A lot of questions on my 70D now, but bare with me, after this I will be quiet (for a while)...
I´m wondering about the two buttons for AF.
I can´t really figure out what´s the difference between them.
I have read the manual, but no...
I just use the one next to the trigger and the select AF-Area and I would be happy if I could use the other one to something else. But it seems that I can`t dedicate another function to that button?
What am I missing?
If you're looking at the buttons I think you're looking at, they're completely different.
There's an AF button on the top of the camera (next to the top LCD display). That buttons lets you switch AF modes (e.g. switch from "One Shot" AF to "AI Servo" AF. That button is simply labled "AF" but you can think of it like an AF mode button.
The button on the rear labeled "AF-on" is the AF back-button. This was created mostly for sports but others find it useful too.
For years, the AF was always performed when you pressed the shutter button down half-way. Sports photographers had an awkward problem... they'd be tracking an athelete (say a football player) running down the field -- camera tracking focus as they run. Suddenly as you're panning, someone else enters the field of view (perhaps a game official or someone standing on the sidelines). The "normal" behavior of an auto-focus system would be to switch focus to this new target because they are closer and you mis the shot.
Canon introduced "back button" focus. That's the "AF-On" button. You do have to go into the camera menus and change the focus so that a half-press of the shutter button no longer activates auto-focus, but the AF-On button will activate auto-focus (it's configurable).
NOW... when that same scenario occurs, as you track focus on your subject of interest and notice a distraction entering the frame, you can release the back-button (so the camera stops focusing but at least focus remains at the last focused distance) and then resume auto-focus once the distraction has cleared the frame (or at least the focus points).
They're similar but somewhat different.
The new AF button (between the shutter release and the main dial) let you change the AF Selection mode from Single Point, Zone Focus or Automatic Point Selection. Think this is the preset. You can quickly switch from one mode of AF to another.
The other AF button, let you choose each single AF point. THis is more like manual selection.
Ahh... I see what you're looking at.
The rear button is the AF point selection button. This is a fairly standard button that all Canon digital EOS cameras have. After pressing that button, you can navigate to the single AF point that you'd like the camera to use... except that since this camera can do zone focus, you can also use it to push the zone around (a zone is a collection of AF points all near each other.)
The front button allows you to toggle the AF area type you want... between "single point", "zone", or all points (auto-select). When you allow the camera to use all points, keep in mind that the computer will take the point that can lock focus at the CLOSEST focusing distance (so if there's a distracting object in the foreground -- closer than your intended subject -- then the computer will prefer that nearer object to lock focus.)..
When you look through the viewfinder, there are three icons near the top (above the focus points). Those icons tell you which focus mode the camera is using with respect to focus areas. They are "single point", "zone focus" (a block of AF points all near each other), and "all" (the camera is allowed to use any focus point it wants.) The FRONT button change the AF Area mode (i.e. it changes which of those three modes is being used.)
The rear button you referred to actually lets you push the AF point (or points as the case may be) around. So if you're in single point mode, then push the rear button once then use the 8-way navigator to move from point to point until you've selected the point you want to move. If in "zone" mode, it moves the zone around (the block or array of AF points being used.) It would serve no function in "all" points mode because you don't get to select a single point or zone of points in that mode.
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