Advanced AF Frustrations: does your 7D Zone AF front focus vs other modes?

Started May 8, 2012 | Discussions thread
Sailor Blue
Sailor Blue Forum Pro • Posts: 11,378
Re: Advanced AF Frustrations: does your 7D Zone AF front focus vs other modes?

One reason that using the central focus point and one away from the central point may focus at slightly different distances is the way lenses work.

A single element lens actually focuses the image on an curved surface, not a flat surface. The human eyeball is a good example of this. The back of the eyeball, where light is sensed, is concave in shape so that as much of the image will be in focus as possible.

Engineers combine convex and concave lenses of different materials and place them at different distances from each other to produce the flattest image possible and to minimize other problems like chromatic aberration. The result is never perfect but a compromise.

The typical result is that for a prime lens the focal plane actually looks something like the circular waves produced when a small objects hits a pool of water.

With center point focus it is normal to see the center of the image in focus with a ring of slightly out of focus image surrounding the center, then a ring back in focus, then the edges/corners slightly out of focus. With non-center point focus that point should be in focus but other areas of the image may be slightly out of focus. The wider the aperture the shallower the depth of field at the sensor as well as at the subject so this problem is greater at wider apertures.

Zoom lenses are even more complicated. They might focus to a concave surface at one focal length, fairly flat at a different focal length, and convex at yet another focal length.

There is no such thing as a perfectly focusing lens. All lenses are compromises. The reason some lenses are more expensive than others is partly due to a more sophisticated design to give the lens fewer aberrations and a flatter plane of focus.

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