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I have started to use my D3100 with my AE/AF-L button set to AF. So in other words I use this button instead of the half press shutter button to AF. I'm new to this technique and came across it here at the Canon website

A problem with the D3100 is that the shutter will not activate if focus is not locked on. If I'm in AF-C using a single focus point and I want to recompose (as in point 1 of the article above) I should be able to just lift my thumb off the AE/AF button after focusing on my subject. Moving the camera slightly to the side I should be able to take the shot but I can't. It wont allow me to release the shutter as the new centre point is out of focus. To get this working I need to be in AF-S (which is fine) and then HOLD the AE/AF button while recomposing and taking shots. Does this not miss the point of the back button AF? In that article the author talks about taking his finger off the back button once focused and then shoots freely. Thats an advantage to me. Keeping my finger down is not. I only want to focus when that back button is pressed and to be allowed to shoot even if the new center point is out of focus.

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That's Matt. After playing a little more I've discovered if I leave the AF as half press shutter and press the AE/AF-L I can always release the shutter, even if not focused. That's the behaviour I would like. I don't like having to keep my finger pressed on a button to keep the focus and be able to shoot. –  Kevin Jan 9 '11 at 15:15
    
With Canon that's certainly the case with back button focus - the camera will always release the shutter as soon as you press it. I assume there must be a setting somewhere to enable this on your D3100. If that's the case I'm sure the experts on photo-se will know it! –  Matt Grum Jan 9 '11 at 15:26
    
This could be useful as a trap focus mode, where you pre-focus and hold the shutter down. The camera will fire once your subject comes into focus. –  Evan Krall Jan 9 '11 at 20:09
    
I do have a D3100 too, and I'm using AF-C most of the time and hold the AE/AF-L button to recompose if needed. This works fine for me. –  apparat Jan 31 '11 at 13:37
    
that's interesting, i never used that button for that. i keep it in exposure lock mode...but i think it has 2 ways of operation, click to lock / click to unlock, and lock while pressed. i don't have my camera here with me, but i remember choosing something like that. –  JoséNunoFerreira Sep 16 '11 at 17:24

2 Answers 2

I think you may be running up against a limitation of the specific camera. Pentax, and I suspect Canon, in some of their products have a menu option to disable requiring focus confirmation before shutter release in autofocus modes, the D3100 doesn't seem to have this according to the manual I downloaded. On page 55 of the manual, all three AF modes state that the shutter can only fire when the camera is able to focus, only manual focus can do otherwise. Normally you would use the AF-L button to lock focus in AF modes which should allow for recomposition and shutter release, but if you've repurposed the button, then this is lost.

I would be surprised if this was a limitation of Nikon in general, so I would have to think that it is specific to the D3100 being an entry level dSLR, it could be seen as "helpful" for the average consumer though irritating for somebody trying to do more than party photos.

So, presuming that there isn't a hidden menu option that I didn't see in the manual and you still want to use the button this way, then one option in AF is to manually select the focus point off the center, targetted to the subject you want to have the focus on. That only works, of course, if that subject falls within the focal point selections. Alternately, you could autofocus on what you want, switch to manual focus, recompose, and then release. This should work, I would think, though a little tedious. The final option I can suggest is to manual focus entirely.

Probably not what you wanted to hear... This is what I determined from the manual, though a D3100 user may have better information than I.

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Thanks John. I suspect too this may be a limitation. Not sure if its intentional or a firmware bug. Leaving the button arrangement as per default and having the focus lock function makes more sense right now. At least I can shoot no matter what the focus is as long as I have it pressed in. –  Kevin Jan 9 '11 at 15:29
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@Kevin - I suspect it is intentional. If the camera released before focus, the average beginner would probably be complaining about out of focus shots. –  John Cavan Jan 9 '11 at 15:36

Kevin your conclusion seems to be correct, the D3100 autofocus modes require the focus point to be in focus unless you hold the AF button.

On my Nikon D700 there is a menu setting to set the shutter to release on press or release on focus. There is also a second AF-ON button on the back. I set a custom setting to release on press and a momentary press of the AF-ON button locks the focus and the camera will shoot after re framing.

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Thanks Don. Your setup sounds ideal. I guess I can live with holding the AE/AF-L button when needed. Not as convenient but gets the job done. Maybe Nikon will be nice down the line and enable a setting like that in a firmware update :) –  Kevin Jan 9 '11 at 19:50

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