You probably have an eyepiece cover that came with your DSLR when you purchased it… but are you actually using it? The answer is most likely no.
Should you use it? No. Not only is that eyepiece cover clumsy to take and remove, doing it too often has a change to loosen the eyepiece mount, making your regular eyepiece easier to fall off and lose accidentally.
However, it doesn’t mean that you shouldn’t cover your viewfinder at all. You can use a microfiber cloth, a cardboard or even by cupping your hands to cover the viewfinder in a pinch. While most cameras are well built enough in regards to light leaks, there are still some situations where covering your viewfinder might be a good idea.
Cover your viewfinder if:
1) There is a great light difference between the front and back of the camera. Sometimes you will have bright street lamps or strobes shining at the back of the camera while the front is nearly pitch black. No matter how “lightproof” your camera is, it is better to be safe than sorry and make sure not to let any undesired light in by covering your viewfinder. Although it is quite an extreme example, here are two pictures from my Nikon D5100. Both are 20 second exposures with a body cap, but the first one was shot with a bright light shining through the viewfinder, while I covered the viewfinder in the second shot.
2) Your camera has known light leak problems. While most cameras will do fine, there are reports that the Nikon D800 and Canon 5D Mark iii all have long exposure light leak problems. In these cases, just cover your viewfinder during long exposures no matter what. Period.
3) You are shooting super long exposures (i.e. 2 minutes or more). Super long exposures have many, many other things that need to be fixed such as fixed pattern noise, but making sure that light doesn’t come from the back of your body will mean one less problem to fix in post-processing. That being said, no shutter design is perfect and light leaks during long exposures can even come from the lens mount contacts. And there isn’t a simple way to fix this…
Don’t cover your viewfinder if:
1) You own a mirrorless camera with or without EVF (for obvious reasons). As far as I know, the electronic viewfinder is isolated from the shutter, so no worries here. And chances are that your mirrorless camera didn’t come with a viewfinder cover in the first place… 😛
2) If you are just shooting regular long exposures. Like I’ve mentioned earlier in this article, *most* DSLR’s are lightproof under normal conditions, under normal shutter speeds (which basically means everything apart from bulb). Just don’t worry.