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01-01-2010, 06:08 PM   #1
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Is my lens busted?

I have a K20D with 3 lenses. Kit 18-55, SMC FA 28-70 and a DA*300mm.
The kit and the 300 are as sharp as I could want but the 28-70 takes nothing but fuzzies. It auto focuses and/or zooms as it should
Shots look almost OK until cropped or otherwise close upped.
I have to assume the thing is damaged.
Bought it used after getting my first DSLR, a K10D.
Didnít have the other 2 lenses till later so was blissfully unaware of what I was missing.
Take 95% of my shots (wildlife) with the 300 and the remainder of other stuff with the kit lens. I am happy all in all but feel I would benefit with the 28-70 as a happy medium.
Or, as I look for a buyer for the K10D, I would offer the thing as part of the deal.
What are the probable causes of the problem and there a remedy short of applying a hammer?
I donít feel it is worth paying to have it repaired.
I know other members have posted questions about the quality of their pictures and have been asked to include a photo and the EXIF data, but I haven't 'cause it ain't the camera right? Or would it help anyway?

01-01-2010, 06:55 PM   #2

still post some shots

It could very well be the camera... I have a k100d k20 and now Kx

My DA 40ltd was sharp as glass on the K100d. I could not get it sharp with the K20 even after lens adjustment at full 10. But it is back to tack sharp on the Kx.

Put the lens on a tripod and use AF to take the shot.

Then mf / live view and magnify the live view image a few zoom stops. Adjust focus to the sharpest you can get it. Then compare the photos. If they both look funky it is probably the lens. If not, your K20 is not AFin' the lens properly.

Last edited by Igilligan; 01-01-2010 at 07:08 PM.
01-01-2010, 07:04 PM   #3
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It would help. Right now, it's impossible to know if it's just a question of the lens not being as sharp as the new ones, or if it is defective, or something else.
01-01-2010, 07:23 PM   #4
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There are two FA 28-70s, an f2.8 model and an f4 model. This is the f4:

Pentax Lens Review and Specification Database - 28-70mm F4 AL

One review mentions the problem suggested by Gus, autofocus doesn't work. It also mentions one other issue with this lens, element separation. Some lenses have elements glued together, and a few of those are prone to glue failure. Here's a photo of what it looks like on a different lens:

If it causes problems with photos, you should be able to see it yourself. If it's an auto-focus issue, it will be more obvious at f4. Something in the photo will probably be in focus, just not what you wanted. You can put the camera on a tripod, point the camera at a subject with high-contrast edges and include something a few feet in front and behind the subject. A constant light source is helpful. Then put the kit lens on the camera and take some photos at 55mm and f5.6, also f11. Then put the FA 28-70 on the camera, set it at 55mm for the same field of view, and take photos at f5.6 and f11. Compare the photos. The lens reviews suggest that the FA should be at least the same, hopefully better.

01-01-2010, 07:30 PM   #5
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Element separation is a well known problem with the FA28-70/4. Most likely, this is the problem with your lens if it is the f/4.

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