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This is what that rubber piece on your camera strap is for

Since the day I owned my first Canon camera — a Rebel xT — I’ve wondered about that little rubber piece that’s securely attached to the camera’s shoulder strap. Now, thanks to photographer Bassam Sabbagh, I know.

Collaborating with DIY Photography for a recent video, Sabbagh explains that this mysterious rubber piece is actually a cap. Specifically, it’s a cap to place over the viewfinder so there aren’t any light leaks or unexpected flares when shooting a long exposure shot or time lapse.

While light leaking through the optical viewfinder isn’t a common issue, it can occasionally occur when shooting long exposures or time-lapse videos in low-light conditions. Yes, the mirror retracts up when a photo is being taken, but considering the prism above the mirror that’s used to redirect the image, it is possible for some light to leak through.

Related: These were the most popular cameras and lenses at the Olympics

I could see this being an issue when you’re looking to take a long exposure of a road where cars are driving in both directions. If your camera is angled toward one direction of traffic, it’s possible that the headlights of the opposite traffic could shine through your viewfinder and potentially alter the image.

Considering I’ve only ever been a Canon user, I was a bit curious to see if Nikon has an equivalent. From my research, I wasn’t able to find any particular products made for this purpose, so it looks like only Canon has an official answer to this problem.

Of course, if you’re a Nikon user who’s concerned about potential light leaks, you could always resort to Gaffer tape, the photographer’s equivalent of duct tape.