Today’s Question: I wondered if you could settle a curiosity for me. Do you know what the rubber rectangle on my camera strap is for? It almost looks like it could fit on the flash hot shoe, but it doesn’t. I can’t figure out what it is for. Help!
Tim’s Quick Answer: The rubber device you’re referring to is a cover for the viewfinder, used to prevent light from entering the viewfinder. You generally need to remove the eyepiece on the viewfinder in order to attach this cover.
More Detail: It seems to be conventional wisdom that during long exposures at night, you should cover up the viewfinder to prevent light from getting into the camera and causing aberrations. In my experience this is not an issue at all. There have certainly been cameras in the past (think large view cameras) where light was an issue in this way. But as far as I’m concerned this is not a concern for modern digital SLR cameras.
However, there is another reason you may still want to cover the viewfinder for night photography (among other situations). While light coming in through the viewfinder won’t impact your photos, it can have a significant impact on your camera’s meter. If you’re using an automatic or semi-automatic exposure mode, or, depending on the camera’s meter to measure the exposure for a night photo, it is a good idea to cover the viewfinder. Of course, it would also be fair to point out that the camera’s meter isn’t incredibly helpful with a lot of night photography anyway.
So, while light can most certainly come in through the viewfinder and affect your camera’s meter, you don’t need to worry about that light in terms of the actual image being captured. More importantly, now you know what that little gadget is on your camera strap!