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04-25-2010, 07:52 PM   #1
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AF Adjusting a zoom

i'm doing an AF fine adjust for my zoom.
at 18, it's a bit front focused, at 50 it's a bit back focused.

since you can only do one AF adjust per lens, what would be the best compromise?
should I:

1. set it at midpoint between the front and back focus adjustment values
2. set it at the AF for the long end of the lens
3. set it at the AF for the wide end of the lens


i'd go with #2 since the long end would be more critical to correct focus than the wide end.

what would you do?



jordan


Last edited by opiedog; 04-26-2010 at 10:17 AM.
04-25-2010, 08:49 PM   #2
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QuoteOriginally posted by opiedog Quote
i'm doing an AF find adjust for my zoom.
at 18, it's a bit front focused, at 50 it's a bit back focused.

since you can only do one AF adjust per lens, what would be the best compromise?
should I:

1. set it at midpoint between the front and back focus adjustment values
2. set it at the AF for the long end of the lens
3. set it at the AF for the wide end of the lens


i'd go with #2 since the long end would be more critical to correct focus than the wide end.

what would you do?



jordan
Where do you use the lens the most? If you use it mostly at the long end, correct it there. If it's at the wide end, correct it there. What I would do though is go ahead with the original idea of #2 and see how it works for the rest. If it's both front and back focusing at the extremes, it would seem to me that in the middle, it's somewhat correct in that range already. No? Yes?

I would also make sure I'm doing these tests in GOOD light (like Sunlight), with a reliable target, at proper angles, not under incandescent or other interior lighting.

04-26-2010, 02:21 AM   #3
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You could also send it in for adjustment.
04-26-2010, 06:09 AM   #4
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QuoteOriginally posted by blende8 Quote
You could also send it in for adjustment.
well, the lenses are not pentax so pentax wont do anything with it.

if i send my lens to sigma, they'll want the body (kx) as well. may be a good idea if sigma can align the lens' AF and match it up with a specific body.

anyone has experience with this?

04-26-2010, 06:30 AM   #5
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QuoteOriginally posted by JeffJS Quote
Where do you use the lens the most? If you use it mostly at the long end, correct it there. If it's at the wide end, correct it there. What I would do though is go ahead with the original idea of #2 and see how it works for the rest. If it's both front and back focusing at the extremes, it would seem to me that in the middle, it's somewhat correct in that range already. No? Yes?

I would also make sure I'm doing these tests in GOOD light (like Sunlight), with a reliable target, at proper angles, not under incandescent or other interior lighting.

jeff,

yes, the lens seems to be focusing accurately at ~ midpoint. i did the tests using daylight, but like someone once said, i wouln't go crazy trying to get this perfected, since there are quite a few variables involved.

the AF adjustments on the extremes are not so much either +/- 20

i'll do another round of tests and i get the same results, i'll probably just set it to

the AF ajust at the long end.

tnx
jordan
04-26-2010, 08:27 AM   #6
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QuoteOriginally posted by opiedog Quote
if i send my lens to sigma, they'll want the body (kx) as well. may be a good idea if sigma can align the lens' AF and match it up with a specific body.
anyone has experience with this?
I can only speak for Germany.
It took about 2-3 weeks and the lens was perfect thereafter.

If you have the Kx, you can only apply a global adjustment AFAIK and this only via debug mode. This is not optimal. If your other lenses are ok, then it is probably the Sigma lens and you should send it in.
04-26-2010, 09:38 AM   #7
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QuoteOriginally posted by blende8 Quote
I can only speak for Germany.
It took about 2-3 weeks and the lens was perfect thereafter.

If you have the Kx, you can only apply a global adjustment AFAIK and this only via debug mode. This is not optimal. If your other lenses are ok, then it is probably the Sigma lens and you should send it in.
did you have to send in your camera body?
yes, i have the kx. if i sent it to sigma, i don't expect them to make any changes to the AF adjust on the camera body so i'll set it at 0. they'll probably adjust the len's data parameters for 18 thru 50.

and 2-3 weeks is difficult to be without a camera

tnx

jordan
04-26-2010, 11:14 AM   #8
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Yes, with camera.

05-08-2010, 06:16 AM   #9
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QuoteOriginally posted by blende8 Quote
You could also send it in for adjustment.
Wieland, do you have any idea how the adjustment works?

I never found a good explanation as to how two lenses can FF and BF respectively on the same body.

Has it to do with rotating lens elements to minimise abberrations? There's not a lot you can adjust in a lens, is there?
05-08-2010, 08:27 AM   #10
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I don't know exactly, but you can program a calibration curve either in the lens and/or in the body.
I know that Sigma can do this in the lens ROM. I don't know about Pentax, but I know that they can program different values for different focal lengths and different distances.
If the standard or factory value does not fit for your body it can happen that one lens shows a BF and another a FF.
05-08-2010, 05:21 PM   #11
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QuoteOriginally posted by blende8 Quote
I don't know exactly, but you can program a calibration curve either in the lens and/or in the body.
Thanks for the info!
Interesting.
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