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04-23-2019, 02:37 PM - 2 Likes   #1
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De-centering test on new DA*11-18 f2.8 lens.

I recently purchased the new DA*11-18 lens and decided that I would test for any de-centering. I have never done this before so with the help of the internet I found a process for performing the test.The following is the process I used, the camera and lens settings, and the results.

Settings
Camera - K3 mounted on tripod
Exposure - Manual (Spot metered on the center of target on first (center) exposure)
Focus - Manual
Shake reduction - Off
Aperture - 2.8
Focal Length - 11mm and 18mm
Shutter speed - As need for correct exposure
ISO - 100

Process
Set the focal length to 11 mm. Positioned camera so the target (building about 120 yards away with sign"FISH CAMP") was in the center of the view finder. Focused on the "FISH CAMP" sign and adjusted shutter speed for proper exposure. Turned on the Focus Clamp (neat option on this lens) to ensure focus did not change between exposures. Took the first exposure using a remote shutter release in the mirror lock up mode. Loosened the tripod ball head and moved the camera so the target was in the upper left of the viewfinder. Tightened the ball head and made the second exposure. Repeated this for the other three corners. Changed focal length to 18 mm and repeated the whole process.

The attached images are 100% crops of the images. The top is the 11mm set and the bottom the 18 mm set. These are straight out of the camera images with no adjustments made. The only thing I did was crop them and make a collage in photo shop of the two different focal lengths

My observations - Upper left is the softest wth the bottom right the sharpest (There was a significant wind so don't look at the trees). Appears to have a little more vignetting on the top corners as compared to the bottom corners (top corners are darker than bottom corners). There is a little chromatic aberration in all four corners but could easily be corrected. CA on the 12-24 is much worse.

Since I have never done a de-centering test before was this the correct process and if so is this an acceptable lens regarding focus de-centering? I believe it is but for the price of the lens I just wanted to get a second opinion. Even though you want it to be there is probably no perfect lens.

Attached Images
   
04-23-2019, 04:35 PM - 1 Like   #2
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Very interesting. I must say, given these are 100% crops, I'd absolutely consider that a keeper in no uncertain terms. Is it perfect? Not quite... but not far off. For a zoom lens at these focal lengths, I'd say it's very good indeed
04-23-2019, 06:29 PM   #3
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I would keep the lens, and think your test was very good. I guess you could do a completely different test to make sure you get the same results, maybe at a significantly different distance.

Having said that I'm not sure why we have to settle for any visible differences between any edges or corners. I'm guessing on a 6mp sensor you wouldn't have seen any differences in your test, but I'm not sure lens manufacturing has kept up with sensor technology. Now that sensor technology has taken a bit of a breather in resolution terms (well, maybe not counting pixel shift), it seems like lens design and manufacturing need to catch up.
04-23-2019, 07:49 PM   #4
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Given its f2.8, I would be very happy with those results.

I tested a copy of a Samyang 14mm f2.8 at f8 and it was far worse than that (it went back as yet again I got a bad copy and given up on them).

04-23-2019, 08:08 PM   #5
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I'm amazed by the absence of any purple fringing.
04-24-2019, 02:35 AM   #6
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QuoteOriginally posted by jim.haase Quote
Since I have never done a de-centering test before was this the correct process and if so is this an acceptable lens regarding focus de-centering? I believe it is but for the price of the lens I just wanted to get a second opinion. Even though you want it to be there is probably no perfect lens.
The approach is fine and in sync what I described here: How to Check Your Lens for Decentering - Articles and Tips | PentaxForums.com

To me it seems to be an absolutely excellent copy, considering you have a 11mm test (=16mm FF) at F2.8 wide open on 24 MPx APSC (= 54 MPx FF) and these are 100% crops.
I actually would consider your results as great reference for anybody else.

Look at the sample shots from my decentered Sigma Art lens in above linked article and you see what real decentering means. Yours is nothing like it.

Congratulations to a great lens.
04-24-2019, 12:17 PM   #7
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Looks like it passed with flying colors to me. Congrats for owning such a fine lens!
04-24-2019, 02:02 PM   #8
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Looks very good to me.

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