See How an SLR Camera Shutter Works

SLR cameras are small wonders of modern technology. This neat video shows a shutter in action, so you can see exactly how the mechanism works.

The general concept behind SLR shutters is pretty simple: cover the sensor/film, uncover it for the proper exposure time, then cover it up again. In practice, coordinating the two shutter curtains to follow each other at precise intervals consistently over the course of hundreds of thousands of exposures is an impressive feat.

So, why two shutter curtains? There are two reasons: first, for faster shutter speeds, the exposure is so short that by the time the first curtain clears the sensor, it's already too late, so a second curtain follows closely behind it, essentially forming a slit that travels across the sensor. In fact, the fastest shutter speed in which the entire sensor is simultaneously exposed for some period of time is known as the sync speed of the camera. Second, having two curtains ensures even exposure across the entire sensor; if there was only one curtain that opened, then reversed direction, the area of the shutter near the bottom of the curtain's path of travel would receive less exposure than the top.

Check out the video above to see it in action! 

[via ISO 1200]

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Most mirrorless cameras have electronic first shutter. It is audible because you hear only one click.
It has its downside but in most cases, it works perfectly. And pretty soon the shutter will become completely obsolete.
The Sony a9 is the first camera that reads the sensor so fast, it doesn't a shutter anymore in most cases.


A solution to these problems is the electronic front curtain shutter. It works as follows:

The shutter stays open at the beginning of the exposure. Instead of having a mechanical front curtain, the image is cleared from the sensor, pixel row by pixel row, in the same direction and with the same speed as with a mechanical shutter. Exposure of each row of photo sites starts immediately after it is cleared. This is possible, because you are not interested in the image that was stored in the photo sites before clearing them, only in the image that is created afterwards.
After some delay, which again is the exposure time, the (mechanical) rear curtain starts to close, trailing the clearing of pixel rows with the same direction and speed. Pixel rows covered by the rear curtain then receive no more light.
When the rear curtain has closed, the camera can read out the entire image from the sensor and process it.
Then the mechanical shutter can open again.
The advantages of this system are obvious:

There are fewer mechanical parts. Specifically, you need only a single set of shutter blades.*2
For one shot, the shutter blades have to move only twice, not four times as before. This roughly doubles the lifetime of the shutter compared to the earlier design, without spending extra efforts in mechanics and materials.
Since there is no movement of any parts for the front curtain, vibrations are reduced.
The delay from release to exposure is reduced. You can start exposure immediately and don't have to wait for the shutter to close first.

Timothy Turner's picture

Focal plane shutters have been this way ever since the focal plane shutters was invented, the gap between the two curtains determines the shutter speed, the only exception is that today shutters are controlled electronically.