I was reading an article in a photo magazine and came across a tip I have seen before and wondered about its importance or usefulness. The recommendation was to close off the viewfinder when making a long exposure. As I considered that tip I remembered that Canon used to include a viewfinder cover which could be attached to the neck strap. In fact, I seem to recall that my F-1 had a switch which would close the viewfinder. But as I considered the necessity of using such an item I wondered if the mirror/body seal was so loose to allow light-leaks.
I rarely, if ever, take exposures of more than a second or two so I wondered if anyone who makes truly long exposures, say thirty seconds plus, has had an issue with light-leaks from the viewfinder?
I also seem to remember, back in the early days of light meters being part of the camera, that the measurement for light value was made in the pentaprism so the recommendation was to not allow any light to enter from the viewfinder side lest the exposure be skewed. Just some random thoughts on this early morning.