Need help with focusing issue.

Started 2 months ago | Questions thread
ttusteveo Regular Member • Posts: 262
Adequate depth of field for Focus/Recompose

Jerry-astro wrote:

Nas from the Valley wrote:

charley5 wrote:

Hi Everyone:

I am having a frustrating time with a focusing issue and I need help to solve it. I focus on the face of the person initially by pressing down the shutter half way. Then, because I want the face to be at the top third of the frame, I move the camera (with the shutter still half-way depressed) usually to chest level, and press all the way down. Their chest then is well focused in the final image but their face is not. This is really frustrating. I have lost several good shots in this way. What may be the problem? I use single point focusing and 9 squares. Thanks for any suggestions. Here are two examples:

I couldn't read all of the comments above but of course you will have a problem with focusing. It is obvious!

When you focus (in single focus mode) and hold the shutter button and move the camera you are changing focus plane! Don't expect camera to remember where exactly you focused and lock that focus spot forever!

Let me give you an example: you focus on the face which is exactly let's say 2.37 meters away from the camera and focus plane will stay on the same distance (2.37 meters) as long as you are holding the shutter button. Then you move down the camera a little bit and the face will be on let's say 2.32 meters away from the camera but the chest now will be on 2.37 meters. So the focus plane will be changed and the face will no longer be the sharpest spot since it's not on the focus plane anymore. Simple as that.

The only way to do it the way you want it is with wide tracking, face/eye detection or the best is just use the focus points situated at the top of the sensor where the face will be.

Focal plane changes when you re-focus.

I know drawing is funny but that's what I wanted to show you. The red line is the focal plane.

The point you're making is spot on, however, it's worth noting again that the focal plane is actually curved, not straight as you've illustrated, and equidistant to the camera.

Question...

An online depth-of-field calculator shows that for the first image:
Focal length 39mm, f/8, that the depth of field

I think he had to be at least 6 feet from his subject when he took the photo, but the OP can confirm.

At a distance of 8 feet, the depth of field would have been:
In front of subject: 1.58 ft
Behind subject: 2.61 ft

Wouldn't that have been sufficient for Focus+Recompose to have captured sharp detail, in spite of the slight change in the focal plane?

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