Mechanical Shutter

A mechanical shutter is a physical barrier that opens and closes to control how long light reaches the sensor or film. In most cameras it is a focal-plane shutter with two curtains that travel across the frame one after the other. The time between the first curtain opening and the second closing is the shutter speed. It makes the familiar click, syncs with flash up to about 1/250 s, and usually tops out at 1/4000 s or 1/8000 s.

Two curtains sit directly in front of the sensor. At rest the first curtain covers it. When the shutter fires, the first curtain drops away to start the exposure and the second curtain follows to end it. At slow speeds there is a moment when the sensor is fully uncovered. At fast speeds the second curtain starts before the first has finished, so the sensor is exposed through a narrow travelling slit. Narrower slits give shorter shutter speeds.

A leaf shutter sits inside the lens instead of in front of the sensor. A ring of thin metal blades opens from the centre and closes again, much like an aperture. Because the whole frame is exposed at once at every speed, a leaf shutter syncs with flash at any shutter speed, often up to 1/2000 s. It is quieter and lower in vibration but harder to make fast. Fujifilm X100 cameras and many medium format lenses use one.

A flash burst lasts about 1/1000 s. For the burst to light the whole frame, the sensor has to be fully uncovered at that instant. That only happens at speeds up to around 1/200 s or 1/250 s, the flash sync speed. Faster than that, the two curtains form a moving slit, and a single burst lights only the strip the slit is passing over, leaving a dark band. High-speed sync works around this by pulsing the flash many times as the slit travels.

Shutter shock is a small vibration caused by the first curtain starting and stopping inside the camera body. The sensor is moving slightly while the exposure begins, which can blur fine detail even on a tripod. It shows most between about 1/15 and 1/125 s and with long or lightweight lenses. Electronic first curtain shutter removes it by starting the exposure electronically, and the full electronic shutter has no vibration at all.

Makers rate shutters for a set number of actuations, from about 100,000 on entry-level cameras to 400,000 or 500,000 on professional bodies. The rating is an average, so some shutters fail earlier and many run far past it. When a shutter wears out, frames show uneven exposure or a dark band along one edge, and the unit can be replaced by a service centre. Shooting with the electronic shutter adds nothing to the count.

Cameras such as the Nikon Z 8, Nikon Z 9, and Sony A9 III use a stacked sensor that reads a full frame in about 1/250 s or faster. At that speed, the rolling shutter skew and flash problems that made the mechanical shutter necessary mostly disappear, so the makers removed it. The result is a silent camera with no shutter wear, no shutter shock, and bursts of 20 to 120 frames per second. The sensor is protected by a cover that closes when the lens comes off.

No. A DSLR has both. The mirror sits in front of the shutter and bounces light up into the optical viewfinder. When the shutter button is pressed, the mirror swings up out of the way first, then the shutter curtains open and close to make the exposure, then the mirror drops back. The mirror movement is louder and causes more vibration than the shutter itself, which is why DSLRs offer mirror lock-up. Mirrorless cameras keep the shutter and drop the mirror.

Use the mechanical shutter whenever the subject moves fast or a flash is involved. Sports, birds in flight, children running, and any studio or speedlight work are safer with the curtains. It also avoids the dark bands that LED and fluorescent lights cause with the electronic shutter. Choose the electronic shutter for quiet venues, long bursts, or shooting wide open in bright sun. On most cameras the mechanical shutter is the default setting.

The shutter count is the number of times the mechanical shutter has fired since the camera was made, like mileage on a car. The camera writes it into each photo's EXIF data, and free web tools or desktop apps can read it from a fresh unedited file. Compare the count with the maker's rating for that model. A camera at 30,000 actuations on a 200,000 rating has plenty of life left, while one near or past the rating may need a new shutter soon.