How a shutter works

The shutter in a Digital Single Lens Reflex (DSLR) camera is sort of the gatekeeper that allows light through to the camera’s sensor and, along with the lens’ aperture and the ISO (the camera’s light sensitivity) creates and exposure. A fast shutter speed can freeze and Olympic athlete in his or her tracks while a slow one can capture the wonders of the heavens in the night sky. One can think of the shutter mechanism as a simple door that opens and closes but it’s a much more complicated piece of machinery.

When you look through the viewfinder of a DSLR camera you are see an image that comes directly through the lens. In other words you’re seeing what the camera is seeing.

To be able to do that and also be able to take a picture, light comes through the lens then is reflected off an angled mirror upwards into the pentaprism, that bulge or hump at the top of the camera where the viewfinder resides. The light is refracted through the pentaprism and out the eyepiece.

When the shutter button is pressed to take a picture a whole set of machinations is set into motion. First the mirror is drawn up and out of the way and at the same time (in some models) a support that the mirror rests on at the bottom flops downward. The shutter is actually consists of two, bladed “curtains” usually made of thin, high-strength metal (or in the case of the more expensive models carbon fiber). The front curtain drops open to expose the camera’s sensor then the second curtain drops down to end the exposure. The mirror drops down and it support pops back up and the shutters reset for the next shot. (Back in the old film days you had the added step of the film being transported frame by frame with each shot as well). When all this happens the viewfinder is momentarily blacked out as the camera takes the picture.

( Here’s a YouTube video of a Nikon D4 DSLR camera shutter/mirror assembly in slow-motion action by Chad Westover)

If you think that this sounds pretty complicated there’s yet more. Consider this: It all happens in a fraction of a second. Additionally, most cameras shoot in some kind of burst mode. Low end DSLRs shoot around 3 to 4 frames per second (fps). Mid level cameras top out at around 5 to 6 fps. Nikon’s top-of-the-line D4 does a blazing 10-fps and Canon’s 1Dx does an astounding 12-fps (it can go even faster – 14 fps – in a special mode). All these cameras have been tested at hundreds of thousands of actuations (Both the D4 and the 1Dx have been rated at 400,000 cycles).

Today’s cameras are basically mini computers and as such are very sophisticated electronic devices. But even in this age of digital wonders they are still incredible machines as well.

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