If there is a light source behind the camera, including a remote off camera flash, light can enter through the viewfinder and affect the camera’s light metering system, thus causing under exposure.

While the severity of this effect can vary from model to model, this is a phenomenon the can happen with all SLR cameras.

In particular, when light from a flash is bounced, the amount of reflected light from the pre-flash is lower than when the flash is directed straight at the subject. This causes a low pre-flash component, making it easier for light that comes in through the viewfinder to have an influence on the camera, thus causing the image to be underexposed.

For this reason, when you use the self-timer, bulb, or a remote switch and do not look through the viewfinder, stray light entering the viewfinder can cause the image to be underexposed. To prevent this, use the built in viewfinder cover (if your body is equipped), or the eyepiece cover that is attached the camera’s strap.

For more information, please see the applicable section of your camera’s manual.