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05-18-2008, 06:21 PM   #1
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req advice - send 16-50 for repair? - warning, large images within (9 img, ~1Mb each)

I wanted to do one more test, so on Friday, I made up my own "test chart" to check the focus on
my DA* 16-50/2.8 lens. I used Excel to create the chart. Each box is one quarter inch square
and printed with a laserjet on white bond stock. I taped the chart to a window on the house in
the backyard. Sun was from the right, but test area was in complete, open shadow.

I shot RAW+ at 16mm, 29mm, and 50mm at apertures f/2.8, f/4.0, f/5.6, f/8.0, and f/11. 29mm is
pretty close to the mid-zoom range for this lens. Only the f/2.8, f/5.6, and f/11 images are posted
here. The images are the full resolution images. All shot at ISO 100 in Av mode, AWB.

I mounted the K20D on a tripod and set the exposure mode to infrared wireless remote with a
three second delay. I reset the aperture, pressed the wireless remote, and moved about 10 feet
(3 meters) away from the camera so that I wouldn't influence the exposure or amount of light
falling onto the chart. At 16mm, the front element of the lens was pretty close to the chart. Pretty
close to the minimum focus distance. Focus mode was AF-C. Without the hood, because of
being in shade and the closeness of shooting at 16mm.

I loaded the PEF files into Adobe Photoshop CS3 which loaded the images into Adobe Camera
Raw. Only thing I did to the images was to change the white balance to "shade" from the AWB
that I normally use. Within Photoshop, I applied an automatic contrast stretch
(image/adjustment/auto levels), changed the bit depth to 8-bits (image/mode/8 Bits per
Channel), then saved as JPEG quality "low", or quality level "3". I did this to keep the images
sizes near or below 1Mb, which is the limit for imageshack. EXIF data should still be attached.

Could those of you who have the 16-50 take a look at the images and tell me if in your opinion,
you think this lens is acceptable and I should just keep it and be happy, or is it a candidate to
send in for "repair". Especially notice the right hand side of the images, and the apparent
vignetting at f/11. Wouldn't think that would be a problem at 50mm. It is to be expected at
16mm, but not the longer focal lengths. Too much time has passed to exchange the lens
at the shot, so I need to send it to Pentax while it is still under warranty.

Thank you for any advice you can offer. Right now, I'm not happy with the performance of this
lens. Is it a lemon, or is this the best I can expect from any copy of this lens? (This quiery
cross posted to dpr because not everybody here visits there also.)

16mm, f/2.8, 1/1250s

16mm, f/5.6, 1/400s

16mm, f/11, 1/100s

29mm, f/2.8, 1/1600s

29mm, f/5.6, 1/500s

29mm, f/11, 1/125s

50mm, f/2.8, 1/1600s

50mm, f/5.6, 1/500s

50mm, f/11, 1.125s

Last edited by volosong; 05-18-2008 at 07:00 PM. Reason: grammar typo in title
07-09-2008, 10:16 AM   #2
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Original Poster

no comments? Is this a trash copy, or acceptable?
07-09-2008, 11:36 AM   #3
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I understand why post them at full resolution but it is really HARD to visualize whats going on.. so maybe thats why you are not getting much responses...

i know it is a PIA, but i'd try to resize them to the biggest size you could fit in the forums for a 19" monitor.. so people can see the images without scrolling down and up and forgeting which image is which etc etc....
07-09-2008, 01:09 PM   #4
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I hate to say it, but it's next to impossible to really judge those images, they're
just too large and cumbersome.

I always suggest doing some real-world tests - go out in daylight and take some
flora/people/landscape/architecture shots. In the evening, do some low light
tests, while keeping in mind that *any lens is going to have at least some
problems focusing in low light, especially low tungsten light.

Then, look at the results. If you didn't underexpose, the daylight shots should be
terrific. Same with the low-light shots, although your keeper ratio is going to be
lower there, so keep that in mind. Try to determine if any 'bad' shots are the
result of the situation/photgrapher vs. the lens.

And when the I-expect-them-to-be-bad shots are weeded out, look at what's
left - whats left should all be really great, sharp, pleasing to the eye, a few
should even floor you. That's how you know you've got a good lens.

Often these grid/brick wall tests are counterproductive. Real-world shots!

Here are some links to shots from a 'good' lens - would my lens pass
your grid test? I don't know, and I don't care, these images make the lens
worth every penny to me --->

16-50 #1

16-50 in the West



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