How can auto focus errors be a lens problem?

Started Dec 10, 2012 | Discussions thread
Helge Hafting
Contributing MemberPosts: 817
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Re: How can auto focus errors be a lens problem?
In reply to guzzibreva1, Dec 16, 2012

guzzibreva1 wrote:

I have read user reviews about Sigma lenses having focus problems on certain cameras. I find it hard to understand how a lens can auto focus wrong.

If you try adjusting the autofocus, you will notice that a camera that is a little off, may work fine with a tele but consistently misfocus a large-aperture wide-angle.

This because of how the optics works. There are several problems:

1. autofocus is supposed to work up to f/5.6, so it uses the light that goes through the f/5.6 part of the lens. This limits the precision somewhat. What is good enough when the lens is stopped down to f/5.6, might not be that good at f/1.4, More expensive systems also uses light from other parts of the lens (typically the f/2.8 area or so), in order to work better with premium lenses.

Note that wide-angles often have larger apertures than tele lenses.

2. With manual focus, you notice that a tele has shallow depth of focus, and a wide angle has large depth of focus - so the wide angle is easier. But this is true only on one side of the glass - the scenery side.

It is the opposite on the sensor side of the glass. The AF sensor can have a wrong distance to the lens, i.e. not the same as the sensor distance. In that case, AF will move the focus motor until the image on the AF sensor is sharp - but then it may not be sharp on the sensor. In such a case, a wide angle will err much worse than a tele. The light rays from a tele is nearly parallel (light comes from a very small area far away). So a small distance difference won't matter that much for the sensor. A small fault in the AF distance won't matter. Sharp focus slightly in front of the sensor will still be pretty sharp.

With a wide-angle, the light rays are not "nearly parallel", for the come in from a very wide area. If they are focused perfectly only slightly in front of the sensor, they will spread out a lot before actually hitting it. You may have a lot of depth of field, but an AF system that is just a little off distance-wise, will make a huge miscorrection and blur things.

When I adjusted focus on my SD15, the 20mm f/1.8 (my widest lens) was the hardest to get right. When that one was ok, all the others were fine too.

Chances are that if you see AF problems with one particular lens - then it is either your widest lens or the one lens with the largest aperture.

3. There are various other phenomena that others have described, such as overshooting motors and glass faults. these can also cause one particular lens to get AF wrong.

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