How Autofocus with Single Focus Point works after Re composing

Question

I own a Nikon D7000 which of course has 39 focus points. I was going through some articles on internet on focusing techniques. One article I came across was say shooting people (portraits) I should use AF-S and Single focus point (central only to be specific) I should point it on eyes preferably and press the shutter button half way once the camera locks it focus, press down the AE-L/AF-L button to keep the focus locked and then re compose my shot. e.g. rule of thirds or something else. I am confused over how come I can not compose my shot first and then chose any of the other 39 focus points which points on my subject. If the central point is more advanced from others, then how come when central focus point is not even on my subject, will help in taking a sharp picture that has sharpest details of my subject.

Any comments?

Answer 1

If you want to choose the point of focus you basically have two options:

1. Select the focus point for each shot

2. Always use the same focus point, focus and then recompose

Both have advantages and disadvantages - so you should use the technique that is better for you and that fits the specific situation better.

Focus and recompose is so popular because:

1. It's faster than choosing the focus point, especially if you have lots of them

2. It works even if you don't have a focus point in the exact place you want to focus on

3. If your camera has different kinds of focus points the center point is always of the best kind

Also, the focus and recompose technique is old, I learned it on a camera with just one focus point where you don't have another choice.

Now, it does have the disadvantage that if you have a very shallow depth of field the recompose can actually move the subject's eyes slightly out of focus - but only if you have really shallow DOF (so in the correctly focused photo the eyes would have been sharp but the nose and ears out of focus)

Answer 2

"I am confused over how come I can not compose my shot first and then chose any of the other 39 focus points which points on my subject."

It's certainly doable, but if you change focal lenths or subject distance, the point of wanted focus and composition may change. So as a general rule, it's easier (for some people) to center focus, then compose.

And this:

"If the central point is more advanced from others, then how come when central focus point is not even on my subject, will help in taking a sharp picture that has sharpest details of my subject."

It's not that it's more advanced, just that it's simpler to compose. But anyway, I'll try my best to explain it.

My personal opinion is that any AF points more than 3 is a gimick. It can be a little fiddley to use the arrow buttons to move the selected points and actually a little slower.

When you have the central point selected, the only way to defocus (after half pressing) is to move the camera back or forth in the direction of the subject. A slight pan to the left or right generally won't ruin the focus point.

If you find it's still slightly out of focus, my suggestion is to close the aperture a little to widen the depth of feild.

EDIT: Just to further comment on the "possible duplicate", multi AF points can be helpful. I guess all I can say is practice and see what works best for you.