Originally posted by disco_owner I can't say that I've experienced this phenomena in really Dim light but then again I may have gotton lucky so far , the issue also seems to get exaggerated when further stopping down beyond F11.The viewfinder cap idea is a really good one. I usually throw on the microfiber lens cloth over the V finder but strong breeze or wind blows it off. Lol.
At times I've been known to hover my hand in front of the viewfinder to prevent light from getting in, lol.
Anyway, the absolute brightness of your shooting environment isn't the major factor; it's how much light is going in through the back relative to the front. So when you stop way down you're restricting the front without restricting the back, and the flare that's always there but invisible due to being overwhelmed by positive signal through the lens suddenly becomes visible. Likewise when I shoot with an ND filter and forget to cover the viewfinder. OR, if you're shooting in the dark but there's a street light behind you flashing in to the viewfinder.
In any event, something we should all be aware of; it has ruined more than a few good shots.