When I close my eyes, I often see what appears to be a donut-shaped afterimage, as if I’d been looking at a similar-shaped bright object. But I had not been looking at anything so shaped. Could this in fact be related to my eye’s iris? How else could this be explained?
They’re called phosphenes, and I see them pretty often, usually in the form of several concentric circles in various colors. I don’t think the shape is directly related to the iris; they are somehow produced by the retina, often after applying gentle pressure to the eyes (as when you have been leaning your forehead against your arm).
Thanks, Ignotus. 
Glad to help!
I get the exact same thing, often associated with sinus headaches (presumably, either because of sinus pressure on the retina or because when I get such headaches I tend to rub my eyes).