We don’t do a to of still photography at Groovy, but I have been a hobbyist since I shot for the yearbook in high school (Go Centurions!) with my father’s borrowd Canon DSLR. For as long as I’ve had autofocus, I’ve often used the trick of focusing on the subject using the center AF then recomposing the shot.
Turns out, in the modern parlance of the Interwebs, I’m doing it wrong.
In most modern SLR cameras, the autofocus sensor located at the center of the frame is generally designed to be more sensitive and more accurate than most or all of the camera’s other AF sensors. In low-light situations, it is possible for the center AF sensor to be the only one that will reliably lock on to the subject and achieve focus lock. This has given rise to the technique of center point focus-recompose, which involves placing the center AF sensor on the portion of the subject that needs to be the most clearly focused, and activating autofocus. Once focus has been achieved, then the camera is reoriented until the desired framing is achieved, and then the shutter is released. There’s just one problem with this idea: it is 100% guaranteed to cause focus to be behind the intended center of focus.
Wha? How? Huh?
Read the entire article to understand the nuances, but in short – simple geometry means that when you focus and then rotate the camera even a few degrees, you’ve now changed the distance from your lens plane to the focal point. When shooting with extremely shallow depth of field, this can mean the difference between a beautiful shot and a useless pile of pixels.
Not much more to say except
- It makes so much sense that my brain hurts for not thinking of it before.
- Full frame sensors are awesome, but awfully tricky to use right.
ᔥ Reddit user csbphoto in response to this question.