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01-09-2009, 09:46 PM   #1
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Sigma 70-200 with both front and backfocus

I've confirmed that my 70-200 is back-focusing up until about 160mm where it starts to develop front focus. I've fixed the back-focusing by setting the AF adjustment to +9 for this lens but this obviously makes the front focus more pronounced. Here are some 100% crops, all shot in RAW and converted without sharpening or any other adjustments. Has anyone had a similar problem fixed before, is it even possible to adjust a lens that has both front and back focus?

135mm @ f/4, - no focus adjustments


135mm @ f/4, +9 focus adjustment


70mm @ f/2.8, +9AF


85mm @ f/2.8, +9AF


135mm @ f/2.8, +9AF


160mm @ f/2.8, +9AF


180mm @ f/2.8, +9AF


200mm @ f/2.8, +9AF


01-10-2009, 08:31 AM   #2
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I have the same problem. My Sigma 70-200 f2.8 (older version) was front focusing somewhat at 200mm and backfocusing quite a bit below 200mm. I usually shoot it at 200mm, so I corrected for that, but obviously that makes the backfocusing worse. I had wondered if I should send the lens to Sigma for adjusting....
01-10-2009, 02:55 PM   #3
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I'd send the lens to Sigma for calibration w/ test pics to show them what's wrong.

I wish listed the ability to adjust focus at multiple focal lengths for zooms...hope Pentax is watching that thread...
01-10-2009, 06:33 PM   #4
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QuoteOriginally posted by sholtzma Quote
I have the same problem. My Sigma 70-200 f2.8 (older version) was front focusing somewhat at 200mm and backfocusing quite a bit below 200mm. I usually shoot it at 200mm, so I corrected for that, but obviously that makes the backfocusing worse. I had wondered if I should send the lens to Sigma for adjusting....
Interesting, mine is the old version too. I'm doing a wedding in June so I'd like to be able to trust the lens at all focal lengths, I suppose a trip to Sigma is in order, I hope it's not going to be a 2 month turn around like my 17-70 was.

QuoteOriginally posted by kenyee Quote
I'd send the lens to Sigma for calibration w/ test pics to show them what's wrong.

I wish listed the ability to adjust focus at multiple focal lengths for zooms...hope Pentax is watching that thread...
It would be great to be able to adjust for multiple focal lengths. I'm probably going to send all my AF lenses to Pentax and have the K20D adjusted anyway since the turn-around should be less than a week. I wonder if Pentax can adjust for multiple focal lengths?

01-12-2009, 01:21 AM   #5
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Okay, why are you going to send all your AF lenses to Pentax? Are they causing trouble too--and not merely the Sigma?

It is an interesting question: Could Pentax (easily) provide AF correction at multiple focal lengths for a zoom lens? Could there be a simple firmware fix for the K20D to provide this capability?

But how would one make such a multi-focal-length correction? I could tell the camera to back focus +2 at 135mm, back focus +4 at 100mm, but what about all the focal lengths in between? Correct for each mm of focal length?
01-12-2009, 06:33 AM   #6
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QuoteOriginally posted by sholtzma Quote
Okay, why are you going to send all your AF lenses to Pentax? Are they causing trouble too--and not merely the Sigma?

It is an interesting question: Could Pentax (easily) provide AF correction at multiple focal lengths for a zoom lens? Could there be a simple firmware fix for the K20D to provide this capability?

But how would one make such a multi-focal-length correction? I could tell the camera to back focus +2 at 135mm, back focus +4 at 100mm, but what about all the focal lengths in between? Correct for each mm of focal length?
I plan all my lenses to Pentax because I'm pretty sure that the body is back focusing. To make a long story short my Sigma 17-70 and FA50 which both focused perfectly on my K100D need a +10 correction on the K20D. The 70-200 needs an additional +9 correction.

The camera can already read the focal length of a zoom lens in any given position for shake-reduction, by the same logic there's no reason why this couldn't be applied for af-adjustment too.
01-12-2009, 05:42 PM   #7
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No doubt it could read the desired AF correction at any focal length. The question is whether we'd want to program in a correction for the 130ish different focal lengths in, say, the 70-200.
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