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07-07-2010, 08:27 AM   #31
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QuoteOriginally posted by JMI Quote
I had one for my D700 and I fixed it at home. Sigma uses metal shims under the rear metal mount. I just unscrewed the rear mount and took both shims out, that was too much, so I put one back in and presto, perfect focus on the D700 with no correction.
This is a brilliant observation. I found out having this "nearsight" problem with 100-300/f4. It was so bad, that full -10 adjustment of K20D was not even near to be enough. I just disassembled the back of the lens barrel and found the shims - they were not right under the mount, but between the lens body and the flange, which carries the aperture ring (the part right under the mount). I had to take all three shims out and now it seems, that this "myopia" is cured to correctable degree.

This can explain the fact, that front focus is much more common, than back focus. Curing the back focus with the same method would required adding the shim(s), that's not there initially. Maybe the third-party manufacturers just do it this way to have the simple way to adjust this thing after the production, because the initial calibration is not as good as expected from the OEMs.

P.S. I disassembled my Tamron 17-50/f2,8 zoom to check for the same thing and it is there - only it is done a bit differently. It seems, that the shims are between the mount of the most rear lens element and the inner barrel - so the adjustment would require the disassembly of optical path, which I would not want to do - but then again this Tamron of mine has only wery slight front focus (corrected by adjustment of -2...-3 from the camera body). I also found out that the Sigma 100-300/f4 has inner structure also made from the metal - all parts related to the attaching of the rear mount are also metal (maybe this is the reason, why Sigma has only 3 screws holding the rear mount, but Tamron has 4, which are screwed into plastic).
I had a Tamron 28-75/f2,8, which suffered from severe front focus. I send it to service - they confirmed, that the lens was sent to Germany in the Tamron's European "headquarters" - if all they're did then was removing of the shims, then it is completely beyond me, why it took almost 4 weeks. But then if Tamron's construction would required this removing of the rear element, then maybe some additional calibration was needed. When looking to this issue this way, it seems, that Sigma's implementation is better service-wise.

Last edited by tim71; 07-07-2010 at 08:35 AM.
07-07-2010, 08:47 AM   #32
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you can disassemble them and remove shims? I have a Sigma 24mm 1.8 and i'm having some crappy focus issues when shooting 10'+. Think removing shims would help that?
07-08-2010, 12:24 PM   #33
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I would not want to encourage anyone, who does not know, what (s)he is doing, because it would not be a good idea to thrash the piece of expensive optics because of some tiny screw falling into the lens inner mechanism, some tiny detail getting lost or something just getting broken, but it looks like this thing can be cured on some lenses this way.

Do you have a front focusing problem? Then it would be possible, if the lens has shims in it - but I do not know, if all of them have this kind of construction. Sigma 100-300/F4 has the shims between the flange of aperture ring and main barrel, lenses without aperture ring may have them straight under mount...

Last edited by tim71; 07-08-2010 at 04:21 PM.

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