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DA21 weird focus behavior

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leekil

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I just got a DA21 off Ebay, and noticed some weird focus behavior the other day. It seemed to have trouble focusing on things that were pretty far away (50-100 feet maybe) but not at infinity (clouds). During AF, the lens would move briefly and then stop, but the green AF light on my K20D would flash instead of indicating locked-on focus.

Today, however, when I went to test this behavior in preparation for this post, it did not happen. I could focus on things in the same range with no trouble at all.

My theory, when the issue was happening, was that perhaps the lens was out of alignment as well as backfocusing, and couldn't focus far enough "past infinity" to get where it thoguht it needed to be.

Anyone have any ideas about this? Have you had an experience like this? Should I try to return the lens?

Lee
 
Try manually focusing the lens. If it feels tight or sticky when focusing near infinity (or near infinity) you might have a failing focusing mechanism in the lens and need to have the focusing mechanism replaced.

On a recent trip I had this happen to me & the lens "broke". It cost $108.00 to have the lens fixed by CRIS Camera Services in the US. They fixed the lens and I had it back in less than 4 weeks.

John
--
K10d, K1000
A35, A50, M135, M200
DA18-55, DA50-200, DA21Ltd, FA43LtdDA70Ltd
 
It didn't seem tight or sticky, it moves smoothly. It seems to focus just past infinity, but I didn't know if that was enough. I assume it would need less room past infinity since it is a wide-angle lens and the focus range is highly compressed near the infinity end.
 
Wow, that sucks! I have had that exact behavior happen when I had that lens. I absolutely LOVED it when it worked--I swear it didn't create any noise--it was the most noiseless, crisp and well-saturated lens I've owned to date, but sometimes it would focus like a champ on everything, and then other times, exactly as you described. I would return that puppy, pronto. I think I bought mine from Cameta and I had the devil of a time getting them to return it at first, but I succeeded and I don't even recall what I bought to replace it, but I now have the 12-24mm and it is a stellar lens indeed. Although I needed to have Pentax CO calibrate it to work appropriately with my K20D, as the focus was out of whack on it terribly--I believe it was back focusing. . . But the DA 21 problems were something completely different. Good luck to you! Cindy
--
My PPG-- http://www.pentaxphotogallery.com/cynthiafarr-weinfeld
http://www.cindysphotoquest.blogspot.com
My photo sites: http://www.cfwphotography.com
and
http://www.cfwphotography.smugmug.com
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leekil said:
I just got a DA21 off Ebay, and noticed some weird focus behavior the other day. It seemed to have trouble focusing on things that were pretty far away (50-100 feet maybe) but not at infinity (clouds). During AF, the lens would move briefly and then stop, but the green AF light on my K20D would flash instead of indicating locked-on focus.

Today, however, when I went to test this behavior in preparation for this post, it did not happen. I could focus on things in the same range with no trouble at all.

My theory, when the issue was happening, was that perhaps the lens was out of alignment as well as backfocusing, and couldn't focus far enough "past infinity" to get where it thoguht it needed to be.

Anyone have any ideas about this? Have you had an experience like this? Should I try to return the lens?
Click to expand...
I don't understand the logic of your theory.

One interesting thing about the 21mm and focusing, the centre focus point for near-ish subjects is significantly below where the focus indicator light is.

Perhaps this is relevant to your situation?

--
cheers!

Gunn

-- Get a big lens and get closer™.
 
Not sure; at one point when I was having a focus issue, I was focusing on clouds and trees out a window and across the street. It was very sunny out, so I would not expect it to have any focusing problem in general. I aimed the center of the viewfinder at the clouds, or at the tops of trees. I guess it is possible that the leaves of the main body of the trees were difficult to focus on?

It would spin the lens to infinity or very near it; it was in the ballpark of where I would expect it to focus. It was not doing the "searching" where it spins all the way to one extreme, then the other.

Lee
 
leekil said:
Not sure; at one point when I was having a focus issue, I was focusing on clouds and trees out a window and across the street. It was very sunny out, so I would not expect it to have any focusing problem in general. I aimed the center of the viewfinder at the clouds, or at the tops of trees. I guess it is possible that the leaves of the main body of the trees were difficult to focus on?

It would spin the lens to infinity or very near it; it was in the ballpark of where I would expect it to focus. It was not doing the "searching" where it spins all the way to one extreme, then the other.
Click to expand...
Vague. Go experiment some more. Keep in mind the unusual characteristic I described for this lens (focus points not being where you expect).

--
cheers!

Gunn

-- Get a big lens and get closer™.
 
chris gunn said:
One interesting thing about the 21mm and focusing, the centre focus point for near-ish subjects is significantly below where the focus indicator light is.
Click to expand...
I don't understand this. The lens doesn't know anything about focus points, it just focuses where told to by the AF system. Sounds like a characteristic the body, rather than the lens.

I have certainly experienced slightly misaligned focus points on a couple of Pentax bodies.
--
Mike
http://flickr.com/rc-soar
 
chris gunn said:
One interesting thing about the 21mm and focusing, the centre focus point for near-ish subjects is significantly below where the focus indicator light is.
Click to expand...
That could just be the camera and not the lens. The indicator lights are not necessarily aligned to be on top of where actual AF points are in the frame. I know they aren't on mine.
 
Use central point AF, and make certain that there is something of high contrast for the sensor to focus on.

Your lens won't know any difference between 100 feet distance and infinity. For a wide angle lens, 100 feet distance is essentially infinity.

Wide angle lenses focused on distant subjects often have trouble focusing because there isn't enough contrast. If it does it again, try the focus and recompose technique (using central point AF).

Joe
 
I definitely recall reading on this forum there was a problem with the earlier batch(es) of 21mmLtd with focus, I think later batches of the lens were fine.
--
-Kathy
 
50-100 ft for a 21mm prime is same as infinity.

Try going through all apertures, slowly, test using different shooting modes too.

Try focusing on different subjects, colours, etc to establish a pattern of behaviour.

It's too early to say anything if you don't spend some time with it.

Pretend there's nothing going on (just to minimise user errors), then test as above, and see how you go. Please share your test results.

Ta.
 
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