Yes—on some cameras and in some situations, covering the viewfinder can make a noticeable difference. Light entering through the eyepiece can sometimes leak into the metering or imaging path, causing reduced contrast, veiling flare, or exposure issues.

The effect is usually greatest when:

• the exposure is long
• there is bright light behind the camera
• you’re using strong ND filters or other conditions that lengthen exposure

In those cases, the difference can be significant, not just slight. In darker conditions, it may make little or no visible difference.

It also depends on the camera design: some cameras are more affected than others. The simplest answer is to test your own camera. Try a long exposure and shine a flashlight into the viewfinder with and without the cover. That gives you a worst-case comparison.

If you don’t have the supplied cover, using your hand, a cloth, or a jacket over the eyepiece can work too. So while a viewfinder cover isn’t always necessary, it’s a good precaution for long exposures or bright light behind the camera.