Dear Canon users,

Looking at various threads in this and other Forums (e.g. photo.net's General Photography Forum) I have many times happened upon the information that in EOS A2/A2E/5 A-TTL is only possible (with dedicated EZ-series Speedlites) in the P and "green" modes. The posters claimed repeatedly that in all other modes the camera/flash switched to "plain TTL".

Unfortunately, this is not what I find when I mount my 430EZ on my EOS-5. I cross my heart that I get TTL flash exposure ONLY AND EXCLUSIVELY in M and X modes. All remaining modes (PIC, Tv, Av, P, Dep) support A-TTL with all due symptoms (a little near-infrared preflash, "A-TTL" displayed on the Speedlite's LCD panel).

What do you think about this discrepancy? My EOS-5 was destined for the European market, maybe there is another difference between the EOS-5 and A2/E besides the well-known thing about the exposure scale in M mode?

I think it's worth clarifying since some otherwise sound advice given to thread originators is based on the fact that one can't have A-TTL in Tv and Av.

Piotr

-- Piotr Mikolajczyk (mikolaj@pcwarsaw.waw.pl), August 25, 2000

Answers

My A2e only gives A-TTL in P mode (don't know about green or pic modes since I never use them). Switches to TTL in any other mode (Tv, Av, etc.) I always assumed - possibly incorrectly - that this was because only in P mode does the camera have full control of exposure (i.e., both shutter speed and aperture control). Jim

-- james simon (jsimon724@aol.com), August 26, 2000.

I just checked my A2/430EZ combo and it is exactly the same as Piotr's gear: A-TTL in all modes except M and X where it is TTL. All A-TTL combinations (PIC, Green, P, AV, Tv, Dep) get the near-infrared pre- flash whereas the TTL (M, X) do not.

I have a 1996 A2 and meter scale 430EZ (grey martket model) maybe the feet scale USA flash is different?

-- doggieface (doggieface@aol.com), August 26, 2000.

Well I busted out my 430EZ instruction booklet (Canon CY8-6518-013, 1989)--the international market (meter scale model) one in Spanish, French & English. Maybe the USA version is different? First, under "Specs" on p. 81 it states that 3 types of flash exposure level control are available:

"ATTL automatic flash, TTL automatic flash (when the camera is set in the manual mode), manual"

There's a footnote that explains that the info in the parentheses apply to the EOS 10, 10S, 100/Elan, RT, 700, 630, 650, 750 & 850. No mention of the EOS 5/A2/A2E but it also defaults to TTL in M mode.

In the instructions under "Shutter-Priority (TV) Shooting" (p. 33) and "Aperture-Prioity Shooting" (p. 39) the author sates that "when ATTL is not displayed in the flash, press the mode button until ATTL appears."

When I switch to Av or Tv mode on my A2, ATTL appears on the 430EZ flash LCD as the default.

No mention is made of the PIC modes.

-- DoggieFace (doggieface@aol.com), August 27, 2000.

Hmmm, isn't that bizarre? And it seems that there is no correlation between this phenomenon and camera models since there are people who have A-TTL functioning on A2, so it's not the A2 vs 5 (European vs US version) difference, apparently.

Unfortunately, it clutters the picture even more and gets us farther away from explaining the variations in operation.

My 430EZ is calibrated in meters as well, I haven't even looked whether (like in 550EX) there might be a little switch somewhere in the battery chamber to toggle between feet and meters.

I bought my 430EZ used (European market again) and have never RTFM'ed- fortunately, the flash is quite simple to handle. All I know that TTL (in M and X) and/or A-TTL (in all other modes) are switched on automatically and displayed at an instant (with distance ranges and other associated data) by pressing the shutter release half-way, without any need to press any more on-flash buttons "until A-TTL sign comes on".

I wonder what Canon would say about it? I wonder whether 540EZ also gives the same variation in TTL/A-TTL?

Perplexed (but not worried too much as I'm lucky to have the full- fledged version)-

Piotr

-- Piotr Mikolajczyk (mikolaj@pcwarsaw.waw.pl), August 28, 2000.

You all make too much out of A-TTL. The only thing A-TTL does different than TTL is that is it (1) pre-flashes, (2) "reads" the pre-flash on the speedlite, and (3) with the camera body, controls the aperture and/or shutter speed. After the pre-flash, the flash duration is controlled during exposure by off-the-film metering (just like TTL).

-- Dave (dherzstein@juno.com), August 29, 2000.

Dave, the A-TTL advancement lies in points 1) and 2) of your post, i.e. sending near-IR preflashes and then receiving and analyzing them by the flash. The preflash is meant to determine the distance between the flash and the main subject as indicated by the active AF point. Taking into consideration the active AF point is one of the advantages of A- TTL. Based on that distance, the camera sets appropriate exposure parameters (in EOS A2/E/5 the algorithm strives to achieve good DOF) which are reasonable w/respect to flash power etc. to properly illuminate the main subject (even in off-center location). With A-TTL, it's less possible for the large, distant background to affect flash exposure readings so it's less likely to grossly overexpose the main subject while the flash tries to illuminate this background

This is correct that during the exposure the TTL cell controls the flash exposure separately. A-TTL is like TTL with the added flash-to- subject distance pre-determination. In-camera TTL is able to instantaneously compensate for some of the errors in A-TTL determinations. How can the errors happen? Well, A-TTL preflashes are received by the sensor in the flash, not in the camera (like in E-TTL). So, the preflashes do NOT measure the film-to-subject distance, but rather the flash-to-subject distance. When the flash is shoe-mounted, it doesn't make much of a difference, these two distances are more or less equal. However, when you have your flash off-camera placed at a different distance from the subject that the camera, then it could be fooled into overexposing (if placed behind the camera, as it "thinks" the distance is larger) or underexposing (if the flash is placed in front of the camera, at a shorter distance). The latter situation frequently occurs in closeup shots and is compounded by the fact that in closeups the distances are generally small so moving the flash around has a relatively bigger effect. Try this by placing your hand in front of the Speedlite's sensor at various distances, then press the shutter release half way and see how the camera-selected aperture jumps around.

Also, placing a filter with a higher filter factor (e.g. 1.5 for the polarizer) over your lens can do the same thing as moving the flash closer, i.e. may result in underexposure.

I've once roughly tested the whole thing by moving the flash around on the sync cord and placing filters on my lens. Most of the shots came out OK because apparently the TTL cell in the camera could (within the limits) compensate for the inaccuracies in preflash distance calculations by changing the duration of the flash burst during the exposure.

Piotr

-- Piotr Mikolajczyk (mikolaj@pcwarsaw.waw.pl), August 30, 2000.

If you put your camera in "P" mode with the flash in A-TTL, your exposure (ambient and flash) will be indentical to that which you would get if you put your camera in "M" mode and dialed in the same aperture and shutter speed. In both cases, the flash output would be determined entirely during exposure by the off-the-film sensor. A-TTL has a practical use as an extension of the camera's "P" mode, but it does nothing to ensure better flash exposure (other than pre-warning of possible under-exposure).

-- Dave (dherzstein@juno.com), August 30, 2000.