What will cause a washed out spot in the centre of an image?
I thought it was light leaking in through the viewfinder, but these were taken with it completely covered.
It only happens sometimes, and
I've only noticed it when I'm trying to get some macro/extreme macro shots. I sometimes use extension tubes and reverse lenses.
For a while, I thought it was my cheap extension tubes. But I bought some expensive ones of much better quality. Every now and then it still happens.
Here are some examples; shot with a reversed Canon EF-S 24mm f/2.8 STM, on tubes (I know they're not great photos, but they illustrate the problem):
I've noticed it in regular wide angle shots too. Here's an example of a photo sequence in which the aberration occurs; taken with a Canon EF 50mm f/1.8 STM: