Why does AF-ON work better?

Started Feb 12, 2013 | Discussions thread
PK24X36NOW
Contributing MemberPosts: 917Gear list
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Re: Advantages
In reply to Mako2011, Feb 12, 2013

Mako2011 wrote:

What jfriend00 said and I would add...you're separating focus form metering/shutter release.

There are advantages to that.

from www.learn.usa.canon.com/resources/articles/2011/backbutton_af_article.shtml
1. Easier to lock focus
If you are shooting something like a series of portraits of a person, and you want them composed off-center, back-button AF makes it super-easy to take as many pictures as you want. Focus on your subject by pressing the rear button (more on which button later in this article). Once in-focus, take your thumb off the rear button. Re-compose the shot to move your subject off-center. Shoot as many pictures as you like. With focus activation removed from the shutter button, you now can fire any time you like, and remove your index finger from the shutter button after a shot is taken. No matter what, the camera makes no effort to re-focus when you press the shutter button half-way down again.
2. critical timing becomes simpler to manage. For example, if you were shooting a speaker at a podium, he or she might periodically look up or make a gesture that would be an ideal instant to capture. If you’ve focused with back-button AF, your index finger is free to shoot at the decisive moment. There are no worries about holding your finger half-way down and waiting, waiting, waiting in that position for your subject to do something interesting.
3. Less risk of focus errors with moving subjects
For sports photographers and others taking action pictures, back-button AF lets you stop focus whenever something might interfere with the moving subject you’re tracking — without requiring you to stop shooting. In sports, for instance, it’s common for a referee or another player to come between the camera and an athlete being photographed. With back-button AF, it’s easy to momentarily pull your thumb off the rear button, and you can still keep shooting by pressing the shutter button fully. The camera instantly stops focusing when your thumb comes off the back button. Once the obstruction is out of your way, you can immediately pick-up your primary subject by pressing your thumb on the back button again.
4. Easier over-riding of AF with full-time manual focus
Many Nikon lenses have a neat feature called full-time manual focus*. Even if the lens’s AF/MF switch is in the AF position, these lenses allow the shooter to instantly adjust focus manually by simply turning the focus ring on the lens. There’s no need to first move the switch to MF.
With back-button AF, this becomes a nearly foolproof feature. Use the autofocus whenever you like by pressing the rear button with your right thumb. Shoot whenever you like by pressing the shutter button. And if you want to touch-up focus, or totally over-ride what the AF is doing, just pull your thumb off the rear button and turn the ring. No matter how many pictures you shoot, pressing the shutter button will not cause the AF to try to kick-in and re-set the focus you just adjusted manually.
5. Easier macro and close-up focusing
Many times, you’ll find that it’s actually easier to get consistently sharp close-up pictures of small objects by pre-focusing, and then moving yourself forward or backward until you see the critical sharp focus appear in your viewfinder. Once again, with back-button AF active, you can use the AF to get within general range (press the rear button with your thumb, then take your thumb off the button), and move a little bit to get things critically sharp. Most important, you can then shoot freely, without AF trying to re-focus each time you touch the shutter button. Finally, touching-up focus with the full-time manual focus feature on certain Canon lenses is simple and quick, and the autofocus never fights you by trying to un-do what you just adjusted.
6. Another way to control the effects of focus point bias in matrix metering.

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My opinions are my own and not those of DPR or its administration. They carry no 'special' value (except to me and Lacie of course)

Good points. I'd also add that I find it very difficult to "half press" the shutter without tripping the shutter, especially if I have to "hold" the "half press" while focusing on a moving subject (it's a pretty light "trigger" on my D3). Use of separate controls for focus vs. shutter makes everything work more smoothly.

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