Imagine heading to a photo shoot and noticing afterward that most images you took are partially out of focus. You are sure you set the focus right, and the depth of field in the photos is also sufficient. The problem might be a decentered lens.
Half of the lenses I bought in the past 15 years were decentered. At first, I didn't know of that problem. The first wide angle lens I got from Canon required focus-stacking all the time. Since I mostly shot landscapes, it wasn't clear to me that this was due to a problem with the lens and not because of the topography of the landscape or the way I composed my photos. Even if I had no element near the lens, I couldn't get everything acceptably sharp. The problem got worse with time.
Interestingly, the out-of-focus areas appeared mostly on one side of my photos. Getting everything into focus in a single image was nearly impossible. So, I kept stacking my images when what I should have done was get the lens fixed.
The problem of decentered lenses is not a problem of cheap lenses, either. When I bought the Canon RF 15-35mm f/2.8 a few years ago, I had to return it twice until I got a perfect copy.
If you're new to photography, you might not have the confidence to return new equipment that doesn't work correctly, thinking it might be something you're doing wrong. I was there myself. But I'll show you now how to test different lenses and build the confidence to return a lens if it's not working flawlessly. When you buy new gear, you typically have a two-week cancellation period you should use if you are not 100% convinced by your purchase.
How To Test Wide and Normal Lenses
In the feature video, I show how I tested the first copy of the Canon RF 15-35mm f/2.8 lens I bought. For lenses up to 100mm focal length, you can perform an indoor test using a target. For longer focal lengths, this can become impractical unless you live in a big apartment.
For a long time, I used a detailed map of the world mounted on the wall of my office. A shelf with some books is also a perfect subject for such a test.
With such a flat subject, you must position the camera so the focal plane perfectly aligns with the target. Otherwise, you can run into depth-of-field issues. To get perfect alignment, a subject containing straight lines is beneficial. Those help you spot perspective distortions, which indicate your camera is not set up appropriately. Use a tripod to allow fine-tuning.
Once you've finished the setup, go through the different focal lengths of your lens and take test shots wide open and stopped down. Don't just check the extreme focal lengths because decentering is not exclusive to those.
After you've taken the test photos, assess them in your editing software. Start with the corners, but don't stop there. My second Canon RF 15-35mm f/2.8 copy looked perfect for the corners, but it turned out to be soft in the area to the right of the center.
While the indoor test can help you spot imperfections in a lens, you should also photograph a 3D subject. Head out into the landscape or find some architecture to photograph. It doesn't have to be a photogenic scene. Just make sure you have enough details filling the frame. You should also avoid foreground elements too close to the lens. The goal is to find a scene you can capture with acceptable sharpness when using the widest aperture of your lens. This hyperfocal distance table can help you with that. Sometimes, it can be as simple as taking photos out of the window of your apartment.
How To Test Telephoto Lenses
The longer the lens, the harder it is to find an adequate test subject. At 500mm, for example, an indoor test will be difficult unless you live in a mansion. But even outdoors, it can be hard to find a flat and detailed subject.
There is a simple solution, though, which I show in the video above. When I bought the Canon RF 100-500 f/4.5-7.1 lens, I mounted a target on a tripod, which I positioned 10 meters from my camera.
Instead of just taking a photo wide open and stopped down for different focal lengths, I took five photos each time. I put my target into the center and the corners of the frame.
Good examples of targets are church towers and other prominent, free-standing buildings. Whatever subject you choose, make sure it's far away from your camera. If it's too close, depth of field can become a problem and cause the corner images to look softer than the center image or vice versa.
Conclusion
I recommend using the first two weeks after your purchase to do some of your typical photography in addition to the tests. Find scenes you'd photograph on a photo tour and use the lens how you'd do in the field. When you return home, do pixel-peeping. If you find the lens doesn't perform as expected after the first round of tests, repeat them. After the two-week cancellation period, you should be confident to use the lens on your next photo trip and not worry about soft photos. And if you have doubts, get a replacement right away.
It's also possible that an initially perfect copy gets decentered over time. If you begin noticing it, do another test and get the lens fixed. A few years ago, I paid around $200 to get my Canon EF 16-35mm f/4 lens fixed by Canon service. After I got it back, I repeated the test and confirmed the improvement. To compare the performance of the lens before and after the repair, make sure your test is repeatable. An indoor test is perfect for that.
Well then explain to us the proper way to do it. Never put something down unless you are willing to articulate a viable alternative.