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12-21-2010, 07:44 AM   #1
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What causes backfocus or frontfocus?

Trying to understand, who is responsible for BF/FF - lens or the body?

I often hear like "this particular lens has focus problem but other lenses works fine", so wondering if lens has any role in this? As I understand about the focus mechanism, the body deploys the feedback control system to rotate the lens element till the focus is achieved - whether it's phase detect or contrast based AF. While I understand that any calibration issue in camera body can cause the bad focus but in that case it should be across all the lenses, not any particular one.

Any insight please.

12-21-2010, 08:11 AM   #2
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It can be both.
The lens can be decentered or the AF modul can be decentered.
Additionally the lens ROM can be programmed wrongly.
12-21-2010, 09:17 AM   #3
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As far as I understand, some degree of unwanted movement is inevitable in any two (or more) mechanically coupled moving parts. Stopping one, for example, stops the other only after some time. The more parts the system has the more difficult it is to 'standardize' the potential errors. Also, cheaper, poorly made systems has more "leeway". This resembles, more are less, the situation with the steering wheel of the cars. It is "normal"to have some movement of it, "before" you move the wheels.
12-21-2010, 10:48 AM   #4
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QuoteOriginally posted by blende8 Quote
It can be both.
The lens can be decentered or the AF modul can be decentered.
Additionally the lens ROM can be programmed wrongly.
What lens ROM or electronics has to do with focus since camera is adjusting the element, isn't lens is just a passive element as far as focus is concern?

12-21-2010, 10:50 AM   #5
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QuoteOriginally posted by yusuf Quote
isn't lens is just a passive element as far as focus is concern?
True for screw-driven AF lenses, but untrue for SDM and other onboard AF motorization technologies.
12-21-2010, 10:53 AM   #6
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QuoteOriginally posted by RBellavance Quote
True for screw-driven AF lenses, but untrue for SDM and other onboard AF motorization technologies.
Agree, may be we can focus on screw driven AF for this discussion.

So if the lens is a passive element, why one particular lens in some cases causes focus issues when other lenses are working fine - with the same body?
12-21-2010, 10:59 AM   #7
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Sigma reprogrammed my screw driven 20/1.8 for correct focus.
12-21-2010, 09:24 PM   #8
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I dont know the answer, and i also have trouble understanding how a lens can be effecting the result seen by the phase sensor... either the sensor sees focus or it does not. It is pretty obvious that is not the case though.

12-22-2010, 02:31 AM   #9
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I see the readers fail to realize that this is more a "mechanical" rather than an optical phenomenon.
The focus sensor and its chip decides that "it is OK" when they sense maximum contrast (or, whatever). Then, the chip sends a signal to the moving parts (outside the sensor, mechanical) to "stop". Every lens (depending on age, brand, focal length, local temperature etc.) reacts just a tiny amount differently. The mechanism comes to a stop "after" a while, where the focused point may no longer be "exactly the same" as that was found good by the sensor&chip. This may or may not alter the focus point noticeably and consistently. Adjusting for back focus and/or front focus can be talking specifically to that lens to account for the delay in its response. If the camera side is weak/faulty/problematic, than you adjust for the camera. Of course, an occasional problem can be out of reach of the supplied adjustment tolerances and sevice may be needed.
The focusing screens of our DSLRs are of no help in showing us the differences in focus resulting due to these events. There are "optical" reasons for that.
Does your car stop the moment you hit the brake?

Last edited by bc_the_path; 12-22-2010 at 03:42 AM.
12-22-2010, 03:08 AM   #10
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@Bulent, you are correct but then there is a feedback system which should correct for any inconsistencies - at the most it should result in a bit of hunting in acquiring focus but should not have BF/FF problem.
12-22-2010, 03:29 AM   #11
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QuoteOriginally posted by WerTicus Quote
I dont know the answer, and i also have trouble understanding how a lens can be effecting the result seen by the phase sensor... either the sensor sees focus or it does not. It is pretty obvious that is not the case though.
The AF sensor is not "seeing focus". It is seeing a phase difference.
A decentered lens element can cause an additional phase difference.
12-22-2010, 04:29 AM   #12
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There is also the "aperture value" of the focusing elements. Like lenses, a wide aperture system is more expensive. A narrow aperture system is easier to build and cheaper. Used along with "like" lenses the (inevitable) focus errors are not obvious as DOF is big enough to hide them. The errors usally become a problem when we try to focus using a narrow aperture system (say, f/5.6) with a tele lens at f/2 or so.
Hunting happens when the focus sensor "notices" that the change in positions of the elements produce a poorer contrast.
All the arguments aside, I do not understand why focus adjustment is possible only in the higher end DSLRs. It is a must especially for wide aperture lenses, anyway...
12-22-2010, 06:09 AM   #13
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It's simple, really. Both camera and lens are made within manufacturing tolerances, which should ensure that every lens delivers acceptable sharpness when used with any camera. But sometimes, if lenses are at one end of their tolerance range, and the camera at another, you end up with out-of-bounds combined results.
12-22-2010, 08:41 AM   #14
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QuoteOriginally posted by bdery Quote
It's simple, really. Both camera and lens are made within manufacturing tolerances, which should ensure that every lens delivers acceptable sharpness when used with any camera. But sometimes, if lenses are at one end of their tolerance range, and the camera at another, you end up with out-of-bounds combined results.
To me, that says that the acceptable tolerances were chosen incorrectly.
12-22-2010, 09:05 AM   #15
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My current theory is that the camera AF algorithm tries to optimize for fast focus and therefore makes assumptions about movement after the power to the focus drive has been cut. This could account for differences between lenses, even with ones of the same make and model (and add a factor depending on the body to AF / image sensor misalignment). At any rate I'd hope for for a built-in calibration procedure with, say, a printable target one might stick on the wall (or at least more adjustment range, the current situation where the existing set of lenses might become sub-standard when upgrading the body is scary).

Last edited by jolepp; 12-23-2010 at 06:45 AM.
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