Unless you AND the subject are perfectly stand still and do not drift/rock back and forth at all, when you stop focusing to recompose, again you're likely not on the same focal plane.
In practice/practical terms...this is actually rarely an issue. The difference in focus plane is almost never noticeable except in cases where one is very close to the subject. So close that one wouldn't take the image anyways...due to distortion from being to close to the subject. I know this...because tested with all my lenses (16mm-300mm) to see how close I needed to be before it was an issue even with large changes in focus point
I see it often enough to not care to ever use the focus and recompose method as have friends.
Unless you are using a very wide lens inside a 5-7 feet...the effect you are seeing is likely more a result of moving in out rather than due to the geometry of recomposing moving the focus plane. Even more likely...the lens is sharper in the middle than the edges and you are seeing the differences in lens sharpness across the filed
Example from Bob's site:
"Let's look at a 20mm f2.8 lens on a full frame 35mm camera and focused at a distance of 1m. A 20mm lens has a horizontal angle of view of 84 degrees, so the largest angle you could turn the camera through to focus and recompose would be half that, or 42 degrees. You'd be focused at 1m, but an object at the edge of the frame which was originally in the center would be about 0.35m in front of the focal plane. Depth of field would be from 0.8m to 1.3m, so in this case focusing and recomposing would result in a significantly out of focus subject. But 20mm at f2.8 and focused at 1m is a pretty extreme situation.
If you run through all the numbers and make assumptions about typical subject distances, apertures and angles likely to be swung through when focusing and recomposing you find that the vast majority of the time it shouldn't give you a significant loss of sharpness. In fact most of the time it probably won't be detectable."
In the cases where it could be an issue...folks learn to slide the camera Left/Right Up/Down rather than pivoting to recompose
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My opinions are my own and not those of DPR or its administration. They carry no 'special' value (except to me and Lacie of course)