Does a viewfinder cover really make a difference? I've heard opinions from people that they are essential for scenarios such as shooting long exposure pictures. I'm just wondering if it is only a slight difference or a major one.
Yes they do but so does your hand placed over the viewfinder.
To take this shot last week I had the sun behind me. Without covering the viewfinder I got a strong veil and very low contrast image. Unfortunately, my viewfinder cover was attached to strap I forgot to bring. Luckily my hands were free during the exposure, so I retook the shot and got a crisp constrasty image. The image you see was cropped to a 2:1 aspect ratio and resized for display but no other changes were made to it.
The different it makes is proportional to the amount of light and the exposure length. In this case it is a worst case scenario because it was bright light behind the camera and a 15s exposure (thanks to an ND400 filter).
Depends on the camera. Can be significant.
Also depends on the exposure time and ambient light level.
Best test is to try it and see with your camera.
Set up an exposure of say 30 seconds and shine a small torch into the viewfinder eyepiece. That gives you worst case result. Then go from there. I occasionally place a coat or cloth or similar over the eyepiece in exceptional circumstances but for most purposes it doesn't matter. If ambient level is "about dark", which is often the case when you are using long exposures, then you may well be OK. With new cameras Minolta used to provide a rubber fitting which mounted on the camera strap and slid over the eye piece when required.
Ever recurring debate...
Personally I would say it does not matter on a new/modern camera, on the other hand, given that the cover is attached to your camera strap, why not use it to be on the safe side?
The only way to really find out is to test it. Shoot with the cover off at night and shine a torch at the viewfinder and then shoot with the cover and point a torch at the viewfinder.
I was taking 7 minute and longer exposures during the day with 16 fstops of ND and the light leak without a viewfinder cover is terriiiibleeee. You have to use a viewfinder cover for longer exposure times. I sometimes take 20 minute long exposures depending on how slow the clouds are moving and if I were to take that photo with the sun behind me and without the viewfinder cover you would not even be able to see the image. What would result is bands of purple and green streaks.