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Photography Question 

member since: 6/27/2006

Canon's Back Button Auto Focus

I've seen several articles about how to use the Back Button on Canon's DSLRs, but they all seem to be written after the 20D was created. Because the articles are newer than my camera, the examples seem to be a little different than the ones I'm seeing. For example, Custom Function IV (4) on the 40D and 50D will read something like this:

0: Metering + AF start (note: there’s no slash here)
1: Metering + AF start / AF stop
2: Metering start / Meter + AF start
3: AE Lock / Metering + AF start
4: Metering + AF start / Disable

But with my 20D, Custom Function IV says this:
0: AF/AE lock
1: AE lock/AF
2: AF/AF lock, no AE lock
3: AE/AF, no AE lock

Here's the link to the page that Canon has about the Back Button.

It sounds really great, I just need to know how it works with an older camera like my 20D. Also, which one is the back button? The one with the asterisk (*) or the one with the metering symbol?

10/27/2008 3:30:50 PM

Jon Close
BetterPhoto Member

member since: 5/18/2000
  I hope this helps:

In the default settings for all the EOS cameras, a half-press of the shutter button starts both autofocus and metering together. There are situations where the user does not necessarily want to meter on the subject being focused, so the custom functions are provided to assign autofocus and metering to separate buttons.

On the 30D/20D/10D... C.Fn4 is used to assign AF start to the * (AEL) button. To make the descriptions more consistent with the new models, the options for the 30D/20D/10D... should be:
0 = Metering + AF start/AE lock
1 = AE lock/Metering + AF start
2 = Metering + AF start/AF stop, no AE lock
3 = Metering/Metering + AF start, no AE lock

On the 40D and 50D there is added a dedicated AF-ON button as well as the * button for AEL. This gives the new models a little more flexibility. C.Fn IV-1 = 0 on the 40D and 50D is the default setting. You're right, the description in the manual gives the function for the shutter button only (Metering + AF start). It should also give the default for the AF-ON button (ie. .../AF start). Regardless of how C.Fn IV-1 is set, the * is always AE lock.

C.Fn IV-2 = 1 is added to swap the function of the AF-ON and * buttons. The 40D/50D with C.Fn IV-1 = 3 and C.Fn IV-2 = 1 will function the same as a 30D/20D/10D with C.Fn4 = 1. This makes it easy for users with both cameras to switch between them. Also, some users find the placement of the * button more convenient than the AF-ON button.

10/28/2008 6:30:22 AM

Jon Close
BetterPhoto Member

member since: 5/18/2000
"...There are situations where the user does not necessarily want to meter on the subject being focused, ..."

Forgot to add, there are also situations where the user wants to trip the shutter without the camera re-focusing.

10/28/2008 6:32:44 AM


member since: 6/27/2006
  Ok. I'm starting to get it. I've always known that the * button is the exposure lock button.

So I have a 20D. If I want one of those buttons to lock the focus, how would I do that? I still can't seem to get that right.

10/28/2008 12:44:13 PM

Jon Close
BetterPhoto Member

member since: 5/18/2000
  Shoot in a Creative Zone exposure mode (P, Av, Tv, M, A-DEP) for the custom function settings to be in effect. Set C.Fn4 = 1 or = 3. Press * to AF, it'll lock focus automatically if you've set One-Shot or AI Focus as the AF mode. For AI-Servo, lifting your thumb from the * button stops focus, effectively locking it.

10/28/2008 2:31:14 PM


member since: 6/27/2006
  Jon, you always have the right answers!

So, when I have C.Fn4 set to 1 or 3, then * will lock the focus. Is there no way for me to lock the exposure too while I have it set up to lock the focus?

10/28/2008 3:24:22 PM

Jon Close
BetterPhoto Member

member since: 5/18/2000
  Yes, With C.Fn4 = 1, then 1/2 press of the shutter button is AEL.

10/28/2008 5:38:29 PM


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