XSi and back button focus..

Hi,
This is an extension of a thread I began and titled XSi and focus issue, but this question is specifically about using back button focus with the XSi and AI Servo. I've read the Canon link on back button focus but it's descriptions of settings cover other, higher end, cameras.

I want to really understand the 4 choices I'm offered in the XSi Custom Function-10 that deals with this. I'm particularly confused by the choice between #2 and #3. The manual says that for choice 2: "AF/AF lock, no AE lock In the AI Servo AF mode, you can press the * button to stop the AF operation momentarily. This prevents the AF from being thrown off by any obstacle passing between the camera and subject. The exposure is set at the moment the moment the picture is taken."

For choice #3 it says: "AE/AF, no AE lock This is useful for subjects which keep moving and stopping repeatedly. In AI Servo AF mode, you can press the * button to start or stop the AI Servo operation. The exposure is set at the moment the picture is taken. Thus the optimum focusing and exposure will always be achieved as you wait for the decisive moment."

If the exposure is achieved at the moment the picture is taken with both settings, what's the difference from an exposure point of view and why would you choose one setting over the other?

What's the difference from a focus point of view?
Thanks!

stillwatergal wrote in post #7782913
Hi,
This is an extension of a thread I began and titled XSi and focus issue, but this question is specifically about using back button focus with the XSi and AI Servo. I've read the Canon link on back button focus but it's descriptions of settings cover other, higher end, cameras.

I want to really understand the 4 choices I'm offered in the XSi Custom Function-10 that deals with this. I'm particularly confused by the choice between #2 and #3. The manual says that for choice 2: "AF/AF lock, no AE lock In the AI Servo AF mode, you can press the * button to stop the AF operation momentarily. This prevents the AF from being thrown off by any obstacle passing between the camera and subject. The exposure is set at the moment the moment the picture is taken."

For choice #3 it says: "AE/AF, no AE lock This is useful for subjects which keep moving and stopping repeatedly. In AI Servo AF mode, you can press the * button to start or stop the AI Servo operation. The exposure is set at the moment the picture is taken. Thus the optimum focusing and exposure will always be achieved as you wait for the decisive moment."

If the exposure is achieved at the moment the picture is taken with both settings, what's the difference from an exposure point of view and why would you choose one setting over the other?

What's the difference from a focus point of view?
Thanks!

Stephanie, I think it does exactly what it says on the tin! In AI Servo with option #2 the shutter button will activate continuous AF but if you simultaneously press the * button it will stop the AF (thus effectively locking it at the last AI Servo AF achievement) but it will not lock the AE (as it would normally if the C.Fn was set to '0'). With option #3 the shutter button will AE and the * button will AF but it won't lock the AE (as it would normally if the C.Fn was set to '0').

In rereading what I wrote, I realize that I should have included what the manual says about choice #1, as well. (Choice #0 is, of course, the default where the depressing the shutter 1/2 way achieves Auto Focus and the * is for Auto exposure lock.

Choice #1 simply reverses choice 0 (and seems similar to choice #3).

So, if I want to shoot on AI Servo, I'm completely confused about what the different choices will do for me....

whoops....talk about timing. Steve's answer came in on my systerm as I was hitting the send button for my last post.

So now I'm clear that #2 won't do what I want....

While I originally thought that choice #1 wouldn't really work with AI Servo, now I'm not sure. What would I gain or lose from each of these choices?
Thanks!

The difference between #1 and #3 is that in #1 a half press of the shutter button will lock the AE to the scene when the shutter button is half pressed. This may not be the desired exposure at the time the picture is actually taken. In option #3 however the AE doesn't lock, the AE being set the moment the shutter button is fully depressed and the picture taken.

stillwatergal wrote in post #7782913
I want to really understand the 4 choices I'm offered in the XSi Custom Function-10 that deals with this. I'm particularly confused by the choice between #2 and #3. The manual says that for choice 2: "AF/AF lock, no AE lock In the AI Servo AF mode, you can press the * button to stop the AF operation momentarily. This prevents the AF from being thrown off by any obstacle passing between the camera and subject. The exposure is set at the moment the moment the picture is taken."

For choice #3 it says: "AE/AF, no AE lock This is useful for subjects which keep moving and stopping repeatedly. In AI Servo AF mode, you can press the * button to start or stop the AI Servo operation. The exposure is set at the moment the picture is taken. Thus the optimum focusing and exposure will always be achieved as you wait for the decisive moment."

If the exposure is achieved at the moment the picture is taken with both settings, what's the difference from an exposure point of view and why would you choose one setting over the other?

What's the difference from a focus point of view?
Thanks!

To clarify, in case it is not understood; as it is written in the manual everything noted before the "/" refers to the shutter function, everything after "/" refers to the "*" button. Hence, Cf 10-2 enabled AF/AF lock, no AE lock will result in the shutter button starting AF and the "*" button will lock AF. Furthermore, exposure will be set at the time of the shutter press. Cf 10-3 enabled AE/AF, no AE lock will result in the shutter button starting metering with no effect on the AF, AF is started by pressing the "*" button. I hope this helps.

I owned an XSi, and I used C.Fn 10-1. This is, to me, the "best of both worlds".

Back button AF, shutter button AE Lock. The exposure will be set at the time of full-press of the shutter button anyway, and you still have the option to exposure-lock with a half-press. For example, metering on the part of the scene you want to expose for, locking it with a half-press, and recomposing.

All the others provide no AE-lock (if I remember correctly), and there are times I wanted it.

Wow, great thread, and thanks so much, Brett and others. I'm using 10-1 for the most part, but at least I now understand the others better!

I think it's your best bet. I want some immediate control over exposure-lock if I ever need it, and I've found that I use it pretty often.

But, Stephanie was asking about use with AI-Servo, and there are times that you'd like the exposure to never be locked until the image is taken. For instance, the subject is in-and-out of dappled shade, or the light is constantly changing, so that you only want AE lock at the exact moment of exposure. In that case, use #3.

I still am not quite getting something, Brett.

If I use C. Fn 10 option 1 with AI Servo and simply press the shutter button at the exact moment I want to take the image (I don't press it down 1/2 way and recompose) won't the camera provide what it believes is the correct exposure? Or, does the shutter have to be pressed in 2 stages: 1/2 way down (which would achieve and lock AE) and then all the way down (to take the picture)?

Thanks!

stillwatergal wrote in post #7802350
If I use C. Fn 10 option 1 with AI Servo and simply press the shutter button at the exact moment I want to take the image (I don't press it down 1/2 way and recompose) won't the camera provide what it believes is the correct exposure?

This is correct. The half-press is just an additional option for you if you want to lock exposure and recompose.

agc1019 wrote in post #7802383
This is correct. The half-press is just an additional option for you if you want to lock exposure and recompose.

stillwatergal, I answered you a bit more thoroughly in reply to your pm, but +1 to agc1010's post. Options are always welcome.