Back button focus

Note that many photographers who set up BBF, also swap it to the AF-ON button because that is a lot easier to reach.

This is all done using special functions, and the way these are documented in the Canon manuals is far from clear.

This is how things are for the 1D and 5D cameras:

First of all, the swap between the AF-ON and the "*" button (which I recommend) is done using Control Function IV-2 (set it to "enable") on the 1Diiim. I find using my thumb on the "*" button (which I do a lot when using BBF) is much handier than trying to use the AF-ON button.

To get focus off of the shutter button and on to the button on the back, you need to use Control Function IV-1 (on both the 1Diii and 5Dii cameras). This control function is setting two both what the shutter button does and what the rear button. In the following "rear button" means either the "*" button or the AF-ON button depending on whether you took my advice on switching them using Control Function IV-2 above.

I use setting (2) below:

• 0 (default) - the shutter button meters and does AF start.
  no telling what the rear button does, perhaps AF-start.
• 1 - the shutter button meters and does AF start,
  the rear button stops AF.
• 2 - the shutter button meters,
  the rear button meters and does AF start (this is BBF)
• 3 - the shutter does AE lock,
  the rear button meters and does AF start (another form of BBF)
• 4 - the shutter meters and does AF start,
  the rear button is disabled!