Understanding/controlling built-in flash on Rebel Xti

[kdnilsen]

I'm an old-school photographer, who just recently picked up a Rebel Xti camera. I have some old lighting equipment and an old Shepherd FM880 flash meter. And I'm seeking some assistance to understand how to leverage my old-school knowledge and techniques with this newer equipment.

If someone can point me to good reference material, I'd be glad to go study that.

I've got a variety of questions:

1. In Av mode, I would expect the flash output to automatically adjust based on distance to subject (as determined by auto-focus) and selected aperture. It seems to do neither. My flash meter reports the same flash output regardless of aperture selection. (maybe my flash meter is measuring the pre-flash intensity?) What am I missing?

2. With an auto-everything camera like the Xti, I would expect some sort of warning if I am asking the impossible. For example, if I am using the camera's built-in flash only, and I ask for f/22 at 12' with ISO 100 film speed, I would expect some sort of indication that my demands are unreasonable. Currently, the only feedback I get is when I review the capture image after the fact, and I see that it is clearly way underexposed. Maybe I just don't know where to look for this feedback?

3. Is there any way for me to manually set the built-in flash output? Sometimes, I would like to use an off-camera key with the on-camera flash as a fill. I'd like to manually specify 1/2, 1/4, 1/8, 1/16, etc. light output. Can I do this?

4. What exactly is the difference between evaluative and average flash metering? Do both modes fire a pre-flash? Since there's no TTL metering during exposure, it seems they would have to.

5. A pre-flash is going to confuse my flash meter and my optical slaves. What do I set to make sure no pre-flash is fired? (I gather that I can use FE lock to set the flash exposure so that I won't get another pre-flash when I actually take the picture. But what if I want to lock the exposure without firing a pre-flash?)

[mjm80]

Hi,
I found the following article useful. The early part is pretty basic but it goes on to give some useful info on how the EOS TTL systems work and should answer many of your questions.
http://photonotes.org/articles/eos-flash/

[mbellot]

I'm an old-school photographer, who just recently picked up a Rebel Xti camera. I have some old lighting equipment and an old Shepherd FM880 flash meter. And I'm seeking some assistance to understand how to leverage my old-school knowledge and techniques with this newer equipment.

If someone can point me to good reference material, I'd be glad to go study that.

I've got a variety of questions:

1. In Av mode, I would expect the flash output to automatically adjust based on distance to subject (as determined by auto-focus) and selected aperture. It seems to do neither. My flash meter reports the same flash output regardless of aperture selection. (maybe my flash meter is measuring the pre-flash intensity?) What am I missing?

2. With an auto-everything camera like the Xti, I would expect some sort of warning if I am asking the impossible. For example, if I am using the camera's built-in flash only, and I ask for f/22 at 12' with ISO 100 film speed, I would expect some sort of indication that my demands are unreasonable. Currently, the only feedback I get is when I review the capture image after the fact, and I see that it is clearly way underexposed. Maybe I just don't know where to look for this feedback?

3. Is there any way for me to manually set the built-in flash output? Sometimes, I would like to use an off-camera key with the on-camera flash as a fill. I'd like to manually specify 1/2, 1/4, 1/8, 1/16, etc. light output. Can I do this?

4. What exactly is the difference between evaluative and average flash metering? Do both modes fire a pre-flash? Since there's no TTL metering during exposure, it seems they would have to.

5. A pre-flash is going to confuse my flash meter and my optical slaves. What do I set to make sure no pre-flash is fired? (I gather that I can use FE lock to set the flash exposure so that I won't get another pre-flash when I actually take the picture. But what if I want to lock the exposure without firing a pre-flash?)


1. Welcome to Canon flash. :rolleyes: Av and Tv meter for ambient light and use flash for fill only. If you want more flash output in a given situation you need to switch to Manual, it "forces" the flash to provide sufficient light (if possible) to expose to your shutterspeed and aperture.

2. See #1, except for Manual. I'm not sure about the 400D, but the 20D will indicate flash power was sufficient with an indicator in the viewfinder.

3. Nope.

4. Yes, both use pre-flash. From what I've read, evaluative is much more aggressive in reducing flash output power to prevent blown highlights. In actual use I've not noticed much difference.

5. FEL is it. There is no way to disable the pre-flash with the on-board (or even most ETTL hotshoe) flash.

When Curtis chimes in I'm sure he'll give you a weeks worth of reading just to get you warmed up. ;)

[Curtis N]

kdnilsen, Welcome to POTN!
The link posted by mjm80 is a rather long but very useful document for anyone wanting to understand Canon's E-TTL metering system. We call it "The EOS Flash Bible". It's worth printing out and keeping for reference.

I would add only a few things to what mbellot wrote. Distance information is reported to the camera only by certain high-end lenses. It's not an essential element of Canon's system. Most likely you are using a lens that does not report distance, hence the camera doesn't know when you're "asking the impossible."

The built-in flash on Canon's DSLRs is "auto only" with no manual options. If you want something you can control manually and use with optical slaves, you will need a 430EX or 580EX Speedlite or an aftermarket flash unit with similar capabilities.

As for Evaluative vs. Average flash metering - Evaluative uses more complex programming to try to determine what the subject is and account for the ambient contribution when calculating required flash power. You'll want to use Evaluative for fill flash. Average seems to give more consistent results when flash is your only significant light source.