How can a lens have defective autofocus?

[Infidel]

This question has been nagging at me for some time now. I've read several instances of how many third party lenses are a great value, often accompanied by the caveat that it must be a good copy, as several copies have front or back focusing issues. How can one lens focus perfectly on a camera, while another lens fails to do so, on the same camera?

It is my understanding that in most DSLR systems, focus sensing is through-the-lens, with phase detection sensors in the body, behind/beneath the mirror. These sensors essentially determine whether or not the image is in focus, and provide data to the focus motor (either in the lens or in the body) to adjust the focus until accurate focus is detected at the sensor. As I understand it, it's a pretty straightforward negative feedback system. Further, when both my camera and lens are in AF mode, I can't release the shutter unless the focus confirmation dot is illuminated.

How then, can a bad copy of a lens result in OOF images? It's the camera, not the lens, that decides whether or not the image is focused.

Could someone either explain this or point to me a resource? My apologies if this has been covered before; my searching didn't yield what I was looking for.

 

[Rekd]

I've been making small, precision parts for about a quarter century. I use state-of-the-art CNC equipment, like Canon, Nikon etc. There is no way to ensure the quality of every. single. lens. they produce. Even if they did, parts will fail pre-maturely, break, be installed incorrectly etc etc etc.

For every way they can make a lens correctly there are a million more ways for it to fail. It's like that with everything in life. You'll just have to get used to it.

 

[Infidel]

I agree that there are varying degrees of tolerance for error in precision manufacturing, but that doesn't really address my question, which is related to the mechanism by which these manufacturing errors lead to focus problems.

 

[KmH]

It is my understanding that in most DSLR systems, focus sensing is through-the-lens, with phase detection sensors in the body, behind/beneath the mirror. These sensors essentially determine whether or not the image is in focus, and provide data to the focus motor (either in the lens or in the body) to adjust the focus until accurate focus is detected at the sensor. As I understand it, it's a pretty straightforward negative feedback system. Further, when both my camera and lens are in AF mode, I can't release the shutter unless the focus confirmation dot is illuminated.

How then, can a bad copy of a lens result in OOF images? It's the camera, not the lens, that decides whether or not the image is focused.

Could someone either explain this or point to me a resource? My apologies if this has been covered before; my searching didn't yield what I was looking for.

Many 3rd party len components are not held to as narrow a range of tolerance as camera maker lenses. Consequently, many of the 3rd party lenses have considerable performance variation, lens to lens. To a large degree the variables will average out. However, from time to time the variables will stack in favor of an exceptionally good performing copy of the lens, and from time to time stack in favor of a very poor performing copy of the lens.

Other considerations are the quality of the materials, particularly the glass used to make the lens elements, how accurately those lens elements are figured, and how well the design corrects the various optical aberrations.

Phase detection systems split an image in two, so one is from one side of the lens and one is from the opposite side of the lens. The two images are then compared so the direction and amount focus needs to be adjusted can then be calculated (essentially a rangefinder).
If there are significant differences in the two images from opposite sides of the lens, sharp focus cannot be acheived by the camera because it cannot accurately calculate the amount the focus motor needs to be driven, because the lens was poorly made. http://en.wikipedia.org/wiki/Autofocus

 

[Infidel]

Interesting...I guess the way the sensors sample image data (from opposite sides of the lens; this part is crucial) and the way the image is recorded by the imaging sensor have some key differences. For instance, a poorly aligned element could "trick" the AF sensor into judging the image to be focused. Now I think I understand...the AF sensor and imaging sensor don't "see" the same image.

 

[djacobox372]

Interesting...I guess the way the sensors sample image data (from opposite sides of the lens; this part is crucial) and the way the image is recorded by the imaging sensor have some key differences. For instance, a poorly aligned element could "trick" the AF sensor into judging the image to be focused. Now I think I understand...the AF sensor and imaging sensor don't "see" the same image.

If the light isn't traveling in a perfectly straight line then it's possible that it will focus differently on the AF sensor compared to the main sensor.

 

[KmH]

. Now I think I understand...the AF sensor and imaging sensor don't "see" the same image.

There you go.

The mirror in most dSLRs lets 1/2 the light through to a smaller mirror that is behind it.

The smaller mirror directs light down to the AF module, which is in the bottom of the camera. The AF module has to do the auto focus functions and drive the lens AF mechanism to focus, before the shutter opens to make the exposure.

In this stop action video Jeffrey Friedl's Blog Nikon D3 Shutter Release in Super Slow Motion drag your cursor iver the image to step through the sequence 1 frame at a time.

From the 8.7 ms mark to the 18.8 ms mark you can see the smaller rear mirror that directs light from the lens down to the AF module.

 

[orb9220]

"which is related to the mechanism by which these manufacturing errors lead to focus problems."

Comes down to manufacturing tolerances between lens & camera manufactures. And lens passes it's QA and Cam passes it's QA both in acceptable tolerances. One on the negative side and other on positve side. Put those two together tho and difference is compounded.

"THIS LENS IS SOFT" AND OTHER MYTHS"
.

 

[Derrel]

Try a Google search on this search string < my Sigma lens will not autofocus right >

and see a few dozen reasons why a lens might not focus properly...in the first three pages of results...