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I bought a second hand lens yesterday, but I'm not sure if it's decentered or not. When buying it, I did some tests to make sure it was not decentered, but when I repeated the test at home I'm not sure of the results.

The lens in question is a Samyang 16mm f/2.0. The images below were shot at f/4.0, at a distance of about 1.75m. The images are 400x400 pixel crops of a 4000x6000 pixel image.

Comparison

The top right corner is obviously less sharp then the other corners, and the top left and bottom right are a little less sharp than the bottom left. But my question is: is this acceptable? Because i heard a little bit of decentering is quite common, and this is my first experience with testing a lens so i'm not sure.

Edit:

I tested the lens according to Michaels answer. I got these results:

Second comparison

These images are 750x750 crops. The top right corner still looks less sharp than the other corners. I still have the same question, is this acceptable?

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    It looks like you moved the camera to put the object into each of the corners for your test, is that correct? If so, how did you control the distance to the subject, and how did you control the focus distance? – scottbb Oct 30 '18 at 22:56
  • I put the object in one position, and the camera on a tripod. I then moved the orientation of the camera so the object was in each corner of the frame for all of the pictures. So the distance should be constant – sFyX67v7Ia1XDGFU Oct 30 '18 at 23:19
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    Okay, the tripod helps. What about focusing, did it stay constant? Or did autofocus engage? – scottbb Oct 30 '18 at 23:20
  • The lens in question is a manual focus lens, and i didn't touch the focus. I took a picture before and after with the subject in the middle to confirm that the focus was the same in all the pictures. – sFyX67v7Ia1XDGFU Oct 31 '18 at 13:34
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    Thanks for being patient and answering my questions. That information should help others craft better answers to your question. =) – scottbb Oct 31 '18 at 13:50
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There's no way to tell using the methodology you used. An improperly aligned lens is just one of several possible explanations for the differences between each of the images. Even among improperly aligned lenses, decentering is only one of several types of improper lens alignment that could produce such results.

When you move the camera around on the tripod, you change the relative distance from the optical center of the lens to different areas of your scene. This affects how sharp a particular thing will look because if the optical distance changes (because the optical center of the lens moved in an arc created by the position of the tripod's axis of movement) and the lens is not refocused, the point of focus will move in front of or behind your object. This is why "focus and recompose" can introduce focus errors that are noticeable when shooting portraits at very narrow apertures.

The only real way to do what you are trying to do is use a flat test chart that is properly aligned to be parallel to your camera's sensor. Then compare each of the corners of the test chart from the same image or series of images taken without moving the camera or target.

If the chart is not properly aligned with respect to your camera's sensor, then the results could indicate lens tilt, lens decentering, other optical alignment issues inside the lens or it might just mean improper alignment of the test chart or a combination of both - there is no visual difference between them in the results.

The easiest way to align a tripod mounted camera with a test chart is to use a stable, non-distorted mirror such as one solidly mounted to a wall. Align the camera so that the center of the lens in the reflection is exactly in the center of the camera's viewfinder. The marking for the center AF point will help in this respect. Then tape your test chart to the mirror without moving the mirror even slightly. You're now ready to start taking test shots as long as the camera does not move in relation to the test chart. For the purposes of this exercise, pointing the camera in a different direction is moving the camera because you have moved the optical center of the lens closer to part of the chart and further from other parts of the chart.

Keep in mind that most lenses demonstrate some field curvature. The field of focus is not a perfectly flat plane, even with a theoretically perfectly manufactured lens. Lenses that are highly corrected for field curvature, such as many macro lenses, still have a field of focus that looks more like a lasagna noodle than a flat plane. They're not perfectly flat, they're just more flat than uncorrected or less corrected lenses.

A few years back, Roger Cicala, the founder of LensRentals.com, wrote an informative blog entry about How to Test a Lens.

For more about the subtleties of field of focus, I recommend reading Roger Cicala's excellent series about it:
Fun with Field of Focus Part 1
Fun with Field of Focus II: Copy-to-Copy Variation and Lens Testing

Also related: Why You Can’t Optically Test Your Lens with Autofocus

I tested my lens according to your answer and edited my post with the results.

Based on the results of that test, I'd say you are still struggling with alignment issues between the camera and test chart. The test targets all skew in the same direction. That is, the top right is the weakest in all four quadrants and the lower left is the sharpest.

If the test chart were square with the camera one would expect the outer corners of each quadrant to be worse than the inner corners. In other words, the top right would look worse in the top right corner but the lower left would look worse in the lower left corner. It's possible tilt could cause the same thing, but a decentered element probably wouldn't.

As it is, the lower left corner is the sharpest, the top left and lower right corners are about equal and just a tad softer than the loser left. The top right corner is even softer. What would have been helpful at that point is to have refocused the lens until the top right corner was brought into focus as sharply as possible and then see what the other corners (and the center) look like.

  • Thanks for the reply. I tested my lens according to your answer and edited my post with the results. – sFyX67v7Ia1XDGFU Nov 1 '18 at 15:35
  • @sFyX67v7Ia1XDGFU Do you have a full field view of the entire test chart? – Michael C Nov 4 '18 at 3:25

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