OK here is a test I have just carried out . I was going to post the pictures but if you are really that interested I suggest you go try it out your self.
I took a plain white sheet of A3 size paper as that in landscape is the same ratio as a K5 and therefore fills the frame. I very carefully measured the centre and drew with a thin black marker pen a vertical line 1/4 of a inch each side of the centre, and stuck the sheet to a door. target area is therefore 1/2 inch high by about 1/8 " wide
K5 on a tripod set to single focus centre point with 50mm f1.7 fitted. and placed so the sheet of paper just filled the viewfinder.
if the black line in
on or
in side the right hand ) in the centre of the viewfinder the focus lock green hex comes on.
if the black line is inside
but NOT on the left hand ( in the centre of the view finder then focus lock is achieved.
if you imagine the ( ) creating a circle the focus lock comes on if the top or bottom edge of the black line touches the imaginary completed circle or any where inside that circle.
nothing new here we knew that any thing inside that ( ) will be found no matter what the dot is on.
but what is very interesting is to try this same exercise with any of the user selectable points.
you will find that in fact the distance either side of the black line where the red dot is placed is not at all equal. if you take the far left one you will see it is much larger and locks a bit left of the line but not at all to the right and also quite some distance above and below the line.
the same applies to the other smaller usable selected points, the distance that they lock on is is not equal either, they are not crosses so much as we have been told.
this is quite hard to explain and either needs images with the red dot added or you you need to test for your self.
what it all therefore indicates is that even if you have put the centre dot on a object say a face, if there is something more contrast in the ( ) area it can lock on that and not the face. So your chosen subject should fill the ( ) area.
What is more important is the distance away from the line that the user selectable chosen point will still lock up can lead to the lock being on completely the wrong thing.
whats worse is that in a small dslr viewfinder with no split screen focus can be very hard to spot.
if you go back to page one of this thread and look at the original image you can possible see why the edge of the crib is in focus and not the face.
it also make no difference what f stop you are using as far as I can see.
as the op said [see below]" I use my arrow keys to move the red dot over what I want to be in focus." if you do that you could easily get the crib edge in focus and not the face.
Try it out you will soon see what I mean.
Originally posted by Deiberson Perhaps I don't understand how it works then. I assumed that if I'm on SEL on the back of the camera and AF-S, I use my arrow keys to move the red dot over what I want to be in focus. I'm "selecting the point" for the camera so it doesn't have to read my mind.