Better focus help

[starbucks]

"You can try auto-focusing on the part you want, then move the camera to the angle you desire (keep holding down the shutter button for the auto-focus as you move it). It will keep the part you want focused."

Can someone better explain this to me?

If I'm focusing in on someone's eyes as my focus point, then can I step back and re-frame the picture and it will stay locked on that point???


SR

[BCampbell]

It helps to understand a bit about how the camera's AF works, and how depth of field works.

Usually, the "recomposition" technique is used sort of in reverse as described here. You frame the shot the way you want it, then move so your focus point is covering what you want in focus, press the shutter halfway to activate autofocus, then return to your desired composition and shoot.

The thing is, this only works parallel to the focal plane. That is, you can move your camera up, down, left, and right, and you are not repositioning your shot in respect to the focal plane. The moment you step forward or back, you are moving your depth of field and changing what will be in focus.

So, short answer, no you can't step back and necessarily expect the same things to be in focus. You can step back, find your preferred composition, and then recompose from there. Or you can select a different AF point in your camera.

[wulf]

Yes - the advice quoted is a little incomplete. It will work if your depth of field is wide enough that the change in angle doesn't move the original point out of the sharp zone.

How much do you understand about "depth of field" (and what camera and lenses are you typically using - that might make a difference as to what you are likely to get away with using "focus and recompose")?

Wulf

[mitchgreene]

A good way to get an idea of what they're talking about with a hands-on method is to try doing what you're describing with manual focus. That is: focus on the item you want to be in focus (manually, not by halfway depressing the shutter release). Then recompose the shot and fire. If what you wanted to be in focus is in focus then the same will happen using autofocus. Chane one thing at a time for several shots; step back farther, step back less, move left, move right, move up move down. See what causes your camera to lose focus (don't bump the focus ring!). This is what was done before autofocus was ever invented, and is a useful skill to learn in case you find yourself without that incredibly useful feature.

But the short answer to your question is Wulf's: not if you step back too far, or more generally, like BCambell said, "no."