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08-05-2009, 10:00 AM   #1
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Help - Infinity focus problem with DA* 50-135mm 2.8



Hey folks... I've lurked around here for a very good long while but I'm not sure I've posted before. Basically seeking some input on a focus problem I'm having with my DA* 50-135mm.... Essentially when using it in manual mode (not AF) which I do frequently, it appears to focus PAST infinity, making the entire image appear soft. I discovered this last night as I was taking shots of the moon and some distant buildings. I have to focus to the end of the ring's travel (as you normally would) then come BACK to infinity just a bit, at which point the moon would come into focus. I really don't think that the moon is going to be need any focus distance closer than infinity. So it seems like something is truly wrong. Looking at the small window atop the lens which displays the distance for your focus, it certainly does look like the infinity symbol goes PAST the little hashmark in the middle of the window. I'm at a loss... don't know if it's always done this. Sound like a warranty repair? (Anyone know how long that would last?) Love this lens to death so this is really disappointing considering I've only had it about 4 or 5 months and take ridiculously good care of it.

Cheers and thanks.

08-05-2009, 11:05 AM   #2
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This is completely normal for a zoom lens. Infinity focus is slightly different at each end, so to get infinity focus at both ends, one end must go slightly past it.

New lenses in general focus slightly beyond infinity anyway. Lots of theories have been proposed for this. It could be to account for thermal expansion, or to work better with AF logic, or allow greater manufacturing tolerances, or less QC.
08-05-2009, 12:19 PM   #3
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QuoteOriginally posted by Just1MoreDave Quote
This is completely normal for a zoom lens. Infinity focus is slightly different at each end, so to get infinity focus at both ends, one end must go slightly past it.

New lenses in general focus slightly beyond infinity anyway. Lots of theories have been proposed for this. It could be to account for thermal expansion, or to work better with AF logic, or allow greater manufacturing tolerances, or less QC.
I concur with Dave on the thermal expansion observation. I have noticed that infinity will be in different positions in cold and hot weather.
08-05-2009, 04:54 PM   #4
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QuoteOriginally posted by Just1MoreDave Quote
This is completely normal for a zoom lens. Infinity focus is slightly different at each end, so to get infinity focus at both ends, one end must go slightly past it.

New lenses in general focus slightly beyond infinity anyway. Lots of theories have been proposed for this. It could be to account for thermal expansion, or to work better with AF logic, or allow greater manufacturing tolerances, or less QC.

The first point should NOT happen if the lens is correctly assembled; focus should not change while zooming!

Focusing past infinity is common on many lenses; particularly telephotos. This is to compensate for thermal expansion/contraction....

08-05-2009, 07:36 PM   #5
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QuoteOriginally posted by Pål Jensen Quote
The first point should NOT happen if the lens is correctly assembled; focus should not change while zooming!
....
I don't know whether my lenses suck or it's my reassembly skills, but every time I service a zoom, I check the infinity setting at both ends, and it's different. Tomorrow I'll test a couple of lenses I have never taken apart.
08-06-2009, 11:44 AM   #6
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Thanks for all the posts guys. I didn't realize focus-past-inf was normal on a zoom... most of my experience is with primes.
08-06-2009, 01:00 PM   #7
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QuoteOriginally posted by chickentender Quote
Thanks for all the posts guys. I didn't realize focus-past-inf was normal on a zoom... most of my experience is with primes.
I don't believe its just zooms, but i have some primes that one has to come back on too. Normal assembly practice, i think.

Isn't that 50-135 great? The only problem i'm having with it is getting it off my camera
08-06-2009, 07:49 PM   #8
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I know exactly how you feel.

See here:

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/60052-da-60-25...servation.html

08-06-2009, 10:20 PM   #9
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QuoteOriginally posted by chickentender Quote
Thanks for all the posts guys. I didn't realize focus-past-inf was normal on a zoom... most of my experience is with primes.
With longer primes you'll often find the same thing.
However I don't think it's noticeably common practice on shorter primes, since there's less material to expand and contract.
08-06-2009, 10:20 PM   #10
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QuoteOriginally posted by philbaum Quote
I don't believe its just zooms, but i have some primes that one has to come back on too. Normal assembly practice, i think.

Isn't that 50-135 great? The only problem i'm having with it is getting it off my camera
My M 400 goes WAY past infinity at times. It's normal.
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