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Recently, I found out that the rubber cover on the strap of my Canon EOS camera is used to cover the viewfinder. I searched on the web to learn why should I cover the viewfinder and I read that it prevents light passing through the viewfinder and somehow getting to the sensor.

  • Is this true?
  • If it is, how does light get to the sensor? Doesn't the mirror prevent that?
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3 Answers 3

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On SLR cameras, the optical path is connected to the viewfinder. Even while a picture is being taken and the mirror is up, blocking the viewfinder path, it's possible the seal/gasket between the mirror and focusing screen will let some light through. It's a very small amount of light, and usually not noticeable.

However, when taking long-exposure or bulb-mode photos, your face probably isn't covering the viewfinder. Coupled with the longer exposure time, the light leak in the mirror seal accumulates, affecting the captured photo.

For example, in this related question, Pink/Purple lines on my photos while shooting long exposure?, the light leak shows up as purplish and green streaks across the wide dimension of the camera's sensor:

This is very typical of viewfinder light leaks in SLR cameras, if the viewfinder isn't covered.

Note that viewfinder light leaks don't happen when using cameras with electronic viewfinders (such as mirrorless interchangeable lens cameras) or separate viewfinders not in the optical path (most older point-and-shoots, and rangefinders).

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  • \$\begingroup\$ I had a plastic molding on the neckstrap of my film SLR for just this purpose. I never used it, but it makes sense that longer exposures would make it necessary. \$\endgroup\$
    – Ross Millikan
    May 4, 2021 at 2:43
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    \$\begingroup\$ @RossMillikan Yeah, I had one on my Minolta X-370's strap. I ruined probably 10 rolls before I figured out what the cover was for. \$\endgroup\$
    – scottbb ♦
    May 4, 2021 at 5:07
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    \$\begingroup\$ As someone who knows absolutely nothing about photography, that picture with the pink and green streaks looks really cool \$\endgroup\$
    – Radvylf Programs
    May 4, 2021 at 14:25
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The only reason I know of to cover the eyepiece is to keep light from throwing off the exposure meter which is built into the prism above the mirror. This is sometimes necessary when shooting from a tripod or other cases where the eye itself isn't covering the viewfinder. Canon discusses this here

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The purpose of covering the viewfinder when doing long exposures is twofold:

  • To prevent light entering the viewfinder from affecting metering in low light situations, particularly if there is a significant light source, such as a flashlight used by the photographer, behind the camera. For more, please see BobT's answer

  • To prevent light from leaking around the edges of the mirror and into the light box when it is in the raised position. For more, please see scottbb's answer

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  • \$\begingroup\$ mmmm, what is the point of this "answer" Michael? Your answer is... see other answers. \$\endgroup\$
    – osullic
    May 4, 2021 at 9:19
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    \$\begingroup\$ To draw attention to the fact that both are correct, but each only includes half of the full answer. \$\endgroup\$
    – Michael C
    May 4, 2021 at 9:21

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