Monday, 6 August 2012

Focal Plane Shutter

Focal plane shutter is used on most modern DSLR cameras. It consists of two metal curtains positioned just in front of the digital sensor. At the start of the exposure, the first curtain opens to expose the sensor and at the end of the exposure, the second curtain moves to cover the area up again.

Focal Plane Shutter
Focal Plane Shutter – source wikipedia

When they first came into use the shutter curtains travelled horizontally from side to side. But now a days most cameras use shutter curtains that travel vertically. They work on the same principle but as the shutter need only cover a smaller distance when it travels vertical than horizontal (24mm rather than 36mm) the shutter blades can travel across the film plane in much less time. Thus providing improved flash synchronization speeds and also reliable performance at speeds as high as 1/8000 of a second.

The speed of shutter blades of a focal plane shutter is always constant. They travel at the same speed irrespective of the shutter speed set in the camera. The camera achieves varying shutter speeds by adjusting the time between the opening of the first curtain and the closing of the second. For slower shutter speeds longer the time gap between first and second curtains and vice versa.

For speeds up to the Sync speed of the camera the second curtain is triggered only after the first curtain has completed its travel. But to facilitate higher shutter speeds the second curtain is triggered before the first curtain reaches the end and as a result the sensor is exposed through a moving slit formed by the two travelling shutters.

Focal plane shutter
Focal plane shutter – The slit formed by the two curtains is visible at the bottom – source Wikipedia

We have discussed about this topic in detail in the article - Flash Synchronization.

Advantages of Focal Plane Shutter

a. The main advantage of having a focal plane shutter instead of leaf shutter is economy. Unlike leaf shutters, Focal plane shutter is built into the body of the camera. Thus it eliminates the need for each and every lens to have its own shutter mechanism built into it.

b. Another advantage of focal plane shutters is their speed. Focal plane shutters could be built to work at shutter speeds well in excess of what leaf shutters are capable of. Normally the maximum shutter speed typically available on a leaf shutter camera is 1/500; but on a camera with focal plane shutter this could be as high as 1/8000 of a second.

Disadvantages of Focal Plane Shutters

a. Focal plane shutters tent to be noisier than leaf shutters.

b. They employ a complex design and are more expensive than other type of shutters.

c. Leaf shutters could sync at any speed but the typical focal plane shutter (1/500 sec) has flash synchronization speeds that are slower than the leaf shutters. (First curtain has to be open fully and the second curtain must not have started to close when the flash is fired).

d. Focal plane shutters distort images of very fast moving objects. This kind of distortion also happens when the camera is panned rapidly. In camera employing the horizontal Leica-type FP shutter, the image appears stretched if the object moves in the same direction as the shutter curtains, and it appears compressed if object is travelling in the opposite direction of the curtains. In modern cameras employing a vertical FP shutter top of the image appears to lean forward.

In the next article we will discuss about Photography - Lighting - Flash - Off Camera Flash

Related Reading

  1. High Speed Flash Synchronization
  2. Types Of Flash Synchronization
  3. Flash Synchronization
  4. Flash Guide Number
  5. Flash Bulb