yes, it is probably caused by the foam below the focussing screen
starting to deteriorate.
It's not like it has deteriorated, it's more like there is NO foam, except small traces that suggest that there had been some foam. When the mirror is up, there is a small gap between the mirror and the upper surface. So I guess this gap should have been filled with foam.
> It shouldn't cost too much to get it replaced.
Thanks for the replies, I won't worry anymore. :)
I'll take it to a camera repairman tomorrow and see what he tells me. Otherwise the camera seems to function just fine and will make a great second body for me.
Best regards, Girts

