Focus and recompose question

achua00 · Apr 16, 2012

Hi, I was wondering when I focus and recompose am I supposed to hold down the focusing button or just let it go?I will use One Shot on my Canon camera if my kid sits pretty still but when she's moving around I will switch it to AI Servo. I think I am supposed to hold it down during AI servo but when I tried to use it in One Shot and recompose I noticed the face is a little out of focus. I use back button focusing.
Thanks

shmne · Apr 16, 2012

I'm going to try and explain this as simply as possible as there is much to explain on this subject.

First you need to understand how focus works - your lens (assuming its a standard, non tilt-shift lens) focuses on a plane perpindicular to the direction your camera is pointing. When you pre-focus in the center of the camera on your child, then move her into the 1/3 sweet spot that plane stays perpindicular to the direction you're pointing. So what used to be in focus, no longer is because your plane of focus no longer bifurvates the subject where it is needed. Instead now it is slightly in front of, or behind the subject.

Now this is assuming you're using center / spot focus. If that is the case you can go ahead and move that center focus point (should be lit up) to any of the auto-focus points on your camera. This allows you to put the subject in the 1/3 zone - then pre-focus and take the snapshot.

I hope this made sense. Try this - take a picture of a stationary object in the dead center of your viewfinder, then take another one where you pre-focus on the subject and then shift it to the 1/3 zone on your viewfinder. Watch how the subject becomes slightly blurred

analog.universe · Apr 16, 2012

I'll also add that the error caused by the plane geometry that shmne described will be more pronounced at shorter focal lengths. Longer lengths require less of an angular movement to move the subject to the edge of the frame, and therefore result in less of a forward-back movement of the focus plane.

For me, I basically don't focus and recompose shorter than 85mm. For shorter lenses, I'll either select a different focus point in camera, or simply focus manually.

Vtec44 · Apr 16, 2012

If AI Servo is the same as AF-C, then focus and recompose method doesn't really work. The camera will track focus automatically in AI Servo based on the focus area mode that you selected, as long as you hold the shutter down 1/2 way. Due to the fact that you're changing the distance when recomposing, you have to have enough DOF so that the change in distance will still be within the focus area.

Big Mike · Apr 16, 2012

With back button focusing (and AF removed from the shutter release button), then yes...when you use the focus and recompose method, you can just release the button. That will essentially lock the focus. By doing this, you can use AI servo mode to mimic One Shot mode (when you need to). Best of both worlds, IMO.