Have you ever used a de-centered lens? Perhaps you’re using one now without being aware of it. It can be a real pain to work with. Focal planes and depth of field behave strangely which makes it hard to get predictable sharpness in a photo.
I’ve had such a lens in the past but I found ways to work around it, the most helpful being focus stacking. With time I also learned where to focus to get the whole image acceptably sharp.
With a proper lens, I’d use hyperfocal distance for this but with a de-centered lens, this usually doesn’t work. With some testing I was able to find another spot to focus on and get similar results. However, most of the time one of the corners needed some extra attention, which is where focus stacking came in handy.
However, that’s a lot of additional work and, for me, not being confident in my equipment resulted in a lot of redundant photos I took just to be on the safe side; to be able to have at least one photo that exhibited enough sharpness for each area of the image.
Testing new lenses
Lenses can become de-centered over time, especially if they’re thrown around a lot in the camera bag during travels. Even new lenses, however, may already show this problem. This doesn’t only count for cheap lenses; premium lenses can have this problem too.
So after picked up the Canon RF 15-35 f/2.8 L lens, which costs more than 2000 Euro, I made sure to test it right away. I honestly didn’t expect to find a problem with it but sure enough, after extensive indoor and outdoor tests, I found a one.
The image below shows a test photo, in which the lower part of the image is roughly the same distance from the lens at around 6 meters. Focusing in the middle of the frame at f/2.8, the right corner is, as expected, less sharp than the center. But it shows acceptable detail, with which I could work in post-processing. The left corner on the other hand is completely blurry.
As you can see in the video below, this problem was even visible at f/5.6. At that aperture I should get everything in the frame tack sharp, which is the case except for the the lower left corner.
So I returned the lens and will soon pick up another one, with which I will repeat the test. I’m sure eventually I will find the perfect copy, because other than the problem with that one corner, this lens is great.
It’s very sharp throughout the complete focal range even at open apertures. And I can use my Kase K9 Kit with two Slim Filters when shooting at 15mm without a vignette. With this lens, there’s no need for large and heavy filters when going wide.
In the attached video I go more into the details of how I tested and I also show more test photos. Hopefully this will help you during your next lens purchase and save you some trouble later on.