ATTL vs TTL Questions

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I have a Canon A2 and the 540 EZ.

I am new to photography, but understand metering techniques, and the aperture - shutter relationship. I am having a difficult time understanding the difference between ATTL and TTL. Here's my question:

If my camera is set to Program (P), the flash is set to ATTL. I am indoors, nightime. The camera selects, for example, a shutter of 60 and an aperture of 5.6. So far so good. Now I switch to Aperture priority, and the flash switches to TTL. So, I select an aperture of 5.6, because I want to be able to control the depth of field. I would think that the camera would select a shutter of 60, and then increase the flash output to provide a proper exposure for that fstop.

Rather, the camera selects a shutter of 1 second, and now I have to move to a tripod to get a proper exposure for that depth of field. But why in ATTL is it able to give me this setting? Am I making any sense?

I mean, I understand that TTL is through the lens, and the camera is metering as if there weren't any flash on the camera. But then how can I control depth of field when shooting subjects (like brides) where I have to be very quick to get the shot? It seems all I can do is put it in program mode and shoot.

The same holds true for shutter priority. If I set the shutter to 60, then the fstop blinks in the camera because it can't provide an aperture large enough to properly expose the shot.

I've poured through the manual, and they explain WHAT to do, but I am looking for more in-depth explanations as to what's going on.

Thank you very much in advance,

A. Elaine Schmitz

-- Elaine Schmitz (amal_k_schmitz@comerica.com), September 13, 2001

Answers

The difference in behaviour between P and Av modes is not because of TTL or A-TTL. The two flash modes are actually very similar - A-TTL simply uses a near-infrared preflash (or visible white preflash in the case of the 430EZ in bounce mode) to do some aperture calculations.

Rather, the difference you're experiencing is between the way P and Av modes handle metering when using flash.

In P mode the camera tries to ensure that the shutter speed is fast enough to permit handholding of the camera - so it goes to 1/60 or 1/125 sec. (X sync) But in Av mode the camera tries to meter for the ambient light (background), which is why you're getting long shutter speed settings. It then uses flash to fill in the foreground.

Another way to look at it is this: in P mode the camera tries to illuminate as much as possible using flash and also tries to ensure the shutter speed is fast enough for handholding the camera. In Av mode the camera just tries to fill in the foreground and assumes you're using a tripod if the shutter speed is low because of low ambient light.

For more information check out:

http://bobatkins.com/photography/eosfaq/flashfaq.htm

It's a bit tersely written, but useful. Ignore the stuff on E-TTL and FP flash, since your camera and flash were designed before E- TTL had been invented.

-- NK Guy (tela@tela.bc.ca), September 13, 2001.


You got a great answer from the NK Guy. However, you may wonder why you'd ever want to use Av mode with flash. When Av mode meters for the background light and subsequently fills in the foreground, the result is a balanced, natural image. This technique is great for night time or twilight city lights portraits where the background is as important as the foreground. On point 'n shoot cams they call this slow sync. Of course, you need a tripod.

In P mode, you get a pure flash picture in dim situations (the caught in the headlights look). Thus, your city lights image will have a correctly exposed foreground subject but the background will be dark or black.

Aloha

-- Puppy Face (doggieface@aol.com), September 13, 2001.


Set your camera to "M". Set a shutter of 1/60 or 1/125 (your choice based on the lens is use.) Set an apature of 5.6. Set flash to +1 stop. Now you can bounce and shoot till you are out of film or batteries. (what ever comes first.)

-- Dan Flather (pincher@mb.sympatico.ca), October 01, 2001.

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