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07-04-2010, 06:01 AM   #1
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Join Date: Jun 2010
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question about testing lens quality

Hi everyone,
I am new to SLR phtography - got my Pentax k-x ~1 month ago. After some research online, I decided that the Tamron 28-75 f/2.8 would best suit my needs at this point (obviously a big debate by itself, lets say I think it would be best to have some zoom while I try to take pics of my kids mainly indoors). I have read about some QC issues with that lens, so I avoided buying a used one - bought from amazon, return options and 6 year warranty. I just got it yesterday. How should I check whether it has decentering issues etc? I found some info online but not much. I dont have a tripod or test charts. I took some pictures at the yard, stucco wall with some flowers etc, which I think is a fairly decent amateur setup. I even took some upside-down shots and tested various apertures. My impression is that there is some edge softness at 2.8, less at 4 and nothing I can appreciate above that. Is that a normal result (I think it should be, given the focusing effects at wide apertures)? I'd appreciate any feedback - and be happy to post pics if I knew how and if anyone cared to see them...
(an unrelated Q, if anybody is using Lightroom, is there a way to delete the pics from the disk after importing? Windows could do this, now LR has taken over and I cant figure it out - other than dleting them or formatting the card)
Thanks
Thomas

07-04-2010, 07:03 AM   #2
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Morning and welcome to the Forums!

I do not have the lens, however several reviews have the lens soft at the edges and in particular in the corners. So if you jump to the second page of this review, you will see a see a section titled "MTF (resolution)" with a number of histograms (charts). What these charts tell you is that the lens center is very good, and you can see at the various focal lengths, as the lens is stopped down from f2.8 up through f8 that resolution improves both in the center and the edges and corners. So for each of the focal lengths you want to find the f stop (aperture) where the center has a reasonably high value, along with both the edges and corners. You might have to give up some sharpness in the center in order to pull up the edges and corners. So it looks like at 28mm its f8, at 40mm its f8 and at 75mm it f8 also. Take a look at Part II Performance section of this review - down at the bottom it walks you through the lens and how it performs.Most lenses are not at their peak performance at the extremes, so stopping down is very normal.

Also, I don't use Lightroom (yet, I am thinking about changing), but have you selected the image you want to delete and hit the [delete] key?

07-04-2010, 10:21 AM   #3
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Join Date: Apr 2007
Location: Oklahoma USA
Posts: 1,502
I'm not an expert but I think decentering will result in the image not having uniform sharpness side-to-side and top-to-bottom. So, flip the camera upside down, or just move the camera so the same object appears in different sides/corners of the image.

You might not find all the sides/corners perfectly identical, but they should be close. You should not necessarily expect to find the corners/sides to equal the center at any aperture, but especially at larger openings.

It is important to test, especially so you'll know what to expect from the lens, and how you'll want to use it. For example, wide open - f4 for most of my lenses, might be fine for portraits, where the corners are generally not important, but you might want to alway stop down to f8 or more for a scenic vista.

Paul
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