Hotspots on IR images are central overexposed areas. They seem to occur more on unconverted cameras than converted ones and only with some lenses.

I’ve been working on a theory for some time that at least part of this issue is caused by light leaking through the viewfinder during the usually long exposures with unconverted cameras. I have found this when testing some cameras myself and it has been confirmed now by several other people that ensuring the viewfinder is covered solved the problem.

While I don’t believe it is the complete answer, I do believe it is a part answer, at least for some people.

I’ll be doing more testing on this and will report the results in full shortly.