Why do screw-driven lenses front/back-focus?

Started Jan 27, 2009 | Discussions thread
SteveL54 Senior Member • Posts: 2,696
Re: Lens is always wide open

Mark53 wrote:

AF with
Live View off always is done wide open.

The lens aperture is wide open during focus -- whether it be phase detection or liveview contrast detection.

Nothing is "hidden" by using
a smaller aperture.

Actually with phase detection AF there is another aperture involved. It's in the AF module.

The phase detection AF module looks through two small sections on either side of the lens. It basically uses these two sections as a rangefinder would. The aperture plate and lenses within the AF module are often fixed and are designed to work with lenses that have a maximum aperture of f/5.6.

Thus the rays coming in from the outer regions of a wide aperture lens are not used by the AF module. But they are the dominant contributor to the image on the imaging sensor when the exposure is taken wide open. With spherical aberration the rays from the outer regions of the lens don't quite come to focus at the same point as the rays from the f/5.6 area of the lens used by the AF module. So the phase detection system will miss focus if this shift is not accounted for.

In the trail of linked messages that I posted previously is an excellent explanation of this by Joe Wisniewski


Douglas Kerr has a good online paper describing split prism focusing and AF mechanisms here:



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