Why AF problem only at wide aperture, wide angle? Any optics experts?

Started Jun 12, 2012 | Discussions thread
Joseph S Wisniewski Forum Pro • Posts: 34,130
Not depth of field, depth of focus...

kliang wrote:

That explains why wide aperture can show AF errors more b/c less DOF. But shouldn't a longer focal length (less wide angle) show AF errors bette b/c longer focal length gives out less DOF? I never understand the wide angle part.

It's not a problem with the "depth of field" in front of the lens. It's a problem with the depth of focus behind it. They're reciprocal. You can have a huge misalignment between AF sensors and image plane, or a skewed AF sensor unit or main sensor image plane, and not see it at all on a telephoto or a normal.

Wide angle lenses have insanely shallow depth of focus, they move very tiny distances when they focus, and even a small misalignment trashes them. When you focus a 20mm lens from 3m to 2m (more or less from 9ft to 6ft) the lens only moves out an additional 0.067mm. That's 67 microns, about the diameter of a human hair.

Try that with a 100mm. That lens moves 1.815mm when it focuses from 3m to 2m, a 27 times greater distance than a 20mm moves. The little 67 micron "human hair" change only focuses you 55mm closer.

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Rahon Klavanian 1912-2008.

Armenian genocide survivor, amazing cook, scrabble master, and loving grandmother. You will be missed.

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