Back Focus Button on A77

Started Mar 14, 2012 | Discussions thread
copejorg1 Senior Member • Posts: 1,626
Re: Back Focus Button on A77

tesch wrote:

I just saw this on Peachpit Press and thought it might make for an interesting discussion. (anything to move us away from OVF vs EVF!)

Scott Kelby's Digital Photography Tips: How the Pros Focus for Sports:

The article talks about seperating the focus from the shutter trigger. Does anyone use this method? I've been using that button more often but only to turn the auto focus off so I can use the peeking. I would like to push this farther.

The higher-end Sony cameras (the A77, A700, A900 & A850) are the only ones having the AF/MF button on the rear, allowing a mode of operation that approximates -- but doesn't exactly duplicate -- the shooting functionality explained in the blog page you linked to. This AF/MF button works somewhat differently than the AF-On button on the back of a Canon or Nikon DSLR.

One thing the author of that piece didn't do a very good job of explaining, IMO, is that the back-button focusing (which he erroneously called "back focusing") technique is most useful when the camera is set to Continuous AF (and of course, with the AF functionality removed from the shutter button). In this case, Single-shot AF can be effectively obtained at any time, simply by pressing the AF-On button to focus, and then releasing the button. Plus, if the lens in use happens to have a ring-type ultrasonic focusing motor, with full-time manual focus available, this setup allows you to easily obtain either continuous AF, single-shot AF or manual focus as desired -- without changing any camera settings -- simply by choosing when to press (or not to press) the AF-On button.

The aforementioned Sony cameras are capable of having the shutter and AF/MF buttons configured in various ways, and can almost replicate the Canon/Nikon AF-On functionality explained above, with one glaring exception: The AF/MF button cannot be set to invoke Continuous AF -- this button can only be set to activate Single-shot AF (even if the AF mode switch on the front of the camera is set to Continuous AF!), defeating a great deal of the potential functionality of that button.

On the positive side, the Sony AF/MF button can provide more-or-less similar capability to that explained above for the Canon/Nikon DSLRs, by using a different technique (one that's essentiall bass-ackwards from the way you'd use a Canon or Nikon to achieve the same result). To do this, you'd keep the AF activation functionality tied to the shutter button, and use the AF/MF button to switch to manual focus when held down (which is basically the default setup), while having the AF mode switch on the front of the camera set to Continuous AF. With this setup, you'd be pressing the AF/MF button at the exact times you'd not be pressing the AF-On button on a Canon/Nikon, and vice-versa. It seems a little counterintuitive to me, forcing the simultaneous pressing of two buttons to take a picture without activating AF, but it gets the job done, and certainly is a technique that can be learned with practice.

And one clear advantage of the Sony AF/MF button, over the AF-On button on the other brands, is that the AF/MF button will defeat AF, and allow manual focusing, even on an "old style" lens with screw-driven AF. " With the other brands, releasing the AF-On button (along with deactivating AF functionality on the shutter button) will allow you to shoot at will without AF, but won't allow you to MF unless the lens itself has built-in Full-Time Manual capability (ring-type ultrasonic AF) . And with the A77, the AF/MF button not only provides AF override/MF capability even with screw-drive AF lenses, it also activates the focus peaking display in the EVF/rear LCD, providing valuable assistance for precise manual focusing.

And finally, here's a link to what I feel is a much better-written piece on back-button AF, explaining it from the Canon POV ...



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