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In lens reviews, ability to override auto-focus manually is often referred to as a good (and important) feature.
I have rarely, if ever used that feature, so I feel I might be missing out on an important technique. I can see how it could be useful when letting the camera decide which point to focus by, but I always select the focusing point myself. Would overriding auto-focus be also useful with selected AF point?
I guess you are referring to the "full-time manual focus" feature which allows you to do manual focus without switching from AF to MF mode.
I find it very convenient if you combine it with back-button autofocus. Then you will never need to fiddle with the AF/MF switch. You can just leave your lens on AF all the time.
If you want autofocus, it is immediately available in the back button (just like with any autofocus lens). If you want manual focus, you can directly turn the focusing ring (as if your lens was in the MF mode).
And you can combine these any way you want. For instance, you can first AF and then immediately after that fine-tune. Or if you notice that AF starts hunting, you can simply give up with AF and use manual focus.
Of course there is a downside, too: it is easier to accidentally change the focus when you are handling the camera.
It is invaluable when accurate focusing is critical to the result as you then need not worry about the sometimes uncertain nature of auto-focus.
This applies particularly in the following situations:
To get the best results from manual focus, you should use live view and magnify the image. It is possible then to get an exact focus (but for some applications this is rather too slow).
A useful technique is to let the auto-focus make the first selection and then to use the manual focus override to fine tune the focus.
From time to time I like to deliberately use manual focus just to keep those skills alive.
Sometimes you must manually focus because there are foreground objects that can't be avoided and will draw off the autofocus.
You can also have a situation where the target has enough noise around it to keep the autofocus from working at all. Say a marine mammal popping out of the water in a cloud of spray.
Lastly, when you can't focus and recompose because there's nobody behind the camera--timer or remote trigger--and the auto is going to go astray for some reason. (Or my old SLR that did the focus when you started the timer rather than when it fired. If you were using that timer to get into the shot it would go astray if there wasn't something suitable there.)
Another one: when the AF can't keep up. I regularly photograph airshows and other aircraft displays. The AF sensor simply can't focus rapidly enough on an aircraft coming in at hundreds of miles an hour, so what you do is you lock focus on a set distance and take your shot when the aircraft gets to that point. Having bright weather so you can use a deep DOF helps a lot in that, as it greatly increases the margin of error you have.
I have never understood this business of making fine adjustments with MF after a lens has focussed automatically. The viewfinders of AF cameras do not show enough detail for precise manual focussing, especially at speed.
What I find more useful is to use the full-time MF of a Canon USM lens to get close to the right focussing distance, then to use AF to lock on to the subject. i.e. the exact opposite of what so many "experts" describe.
