Copy/copy variation - 'SOP' how to test lenses after purchase?

Started Feb 14, 2020 | Discussions thread
Rightsaidfred
OP Rightsaidfred Senior Member • Posts: 1,083
Copy/copy variation - collection of 'SOPs' - Quick decentering test
3

I would like to thank Foxjet (see here) and add a quick decentering test to the collection in this thread that I've so far always called the 'tree test'.

Fred Miranda described the principle very well and also suggested a good way how to perform the test. However, the test can still be done without turning the camera upside down. Here's how I'd recommend to take the images:

  1. Choose a small subject that fits to the corner of an image with some reasonably fine structure in it in such a distance that alignment plays no role (let's say 'dozens of meters' away). Can be a tree (but only with short ss), doesn't have to be a tree, e.g., a church clock, you will find something.
  2. Select the lowest ISO
  3. Select a reasonable aperture (I recommend f/4 or f/5.6 w/ APS-C to avoid loss of resolution due to diffraction and because lens failure is more apparent wide open, but there can also be a reason to stop down and have a better corner sharpness, e.g., f/8 or f/11). Not really a need to do the test at many different apertures. One should actually be enough. If the lens is decentered, it is decentered at all apertures.
  4. Turn OIS off
  5. Focus (AF or MF) at the subject in the center. No need to take a picture! Now make sure to keep the focus (e.g., switch the camera to MF). If necessary for reason of greater corner sharpness, you can alternatively focus to one of the corners. Either way - it is key not to change the focus between the four images.
  6. Keep ss constant throughout the following four images. I always set ss manually at this point.
  7. Now pan the camera in a way that your small subject appears in each corner and take a photo each. Depending on the fl and ss, do it handheld or use a tripod, and use ES instead of MS. You may follow Fred's precedure (turn camera upside down for two of the images) but you don't have to.
  8. Maybe repeat the test with a different subject, particularly if there is a suspicion that the lens is decentered
  9. Test zoom lenses at multiple fl

It's all about comparing the IQ in the four corners. Is it reasonably uniform or not? How large is the difference? Comparing center sharpness with corner sharpness is not the goal here.

Now comes the best. Scrolling down Fred Miranda's thread, I found a link to a tool provided by someone called keepcoding. This tool greatly facilitates the arrangement of the photos. No need to crop manually! Just upload (no worries it'll run in your browser only). Many thanks to you, keepcoding!

Here's an example:

Quick decentering test, arranged w/ keepcoding's browser tool (I made a screenshot). This example was taken with my copy of the Fujinon XF 18-55 mm at 18 mm, f/5.6 on my X-T20. The distance between subject and camera was about 20 m in this case (with a larger fl, I'd choose a greater distance). No significant decentering at this fl in my opinion.

Now comes the most difficult part of the exercise. Should you send the lens back or not? Is it normal or not? Is it to be accepted or not? You cannot expect a lens to be perfect. All the more with zoom lenses, at least not at all fl. Fred's thread (link see above) is instructive on this ('Warning: If you already love your lenses, don't put them through this test. You may get disappointed') and there are many voices with regard to expectation management. Someone in Fred's forum said rightly, the more lenses customers send back, the more we pay in the end for a new lens. On the other hand, if one purchases a lens, it has to be faultless. But again, what means 'faultless', what level of uniform performance in the corners can a customer ask for?

What are your opinions?

Would be interesting if a few of you guys here in the forum could do this quick test and share your results. Maybe you can spend a little time with your gear while you're contributing to slowing down Corona spread. This way we could get a feeling for a 'normal' lens performance.

BR,

Martin

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Fujifilm X-T20 Fujifilm XF 35mm F1.4 R Fujifilm XF 18-55mm F2.8-4 R LM OIS Fujifilm XF 55-200mm F3.5-4.8 R LM OIS Venus Laowa 15mm F4 Macro +5 more
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