash.

In general, if a lens uses a ring type USM motor, it returns distance information and so is E-TTL II compatible. Not all USM lenses use ring USM motors. Those which do not offer full time manual focus ( e.g. the Canon EF75-300/4-5.6 USM), do not return distance information. Lenses using AFM (arc form motors) do not return distance information (e.g. 50/1.8, 24/2.8).
Notable lenses which you might think would return distance info but don't are the 50/1.4 USM and the 85/1.2L USM. The tilt-shift (T/S) lenses are manual focus, and so of course do not supply distance information.

Non-current lenses which should return distance info include the non-IS telephoto lenses such as the EF 500/4.5L USM, 300/2.8L USM, 600/4L USM

Note that E-TTL II does not use distance information when a direct, camera mounted, flash is not used, even with E-TTL II compatible lenses. These situations include:

• When bounce flash is used
• With ETTL wireless flash
• With macro flash

The reason for this is presumably that under those circumstance the flash-to-subject distance will be different from the camera-to-subject distance (which is the information that the lens returns), so the camera cannot know the flash-to-subject distance - and that is the information it would need to fine tune the E-TTL II algorithm results. The bottom line seems to be that the new E-TTL II flash algorithm will give better results with all lenses, but if your current lens isn't E-TTL II compatible, it's no big deal. You'll still get better results than you would with the older E-TTL system.

Here is a list of current Canon EF series lenses which fully support E-TTL II by supplying distance information. It was updated on 03/04/2004 based on information supplied by Chuck Westfall of Canon

Fixed Focal Length EF14mm f/2.8L USM EF20mm f/2.8 USM EF24mm f/1.4L USM EF28mm f/1.8 USM EF35mm f/1.4L USM MP-E65mm f/2.8 1-5x Macrophoto EF85mm f/1.8 USM EF100mm f/2 USM EF100mm f/2.8 Macro USM EF100mm f/2.8 Macro* EF135mm f/2L USM EF180mm f/3.5L Macro USM EF200mm f/2.8L II USM EF200mm f/2.8L USM* EF300mm f/2.8L IS USM EF300mm f/4L IS USM EF300mm f/4L USM* EF400mm f/2.8L IS USM EF400mm f/4 DO IS USM EF400mm f/5.6L USM EF500mm f/4L IS USM EF600mm f/4L IS USM EF1200mm f/5.6L USM

Zoom Lenses EF16-35mm f/2.8L USM EF17-35mm f/2.8L USM* EF17-40mm f/4L USM EF20-35mm f/3.5-4.5 USM EF24-70mm f/2.8L USM EF24-85mm f/3.5-4.5 USM EF28-70mm f/2.8L USM* EF28-80mm f/3.5-5.6 USM* EF28-105mm f/3.5-4.5 II USM EF28-105mm f/3.5-4.5 USM* EF28-105mm f/4-5.6 USM EF28-105mm f/4-5.6 EF28-200mm f/3.5-5.6 USM EF28-200mm f/3.5-5.6* EF28-300mm f/3.5-5.6L IS USM EF35-135mm f/4-5.6 USM* EF70-200mm f/2.8L IS USM EF70-200mm f/2.8L USM EF70-200mm f/4L USM EF70-210mm f/3.5-4.5 USM* EF70-300mm f/4.5-5.6 DO IS USM EF90-300mm f/4.5-5.6 USM EF90-300mm f/4.5-5.6 EF100-300mm f/4.5-5.6 USM EF100-400mm f/4.5-5.6L IS USM EF-S18-55mm f/3.5-5.6 USM EF-S18-55mm f/3.5-5.6

*Indicates discontinued product

If your Canon EF lens doesn't show up on this list, it does not feed distance data back to the camera through the electronic mount.

As a reminder, distance data is never the only factor that controls flash exposure in E-TTL II. It is an optional data point, no more and no less. E-TTL II remains functional with *all* Canon EF lenses, but when using direct flash, E-TTL II is at its best when distance data is available.