How is backfocusing the fault of the lens?

[JerryPH]

Haha... I clicked on the link as soon as I saw it and read... and read... and also discovered they gave no answer. I felt as if they were trying to baffle me with BS.

I should have read 1 post further down here and not wasted my time.

 

[usayit]

So why do some Sigma lenses, including my 30 mm f/1.4, need to be modified by Sigma in order to autofocus correctly?

Remember.. nikon, canon, and most camera manufacturers never made the specifications on their lenses' interfaces public. Canon is especially strict about keeping that information a secret. Third-party manufacturers rely on reverse engineering in order to maintain compatibility, which means updates if Canon decides to change something. This was the case with older AF sigmas and newer Canon DSLRs.

 

[Garbz]

So why do some Sigma lenses, including my 30 mm f/1.4, need to be modified by Sigma in order to autofocus correctly?

I can't speak for other cameras, but the Nikons I've seen have a different AF arrangement to that shown by Garbz. There are partially silvered areas on the main mirror, allowing some light to pass through to a second mirror that reflects the light down to the AF sensor. Therefore the manual focus and AF have two different mirror stops that can be individually calibrated.

This also means that the light travelling to the AF sensor has to pass through the back of the mirror. This affects the focus position, and the effect varies slightly with the angle the rays make to the main mirror. This is similar to the use of a filter behind the lens.

My drawing was oversimplified but the basis is the same. THere's an autofocus module calibrated in the same way to the lens that the film plane is calibrated. Even if the AF module was behind the mirror (and I was certain my D200 contained it next to the pentaprism and I don't see slots in the mirror either), the question I'm wondering about in this thread is still the same. Any deviation would be caused by the camera not the lens.

If a sigma lens were attached to my D200 and didn't autofocus correctly then as above either the focus would hunt because the camera can't get an image in focus, or the camera wouldn't take a picture anyway because of a focusing error.

I really am beginning to think that these "back focusing" issues are caused by people who don't know how to use their cameras properly and in a fit of "not my fault arguments" blame the lens.

/EDIT: My mistake you are right the AF unit is in behind the mirror. but the deal is very similar. Except the difference is now that instead of the red lines diverting up they divert down into the base of the camera.

 

[Helen B]

Garbz,

I agree with you, and I have exactly the same puzzlement about why the lens should be to 'blame'. When I had the problems with the D40x and the Sigma 30 mm, I did not expect Sigma to offer to fix the lens, I only expected them to offer advice about what to do about an inter-brand problem. I thought that the camera alone was deciding when the lens was in focus, and if the lens misinterpreted the instructions the camera would not report correct focus.

It's easy to see why a lens needs to be collimated/calibrated for a rangefinder or for a reflex camera when accurate scale focusing is required (as it often is in cinematography), but why for autofocus if there are no issues about the exact relative position of internal elements? I'm new to autofocus in still cameras, and I wonder how much of the focus computation is done by the camera and how much by the lens. Does the lens simply respond to the camera's instructions? How, and where, is the acceptable error set in the feedback loop? Is it variable or fixed? Is the final adjustment made by dead reckoning without subsequent feedback confirmation (ie 'if the lens is moved 0.04 mm out it will then be in focus and there will be no need to check')?

Nothing I can find on the web or in optical textbooks gets this deeply, so I would be very grateful for any pointers. The technical guys at Zeiss are always willing to discuss things in great depth and without simplification, but I am reluctant to bother them with a non-Zeiss nitty gritty question. It's easy in motion pictures, because you put all the bits together yourself: the laser rangefinder, the controller, the lens motor...

Best, and thanks for any insight,
Helen

 

[Garbz]

Yeah I don't know. I doubt the lens has much of an input with regards to focusing as the camera is the only part which knows if an image is in focus or not. The lens just blindly follows orders. There could ofcourse be problems where the orders aren't understood, but this wouldn't cause back focus. I did more searching on the internet and I am now 100% convinced that

- Backfocusing with lenses on DSLRs anyway is the result of people not taking care before pushing their shutter button.
- Backfocusing issues which do exist are reported with cameras like the D70 and I have seen a few posts where a D70 body was sent back and it fixed the problem.
- Most backfocusing problems aren't even backfocusing problems. People seem to not know what this means and just apply this name to other focusing problems.