Focal Plane shutter

A focal plane shutter is commonly found in most SLRs is located very close to the film or the sensor plane covering it completely so that no light is striking it till the shutter is released.Earlier it was in the form of a pair of blinds made up of very fine fabric or thin metal which roll up like a window blind. Modern complex shutters are made up of thin delicate alloy blades which fan out to to cover the shutter and collapse together to expose. Shutters can be controlled electronical as in Av or Tv modes or mechanically as in Bulb mode. 

The sensor is exposed progressively and not really in one go.One shutter curtain opens up and the other one closes down after the set shutter speed. The second curtain starts after the set shutter speed. In other words if the shutter speed is set at 1/60th of a second, the second curtain would start 1/60th of second after the first curtain started. In case of a slow shutter speed the second curtain would start covering the sensor the moment the set shutter speed elapses. The shutter speed is actually the duration for which the sensor is being exposed and not the speed with which the shutter travels.

In case of high shutter speeds the duration lasts so less that the second curtain starts almost at the same time, the difference being 1/8000th of second if that is the set speed. So the exposure happens through a very narrow slit. Like a scanner the whole scene is exposed or "scanned" across.

The shutters are very reliable and accurate. More professional and expensive bodies are tested to give a higher number of accurate shutter performance compared to cheaper cameras.

The sequence of images above illustrates the appearance of the shutter as it moves across. A quick recall that the image is formed upside down.

1. The first shutter covers the focal plane so that no light is entering. 2. The curtain falls and exposes a strip of image on the sensor.

3. After the set shutter speed the second curtain falls, following the first curtain. The difference between the two equivalent to the set shutter speed. The traveling speed of both the curtains is same and is a constant. 

4. The first curtain has almost finished exposing the sensor and the second curtain is covering up the exposed part. Effectively all points on the sensor get the same amount of light.

5. The second curtain covers the focal plane /sensor until the shutter is cocked, that is pull the first curtain back. 6. back to where it started, ready for the next exposure, depending upon the shutter speed set, which could last longer or less.

The faster is the shutter speed the sooner the second curtain starts and the exposure happens through a very small slit compared to the one at a slower shutter speed. At very slow shutter speed nothing physically happens till the next curtain covers the sensor.