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11-10-2010, 10:21 PM   #1
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Da16-50mm vs Tamron 17-50mm .. or is my kx faulty?

I've been unhappy with my DA*16-50 from the start. It seemed to produce smudgy images. About 10% of the time, I'd get a sharp one, and second guess that it was the idiot holding the lens that was the problem... after all, with the bad rap and the fact that the first copy I had was DOA out of the box, perhaps I was prejudiced - and I know I'm not a good photographer, so there's that ..

After a couple quick tests against the kit lens, however, I decided the DA* was crap (or at least no better than the kit for 3x the price and weight) and ordered a Tamron with the intent of doing a 3 way test. If the Tamron was better I could sell the DA*, have sharper pictures, and come out $300 ahead.

I took a "brick wall" series of pictures across 7 focal lengths and two apertures with the three lenses and intended to post the dirt on the DA* when I saw that almost - but not all - the Tamron pictures were out of focus. I had AF-S on, so the picture shouldn't even exist of the camera didn't think the shot was in focus, but here's a center crop f2.8@~24mm

I tried again on a different wall, and this time, I reset the focus before every shot and made sure the AF activated. The results were puzzling. I have only posted a few of the f2.8 samples from the DA* and the Tamron, as the other photos were similarly lacking usefulness.
Here is the wall:

Center crops:
pentax DA* @ 16mm on the top. Tamron @ 17mm on the bottom. Neither impress me.

DA*@24 on top, Tammy@24 on bottom. Well the pentax looks a little better, but what's with the Tamron - isn't it supposed to be ridiculously sharp?

DA*@35 on top again. Well the Tamron crisped up, but now the Pentax sucks..

The lower right corners, just to add to the mystery:
DA*@24 on top again. How the heck is this even close to in focus?

DA*@35 on top

So, if you've managed to stay through all that, what's the verdict? Did I win the POS lottery and have two bad lenses on my hands, or is it time to suspect there's something wrong with my camera?

11-10-2010, 10:57 PM   #2
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This is a bad example of BF. If the lens or camera are under warranty, this should be tuned by the service for free. At least here in Czech republic this is the case...
Also, when you compare with kit lens, do not compare wide open, but stopped down to the kit lens aperture. Apart of that - the kit is I believe tuned in factory for proper focus. As the kit lens is not a great performer it may happen that the camera is tuned to compensate problems of the kit lens. With my K10D that was certainly the case, but in the service I had it alligned with several lenses I have - except the kit lens, that showed very bad FF then.

Last edited by elho_cid; 11-11-2010 at 12:03 AM. Reason: additional info
11-11-2010, 06:30 AM   #3
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Ideally, your wall should fill the entire frame. You really can't compare the corners at all here because they are supposed to be out of focus. As to the center, you should try changing your focus adjustment in the camera.
11-13-2010, 04:53 PM   #4
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Added focus test of the DA*16-50mm
I think it may need service. Will try some other tests for replication.
It may appear that at the wider angles the camera focused on the lower portion of the square (with text) and is backfocused.
However, I verified using the moire/liveview method that the lens is frontfocused at wide angles.

16mm Frontfocus of ~16mm

20mm Frontfocus ~16mm

28mm Focus spot on

35mm Backfocus ~6mm

50mm Focus spot on. Soft overall.

Last edited by kxr4trids; 11-13-2010 at 05:02 PM.
11-13-2010, 05:14 PM   #5
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By not lining it up directly in front of the camera, you are throwing the results off.

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