Traveling with photography gear can be tricky. With a camera (or two), a few lenses, a flash, filters, batteries, memory cards, and a handful of other peripherals, you end up walking around with a bag full of fragile items that are at an increased risk of getting bumped or dropped. Sadly, even minor bumps and drops can negatively affect the performance of lenses and create all kinds of problems which includes a very common problem: the decentering of lens elements.
A decentered lens occurs when one or more optical lens elements move or tilt away from the principal axis of the lens. This kind of movement can cause a shift in light rays which reduces sharpness and blurs parts of the image. If your lens is severely decentered, it could blur the entire image.
So, what do you do if you suspect you have a decentered lens? The easiest, and most cost effective way of testing your lens for decentering is to shoot very fine detail at infinity by following these steps:
Step 1: Make sure that both the front and the rear lens elements are clean.
Step 2: Find some trees or anything with very fine detail located relatively far away from you so you can focus at infinity.
Step 3: Frame your camera so that the details cover the entire frame or at least cover from bottom left to bottom right of the frame.
Step 4: Set your lens to maximum aperture (around f/1.4).
Step 5: Focus on the center of the area that contains the fine detail using live view.
Step 6: Take a picture.
Step 7: If the fine detail does not cover the entire frame (or the bottom two corners at a minimum), then reframe your shot and take another picture.
Step 8: Analyze images at 100% on your computer. Focused at infinity, the area that you focused on is going to be sharp. Now look at the fine details from the left to the right side of the frame. The extreme corners should be very similar on both sides (either similarly sharp or similarly blurry). If one side is sharp and the other is noticeably blurry, you have a decentered lens.
If you don’t have a decentered lens, commence celebration by gently placing your lens back in its protective case.
If you do have a decentered lens, it’s not the end of the world--but whatever you do, don’t try to open the lens and fix it on your own, as it may void the warranty and you could actually make it much worse. If the decentering issue is minor (you can only see a slight blur), you may not want or need to worry about it. But if the decentering is severe (you see a heavy blur), you may want to repeat the test with apertures you’re more likely to use. If the blur is still noticeable with different apertures, you probably want to return the lens (if possible) or get it serviced by an accredited professional.