I have a particular problem when focusing with any camera, but let's just say for example my Canon EOS 70D.
If I am shooting two objects in close horizontal proximity to each other but in totally different focal planes (i.e. one object is way behind the other object), the camera will always tend to focus on the closer one. That is fine a lot of the time, but sometimes I wish to focus on the behind object. Occasionally, releasing the shutter and depressing it again will lock on to the correct object (the one behind) but not always, especially if the two objects are really close or even overlapping horizontally. Manual focusing works but I do not always want to rely on that particularly if the scene is fleeting.
I know that sports photographers use a focus tracking technique (that honestly I have never tried since I seldom shoot sports) that can lock on to a moving subject even if it moves behind another. I have yet to try this out, but I suspect that this technique only works if you are in a particular mode and shooting a moving object anyway (so it wouldn't be useful if the objects start off already in close proximity horizontally).
Touch screens sometimes help but only if there is enough separation for the AF points to be distinguishable to the camera, and then again only if you are shooting in live view (which I am seldom in).
I guess what would be ideal is a sort of AF "toggle" button where you can toggle between objects. I am surprised that this hasn't been implemented. However, is there any other way to tell the camera to "focus on the bloody object in the rear"?