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12-11-2013, 02:00 PM   #1
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20-40 vs 17-50

I got my 20-40 LTD yesterday. I did some quick optical tests today with my K5IIs using the 20-40 and the Tamron 17-50. The test pics were images of my shed from about ten feet away, tripod mounted, taken at f4.0 and f5.6.

Tamron: 16.5 ounces with caps
Pentax: 11.25 ounces with caps

-Pentax is slightly sharper in the center.
-Pentax is much sharper across the field, edges and corners much better than the Tamron, at f4 and f5.6.
-My Tamron is noticeably softer on the right side. I hadn’t noticed this before, so a better copy of the 17-50 may provide different results.
-Pentax has more CA wide open (more at 20mm than 40mm), decreases at f5.6.
-Pentax has noticeably more distortion at 20mm.
-Focusing is obviously quieter for the Pentax, but the speed improvement didn't seem too noticeable... but I wasn't testing for that. I'll play with focusing later, as I was just doing a quick optical test today.

I’ll try to post some test pics when I’m done playing. I noticed the exposure was longer for the Pentax test shot, so I may reshoot. Could have been the sun peeking through the clouds for a few seconds. Test shots were taken using a tripod. I'll also try to test flare-resistance later.

-Dan K

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Last edited by wildboar; 12-11-2013 at 03:24 PM.
12-11-2013, 02:09 PM   #2
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Good to know the 20-40 is much better than the Tamron at twice the price. We would hope so anyway

I would expect the contrast to be better as well with the Pentax. Have you seen the elusive "pixie dust" with this lens yet?
12-11-2013, 02:09 PM   #3
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And the Tamron is under $300......
Amazon.com: Tamron 17-50mm SP AF F/2.8 XR DiII LD Aspherical (IF) Lens for PENTAX Digital: Camera & Photo
12-11-2013, 02:19 PM   #4
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Corto-PA... from reputable dealers in the US it is still ~$500. I paid $900 for the Pentax. The increase in sharpness across the frame, the weather-sealing, and the weight difference are worth it to me. I'll probably be keeping the Pentax.

Driline... No pixie dust yet, but this is the first day I've used it. My purpose for this lens is hiking/backpacking/travel/etc, mostly landscapes/cityscapes at smaller apertures.

Also, I plan on pairing this with my 15mm and 70mm as travel kit.

12-11-2013, 02:32 PM   #5
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It seems the hood is extended (or the lens shaft is?) on the 20-40 on your pic. I might be wrong, I was under the impression that it was smaller.

Thanks for a nice review, it's an interesting comparison choice, very relevant I would think.
12-11-2013, 02:32 PM   #6
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Amazon remembered that I bought the 17-50 from them years ago, and "twice the price" was accurate at the time.

Much as I like the Tammy, the tendency for the hood mount to loosen and rattle also increases the "curb appeal" of the Limited. It seems to be a common problem with the 17-50, and isn't hard to fix, but it's there.
12-11-2013, 02:32 PM   #7
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I expect there should be some rendering differences ("pixie dust") once you get out and shoot some different scenes. Probaby bokeh, colours.. will at least be different, even if "better" is subjective.

I am a little saddened that the 20-40mm has lot of distortion at 20mm. And that it has some CA.

QuoteOriginally posted by bdery Quote
It seems the hood is extended (or the lens shaft is?) on the 20-40 on your pic. I might be wrong, I was under the impression that it was smaller.
I think the DA 20-40mm ltd is also focused at infinity (biggest extension) and the Tamron is focused near (smallest extension).
12-11-2013, 02:38 PM   #8
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QuoteOriginally posted by Na Horuk Quote
I expect there should be some rendering differences ("pixie dust") once you get out and shoot some different scenes. Probaby bokeh, colours.. will at least be different, even if "better" is subjective.

I am a little saddened that the 20-40mm has lot of distortion at 20mm. And that it has some CA.



I think the DA 20-40mm ltd is also focused at infinity (biggest extension) and the Tamron is focused near (smallest extension).
The Tamron is internal focusing, no shift in barrel length. The Pentax is longer at 20, shorter at 35, then very slightly longer again at 40.

12-11-2013, 02:42 PM   #9
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One could hope that the distortion on the DA 20-40 is very easily corrected like it is on some of the other lenses (like the DA 18-135 I think).
12-11-2013, 03:02 PM   #10
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QuoteOriginally posted by wildboar Quote
-Pentax has noticeably more field curvature at 20mm.
Do you mean linear distortion or curvature of the plane of focus?
12-11-2013, 03:10 PM   #11
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The Pentax is slower, has less range, and is considerably more expensive. It should be sharper. Still, sharper is sharper. It would be interesting to see these two compared to the Sigma 18-35/1.8.
12-11-2013, 03:21 PM - 1 Like   #12
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civiletti... For my use, the 18-35 is not an option due to size and weight. With the 15/20-40/70 combo, I have have a much greater focal-length range of high-quality glass that still weighs less than the 18-35. I'm curious about how the 18-35 compares for CA and distortion though.
12-11-2013, 03:23 PM   #13
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QuoteOriginally posted by civiletti Quote
Do you mean linear distortion or curvature of the plane of focus?
Actually, I guess I am referring to linear distortion. Thanks for pointing that out. I changed that in the original post.
12-11-2013, 03:24 PM   #14
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The Tamron is so affordable right now, It would make sense for people to own both lenses.
12-11-2013, 04:02 PM - 1 Like   #15
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QuoteOriginally posted by Na Horuk Quote
I think the DA 20-40mm ltd is also focused at infinity (biggest extension) and the Tamron is focused near (smallest extension).
Actually, when focused at infinity, the 20-40 is at it's least extension. The zoom extension is least at about 30-33mm. Most extended at 20.
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