Originally Posted by zclin
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Here is the email I got from Sigma:
Here is more info on this.
"Back/Front focusing problems are mainly caused by a variance of the digital
camera body".
In the case of a film camera, the flange focal distance has and
insignificant deviation, since the image sensor is just a single peace of
film.
On the other hand the image sensor of digital camera consists of 5
components each having a tolerance of plus or minus 0.01mm. These
tolerances could cause a variance of up to plus or minus 0.05mm.
In the case of the lens, the variance of optimum resolution point of an AF
lens is generally very small. This is why you have focus problem with a
given lens.
In this circumstance, most of Sigma's distributors provide services for
focus adjustment using the customer's camera to obtain the focus point
variation. Once this variation is obtained the CPU of the lens can be
rewritten to compensate for any variation.
For U.S. customers only.
Sigma Corporation of America
15 Fleetwood Court
Ronkonkoma, NY 11779
Please include a letter with your name, phone # and address explaining the
problem with your product. Also include copies of your warranty card and
sales receipt.
Rory O'Donoghue
Customer service/technical assistance
Sigma Corporation of America
wimg wrote in post #4011589
Total and utter rubbish, and I am being extremely polite here.
First of all, the AF and sensor distance are calibrated by the camera manufacturers in unison, as a combo IOW. What is the use of calibrating a camera's AF if the sensor distance isn't taken into account during the process? Note that cameras and lenses are calibrated independently, and that is for very good reasons.
Secondly, there is much less variation in camera AF, than there is in lens AF. I've had 4 dslrs myself now, and tried quite a few others, and I've tested all of my own and quite a few others for AF, with the same lens for reference. No difference whatsoever. However, different copies of the same lens do vary, and they do vary a fair amount to a lot. And that is visible even between just two copies of the same lens! And this includes Ls. It is one of the reasons why the 85 F/1.8 I have was the third copy I got.
Finally, there is AF variance between shots, even with the camera on a tripod. This is caused by a bunch of factors, but the main one is the fact that the AF motors and the control of the AF motors are not as precise as any other part in a lens-camera combo. It is a moving part after all.
Ok, a sensor is not very forgiving for slight misfocusing, compared to film, but misfocusing is part of life and due to the size of the sensors and the electromechanical aspects of the lens focusing mechanism, and the latter especially.
Kind regards, Wim
Total and utter rubbish, and I am being extremely polite here.
First of all, the AF and sensor distance are calibrated by the camera manufacturers in unison, as a combo IOW. What is the use of calibrating a camera's AF if the sensor distance isn't taken into account during the process? Note that cameras and lenses are calibrated independently, and that is for very good reasons.
Secondly, there is much less variation in camera AF, than there is in lens AF. I've had 4 dslrs myself now, and tried quite a few others, and I've tested all of my own and quite a few others for AF, with the same lens for reference. No difference whatsoever. However, different copies of the same lens do vary, and they do vary a fair amount to a lot. And that is visible even between just two copies of the same lens! And this includes Ls. It is one of the reasons why the 85 F/1.8 I have was the third copy I got.
Finally, there is AF variance between shots, even with the camera on a tripod. This is caused by a bunch of factors, but the main one is the fact that the AF motors and the control of the AF motors are not as precise as any other part in a lens-camera combo. It is a moving part after all.
Ok, a sensor is not very forgiving for slight misfocusing, compared to film, but misfocusing is part of life and due to the size of the sensors and the electromechanical aspects of the lens focusing mechanism, and the latter especially.
Kind regards, Wim
let me add the the two best focusing lenses on my Mark II are the ones that were calibrated, even without the body
