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04-27-2019, 03:29 AM   #1
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Decentered DFA*50?

These are 100% crops and F1.4 aperture. First is left edge and second one is upside down left edge. Softness can been seen from 1/4 of the left side when watching original resolution photo. I'm thinking to send it to the warranty, but they have strict rules that if they think it is fine, I have to pay checking costs and shipment. Sounds a bit ridiculous considering the price of the lens.






04-27-2019, 04:20 AM   #2
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Trees are a very bad choice for target to test anything related to optics. There is tutorial how to perform basic decentering test in articles section of this site.
04-27-2019, 04:45 AM   #3
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Here is the tutorial, good luck.

How to Check Your Lens for Decentering - Articles and Tips | PentaxForums.com
04-27-2019, 06:34 AM   #4
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I seriously think you're splitting hairs. What does the whole image look like?

04-27-2019, 06:59 AM   #5
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It is just a test image and it was calm weather. Shutter speed was like 1/5000s so no motion blur.

Here is another example. Sorry, not something you asked, but I started notice these in old photos. This is pixel shifted and F5.6, so I think the difference is obvious, but I don't know warranty tolerances. When infinity focus is sharp, it seems to only happen then. Closer focus not so obvious.

Left is right middle edge and right is left middle edge. 100% crop.

04-27-2019, 07:20 AM   #6
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Yes your lens looks decentered, but it is not a severe case and so I doubt that they will consider it a defect. You cannot rely on a manufacturer's warranty to address decentering issues. If you want to avoid the problem then you need to test each lens you buy while it is still within the return window, and then return any lens that is not up to your expectations. Of course this also means that you need to restrict your purchases to sellers that have a good return policy. When I see a seller adopt a "no returns" policy I run the other way.

As for the price of the lens, it doesn't matter how much a lens costs, some copies will suffer from decentering. I used to shoot Nikon gear, and even very expensive lenses had about a 50-percent failure rate in my decentering tests (a bit worse for zooms, a bit better for primes). In my experience Pentax is no better, with the exception of the "DA limited" lenses which seem to have a lower rate of decentering. This may be due to their relatively simple design, or maybe my sample size simply isn't large enough. In any case, you need to test a lens thoroughly when you first receive it if you want to avoid unpleasant surprises down the road. It doesn't pay for manufacturers to sell only well-centered lenses. It would be very costly, and most buyers don't seem to care.

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04-27-2019, 08:41 AM   #7
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QuoteOriginally posted by Dan Quote
Yes your lens looks decentered, but it is not a severe case and so I doubt that they will consider it a defect. You cannot rely on a manufacturer's warranty to address decentering issues. If you want to avoid the problem then you need to test each lens you buy while it is still within the return window, and then return any lens that is not up to your expectations. Of course this also means that you need to restrict your purchases to sellers that have a good return policy. When I see a seller adopt a "no returns" policy I run the other way.

As for the price of the lens, it doesn't matter how much a lens costs, some copies will suffer from decentering. I used to shoot Nikon gear, and even very expensive lenses had about a 50-percent failure rate in my decentering tests (a bit worse for zooms, a bit better for primes). In my experience Pentax is no better, with the exception of the "DA limited" lenses which seem to have a lower rate of decentering. This may be due to their relatively simple design, or maybe my sample size simply isn't large enough. In any case, you need to test a lens thoroughly when you first receive it if you want to avoid unpleasant surprises down the road. It doesn't pay for manufacturers to sell only well-centered lenses. It would be very costly, and most buyers don't seem to care.

Dan
And most repair facilities don't have the ability to repair decentered lenses. LensRentals' Roger does trial and error fixes with shims. Those of us who shudder at the prospect of taking our lenses apart are pretty much out of luck.
04-27-2019, 11:42 AM   #8
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I would say also run better tests. Use well-lit scenes with a hard, well-outlined subject, photographed preferably using a tripod, or at least at high shutter speed, and with low wind. Put that subject at each edge, and in each corner. Double check focus accuracy. Then put it a bit further into the frame in each area.

It might not be that bad after all.

But if it turns out not so good, especially with a lens of that ranking, Pentax might do something for you.

05-01-2019, 10:21 AM   #9
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Show us a brick wall
05-04-2019, 03:12 PM   #10
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From my limited experience with manufacturers, the "edge" is about 1/3rd in from the actual edge, and if both sides in your photos look sort of like a tree, it's a winner.
05-09-2019, 04:49 AM   #11
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Here is nine shots at F4.0 and 100% crops, focused to infinity, no refocusing between shots and same exposure settings. Upper left photo is upper left corner, etc.

Forum does not show full size. You need to view the image in a tab and zoom to 100%. Resolution is 1458x1458


Strange thing is that upper middle is sharper than bottom middle.Looks like this lens has some flaws on left middle corner, left bottom corner, bottom middle and right bottom corner Those problem areas have more CA too. No post-processing corrections.

05-09-2019, 05:48 AM   #12
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IMO, the results show an excellent lens, well within reason.
05-10-2019, 12:49 PM   #13
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QuoteOriginally posted by rogerstg Quote
IMO, the results show an excellent lens, well within reason.
I agree.

I recommend shooting any single hard-edged subject, like a statue, or a lamppost, flagpole, flower pot, birdbath, knob at a corner fence pole etc. putting the object at each of the 4 edges, and then each of 4 corners. Get good focus on the object. Do more than one run of the test and use the half press to double-check your AF. I use center-only spot AF, use the half press to hold focus, then reposition the camera to frame my shot.

You can also do a round the test with the object somewhat more into the frame.
05-14-2019, 02:37 AM   #14
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Very excellent indeed. Here is at F2.8 and 1/1600s. Somewhat cloudy weather. The thing is that top middle edge is so sharp and almost same as center, but all other edges are very soft in comparison. Top right edge and middle right edge are second best and the rest are just mess. I'm sending this sample image to Pentax service and see what they say. It would be absurd to think that this is a good lens.

Please, open the image to separate tab if you cant view it at 1458x1458. Same thing can be seen with F4.0 I posted before, but it has less softness.

05-16-2019, 12:30 PM   #15
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These are dark, dull photos and not easy to make a judgment based on this one shot. The shot contains numerous elements at varying distances. A much better test is for a well-defined single object in good lighting, well-exposed, and well-focused. It is not difficult to reposition the camera to put the same object as described in various parts of the frame.

In most cases, the center will be always somewhat more sharp than the edges, but the edges should be close to each other in quality.
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