dbgrizzzz wrote:
I found that AF-A works fine with back button focus, and preserves the focus beep and the focus illumination light, which is disabled using AF-C. In AF-A, it starts as an AF-S, but if the subject moves and you keep pressing the focus button, the focus changes exactly as you would expect.
AF-A is "automation".... it really isn't a separate focus mode.
In AF-A the camera is supposed to detect whether a subject is moving or not, and then switch to and use what the camera thinks is the correct focusing mode: AF-S for stationary subjects, or AF-C continuous focus mode for moving subjects. I don't have D5000, so can't say from personal experience with it, but having experimented with similar to AF-A on other cameras in the past I found that it slowed autofocus slightly, didn't always choose the correct mode and often didn't switch modes properly if a subject started moving. The percentage of my shots that were mis-focused went up significantly while I was using the equivalent of AF-A... And, conversely, when I stopped using it, started using BBF and the equivalent of AF-C most of the time, my percentage of missed focus shots was hugely reduced. So I avoid the equivalent of AF-A on my cameras (it's essentially a "point n shoot" focus mode).
One of the primary reasons to use BBF is to be able to use AF-C as your default focus mode. Sports and action photographers in particular like to keep their cameras set to continuous focus mode to be ready to track a moving subject. But it actually can be used by anyone for any type of subject, moving or stationary.... though not without BBF because that will get you in trouble when using techniques such as "focus and recompose".
Another minor "problem" with continuous focus is there's no focus confirmation (beep or illumination). It's not "disabled"... It's simply not possible because continuous AF never stops (the way AF-S does), so there's never anything to confirm! Once you learn the camera and lens' focusing capabilities well, learn to trust them and yourself, focus confirmation really isn't needed. (P.S. while it's possible to turn off the audible "beep" of focus confirmation, I leave it on because if I hear it while shooting moving subjects I'm immediately alerted that my camera is set to the wrong focusing mode and most shots will miss focus unless I change it to the correct mode). In AF-A, focus confirmation should only work when you're photographing stationary subjects. If your subject is moving you hear the beep or see the flash of illumination, your camera has self-selected the wrong mode and you'll miss focus a lot!
But besides needing BBF and the lack of a confirmation beep and flash, AF-C (or Canon AI Servo continuous focus mode, etc.) has distinct advantages and can be used with both stationary and moving subjects.
For example, some modern zoom lenses are "varifocal" because that makes for a simpler, more compact design and lower cost. This means the lens doesn't maintain focus when you adjust the focal length. If using AF-S (or if AF-A has selected that mode for you), any time you zoom a varifocal your images will be mis-focused unless you consciously make a point to re-focus by lifting off the button and reapplying it. But if instead you are in AF-C continuous focus mode (possible thanks to using BBF), it will instantly and automatically correct any focus lost to a varifocal zoom.
While focus-related, "Focus/Release Priority" serves another purpose. If a camera allows this to be adjusted, it simply lets the user decide what's important: Whether they want the shutter to fire immediately, even if exact focus hasn't been completely achieved in every instance... Or if they're willing to sometimes accept a slight delay in shutter release in order for more (most? all?) of their shots to be precisely focused. Personally I set Focus Priority high on my cameras... missed focus shots are of little value to me. (If I were a photojournalist I might feel the opposite and set shutter release priority high.)