FF/BF related to a lens

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Steppenwolf1
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FF/BF related to a lens

Unread postby Steppenwolf1 » Sun Jun 19, 2011 1:59 pm

This is a technical question related to FF/BF. I always thought this was caused by the camera being inaccurately assembled (i.e. the light path length from the lens to the AF sensor is of a different length than from the lens to the photo sensor). So why is it that camera manufacturers give the option to fine-tune focus relative to each lens. I guess it's because people say that one lens works fine but another back-focuses, etc. but how can this happen - from a technical point of view? How does one lens cause a camera to focus incorrectly while another doesn't?

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InTheSky
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Re: FF/BF related to a lens

Unread postby InTheSky » Sun Jun 19, 2011 2:37 pm

Welcome to the forum.

For the answer, I'm not enough technical to answer properly, but pretty sure David will have all the answer.

Regards,

Frank
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A7 (R, S & R II) + NEX 3N ( and few lenses )

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UrsaMajor
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Re: FF/BF related to a lens

Unread postby UrsaMajor » Mon Jun 20, 2011 1:55 am

Steppenwolf1 wrote:This is a technical question related to FF/BF. I always thought this was caused by the camera being inaccurately assembled (i.e. the light path length from the lens to the AF sensor is of a different length than from the lens to the photo sensor). So why is it that camera manufacturers give the option to fine-tune focus relative to each lens. I guess it's because people say that one lens works fine but another back-focuses, etc. but how can this happen - from a technical point of view? How does one lens cause a camera to focus incorrectly while another doesn't?
As I understand the subject, the issue is that the image can be out of focus if the distance from a convergence of the light rays within the lens to the focal plane of the camera is not correct. This can happen because there is a tolerance during manufacturing as to what is acceptable variation from the optimum. This is needed to make it economically practical to manufacture the parts. That variation can occur in the distance between the face of the lens mount on the camera and the sensor (or film) plane. It can also occur within each lens.

The acceptable variation from ideal assumes that most of the time the camera tolerance and the lens tolerance will either be in opposite directions and tend to cancel each other out, or will be small enough that if they are in the same direction the sum will still be within acceptable limits. Since there are variations between lenses, that assumption may be valid for one lens and not for another.

With best wishes,
- Tom -

Steppenwolf1
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Re: FF/BF related to a lens

Unread postby Steppenwolf1 » Mon Jun 20, 2011 1:49 pm

I'll explain the problem a bit more clearly. When you focus a lens on a DSLR the focus is checked at the AF sensor and, if the camera confirms that the lens is in focus that should be as near as that lens can be made to focus. There may be then a problem with focus at the photo sensor if the camera is inaccurately assembled, but this should be taken into account by the camera (in the final focus correction). Yet the manufacturers offer an option of fine tuning the lens focus in some cameras because many people complain that some lenses consistently back-focus and others front-focus, etc. So it would appear that a lens can cause inaccurate focus. I don't understand why this is because the fundamental reason for FF/BF is that the camera is inaccurately assembled, so how can a lens cause focus problems?

There are tow ways that I can think of:

- Differential focus, where the outside of the lens focuses to a different point than the middle of the lens. Since AF preferentially uses the outside of the lens it will cause incorrect focus if the lens is stopped down. This tends to happen with big aperture WA lenses.
- Wrongly chipped lenses. If the lens reports wrong data (like gearing ratios) then the final focus correction will go wrong.

The trouble is that these are both basically equipment errors, the first of which can't be put right and the second of which will need the lens re-chipping. Yet people talk of having their lenses corrected by "recalibration".

I'd like to understand what's going on here and I find nothing on the net that helps.

Steppenwolf1
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Re: FF/BF related to a lens

Unread postby Steppenwolf1 » Thu Jun 23, 2011 3:20 pm

Has anyone any ideas on this subject?

For instance, do the new Micro 4/3 interchangeable lens cameras (which use contrast detection AF at the picture sensor and therefore don't have camera body assembly error problems to contend with) suffer from FF/BF due to their interchangeable lenses affecting focus? If not, why not? And if they don't why then do conventional DSLRs suffer from lens-related problems?


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