Lately I have been doing a lot of long exposure photography, both digital infrared and visible light.
In the process of doing this work I have had shots that have been spoilt by flares, fogging and other artifacts. It turns out that these effects were cause because I did not have my eye blocking the viewfinder and had not used the eyepiece blind that most cameras offer.
In the shot below I was using a Nikon D3 with a Hoya R72 filter to shoot digital infrared. With a 15 second exposure there was plenty of time for moving objects, in this case water and clouds, to produce movement. With the eyepiece shutter activated (the Nikon D3 has a lever next to the viewfinder that activates a proper blind), the result in a smooth image that can be converted to monochrome using one of the channels (in this case green).
With the eyepiece blind open the result is very different. Light has leaked in and fogged not only across the center of the image but also down one side. You can see this even more clearly when you examine the three channels individually and see the strong fogging in the blue (the difference in exposure is typical of unmodified digital cameras when shooting in infrared).
All cameras can be affected. Below my Canon 400D had the artifact on the right of the image when the klutzy eyepiece shield on the camera strap was not used.
On some cameras I have seen no image artifacts but rather the exposure has been way off. So get in the habit.
Some cameras make closing off the eyepiece hard and others make it easy. Some will have a proper eyepiece blind. Others, like the Canon Rebel (350D, 400D and 450D) will have a small plastic or rubber blind on the neckstrap that can be slipped over the eyepiece when the rubber surround is removed. Some may have no provision and you will need to make something up. But on every camera it is important to do this when taking longer exposures.