Originally posted by FHPhotographer Sometimes it just isn't practical to use MF and only AF will do, so, I would like to pin down exactly what the center focus on my K100 is focusing on, and how big that point is.
It might not be "practical", but sometimes it's simply the only viable option - there is no way to pinpoint the focus with the sledgehammers that are the AF sensors.
As for pinning down how big the sensors are, I've descriebd several times how to do that - try focusing on a black dot on a white piece of paper. If it locks, you're in range. Move paper around and try again; repeat until you've figure out the boundaries
Knowing the exact size of the sensor doesn't really help that much, though. If your subject is smaller than the AF sensor, there is simply *no way* to guarantee you will focus on it.
Quote: Personally, I'd like to know that if I have the Pentax focus screen ( o ) centered on an object and the object was filling the ( o ) in the viewfinder, that the focus point is in there
Would be great if true, but as I've explained, it isn't. The AF sensor is much taller, as you can find out for yourself by performing the simple experiment I described. Only if your subejct is large enough to cover the entire AF sensor can you rely on AF to focus on what you want. Otherwise, it's a crapshoot.