This series of articles is excerpted from Rocky Nook’s Enthusiast’s Guide to Exposure by John Greengo covers the controls — shutter, aperture and ISO.

The shutter speed describes the amount of time each pixel on the sensor is exposed to light. The actual speed of the shutter movement doesn’t change at all; the amount of time the shutter is left open changes. “Exposure time” is a better description of this aspect of photography. Most photographers understand that the shutter speed describes the shutter unit opening and closing for a specified period of time. The actual workings of the shutter are a bit more complicated. The shutter is like a door that opens and closes, exposing the sensor to light. But it’s not like your everyday hinged door; it’s more like sliding doors.

Sutter in a pro 35mm film camera shows the individual blades.
A 35mm film camera with its back open shows the shutter blades.

The camera’s shutter comprises a group of quick-moving blades (generally 3–5) that are made from very lightweight materials. The blades overlap slightly, like shingles on a roof, to block light from passing through. When the shutter release is pressed, all the blades recede quickly out of the way in order to let the light through. The sensor is then exposed to light coming through the lens for the specified amount of time. To end the exposure, a second set of blades from the opposite side will follow the motion of the first set. This two-curtain system ensures that each pixel is exposed to light for the exact same amount of time. If there were only one set of blades, they’d have to make a return trip, thus exposing the pixels on one side of the sensor for more time than those at the other. There is a slight problem with this system. Even with the best of technology, both the blades and the motors that power them have a limit of how fast they can go. If the second curtain waits for the first curtain to open completely before starting its closing motion, the fastest shutter speed possible on most cameras would be about 1/250 of a second. In order to get faster shutter speeds, the camera employs a little trick: the second curtain begins to close shortly after the first curtain has begun to open to let light on the sensor. The two shutters working in tandem are, in effect, scanning light onto the sensor in a narrow strip. Mirrorless Camera Shutters This process of exposing the sensor to light describes how a traditional SLR camera works. Mirrorless cameras, on the other hand, must first have the shutter open in order to gather information on the sensor to be displayed in the electronic viewfinder or on the back LCD monitor of the camera. When the shutter release is pressed on a mirrorless camera, the shutter unit moves into position to block the sensor. The sensor is then prepared to receive light. The first shutter opens and the second shutter closes, just like in an SLR. In order to return an image to the viewfinder, the second shutter needs to open so that light may strike the sensor and the information can be sent to the viewfinder. This extra step at the beginning and end is necessary since SLRs use a mirror to view through the lens and a mirrorless camera uses the image coming directly off the sensor

Photo by chuttersnap on Unsplash

Opening photo: ©Kevin Ames

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