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02-25-2013, 11:52 AM   #1
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Is this a bad copy of the 35mm ltd?

I have had the 35mm LTD for about a year and While it has gotten me many great shot of my 1 year old girl I have been suspecting it for not performing at its best.

So today I tried to compare it to mit kit lens. I compared the kit linse wide open at f4.5 to the ltd at f4. The subject is a news papers on the wall and the camera mounted on tripod at a distance so the news paper filled the FOV. I took extra cared to align the sensor plane and the news paper and used contrast focus in LV. No processing is done to the picture

I have Attached a 1:1 of the extreme upper right corner and lower left corner. The "Candidate" is 35mm ltd and the "select" is the kid at 35mm f4.5

My own observations are: The CA's are much more pronounced on the ltd and the contras is lower.

The opposite observation is done for the lower part of the picture e.g. the lower left corner (also attached)

This lead me to think the focal plane of the LTD is misaligned.

Could the performance of the 35mm ltd be improved by sending it in to a authorized pentax dealer for repair? I bought it from srsmicrosystems (uk)

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02-25-2013, 12:09 PM   #2
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Hm, a couple things. First, the kit lens performs best at 35mm. But the ltd should still be much sharper and definitely not worse.
How did you do that test? Did you use a tripod and M mode, with live view AF and 2 sec timer? It seems like the 35mm ltd test shot is a little out of focus.
But definitely, you should send it back in if you still have warranty, just tell them you only noticed it now. If you say you knew all along, then they may try to refuse warranty because you didn't complain soon enough.
02-25-2013, 12:21 PM   #3
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It was 2 sec timer on a sturdy tripod in in 'Aperture priority' mode.
02-25-2013, 03:47 PM   #4
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There looks to be something strange with both lenses. All of the letters on the page have been jumbled up. I can't even read them. ;-)
I wouldn't trust the AF to be spot on every time, try in liveview with MF where you can magnify a small portion of the frame in the LCD to ensure the focus is accurate.

02-26-2013, 07:16 PM   #5
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I've made another post about the general topic of lens performance expectations. From the posted images it seems like you have an annoyingly marginal case, as I've encountered a few times. Possibly bad enough to annoy you, but possibly not bad enough for it to be fixable. I have some lenses that exhibit discrepencies similar to what you're seeing, between edges/corners of the same lens. And when I compare it to another same-length lens with a sub-par reputation, in at least some frame areas the older, "worse" lens outperforms it.

I'd suggest you make more tests using different subjects, subject distances, apertures, etc. I've made several hundred test images with just one lens. When I've had truly clunker lenses I've just sent them back after a few test images, but then if you get something close to ok (which this appears to be) after you've tried several copies, it's hard to know what to do. It would be interesting to hear from anyone who's had experience with having any Pentax lens improved by services, where it was at least fair to begin with.

Just based on the photos you posted, I'd probably settle for what I had. It's always possible that a "repair" could make things worse. The difference your seeing is pretty similar to what I'm also experiencing - although I have some decidedly worse comparisons, and others that are similar. That's one reason I'd test more. If all I had was my worse tests, I'd send my lens back. If all I had was the best ones, I'd probably be (almost) happy with the lens. It turns out that it's either very difficult to get consistent results, or... these cameras/lenses are actually giving erratic results.

Paul

Last edited by tibbitts; 02-26-2013 at 07:38 PM.
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