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 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 (I know they're not good photos, but they illustrate the problem):