I agree. Take a rest now and then. I have the same problem.It is possible that you may need some eyepiece correction in your viewfinder (some camera models have a system for adjusting this). Talk to a good camera dealer or visit an optician for an eye test. I don't think it should matter whether you keep both eyes open or close your non-dominant eye. If your eye(s) get tired, sit down and rest them for a few minures.
Have you tried using the other eye? I met someone in the passed that couldn't close his left eye on its own but managed to use his camera by closing his right eye instead. In fact many videographers work with both eyes open so they can see where the action is coming from.I can not close one eye! I keep on trying to do it but I just can't! Sometimes I can focus my eye on to the viewfinder but that takes too long and I miss things. Is it just me who can't close one eye and thus struggles with photography?
I agree. If the screen is used then you should not need to close one eye. I prefer to use the viewfinder though!what kind of camera do you have? many digital SLRs have preview screens on the back that are touch screens and will focus on whichever part of the image you touch. i do a lot of wildlife photography and rarely look into a viewfinder anymore.