Shutter – How it works

Shutter is what keeps the film or sensor hidden, and when the shutter opens, the light falls on them, creating the photograph. The time duration for which the shutter opens, to let the light in, is called the shutter speed. Shutter speed is one of the basic camera settings. Here is a short guide to understand the workings of a camera shutter and its various limitations.

 

What exactly is a shutter?

This is a mechanical device that prevents light from reaching the sensor or the film. When the shutter-release button (the click button) is pressed, the shutter opens up for the set duration to let in light. These mechanical shutters can either be like aperture blades in their shape or these can be a pair of curtains.

The shutter in the shape of aperture-blades is called leaf shutter in common usage. They consist of multiple metal blades or ‘leaves’ (The name ‘leaf shutter’ actually comes from very old cameras that had a single leaf which was moved to one side for letting the light in). These leaf like shutters used to be quite common in rangefinders and many other old cameras. View cameras (large format) almost always exclusively use leaf shutters.

The shutter that has a pair of curtains is common in SLRs and now DSLRs. Usually the shutter is located just in front of the film or sensor and so it is also called focal-plane shutter. In common usage this location based name of the curtains based shutter (focal-plane shutter) has become synonymous with each other.

 

An ideal shutter

An ideal shutter should be the one that opens instantaneously and completely as soon as the shutter release is pressed and then close instantaneously. There should be no time taken in opening and closing of the shutter. In real life however the shutter leaves or curtains have to physically travel a short distance to open or close, and so shutter does not open and close as ideally we would like it to. There are compromises to be made.

 

Focal-plane shutter

These are placed just in front of the sensor or film and consist of a pair of curtains. In the ready to fire state, one of these curtains (the leading curtain, front curtain or the first curtain) remains closed and the other curtain (rear curtain, trailing curtain or the second curtain) remains open. As soon as the shutter release button is pressed, the front curtain opens up and after the exposure is complete the rear curtain closes, restricting the light entry. Advantage of the focal plane shutter is that these are capable of providing very high shutter speeds. The shutter curtains typically move vertically since there is less distance to be covered (In some earlier cameras, the curtains did move horizontally but they were restricted to lower flash-sync speeds, explained below) Beyond a certain shutter-speed, it becomes difficult to open the front curtain completely and then close the rear curtain. So what the camera manufacturers have been doing for the past many decades is that at high shutter speeds, when the front curtain opens, the rear curtain also starts closing down. This effectively turns the shutter opening into a ‘slit’ which moves from one side to the other. The slit is formed by the end of the front curtain and the beginning of the rear curtain. So far so good.

Now what if a person was to use a strobe (flash or speedlight). No problems with that till the time the shutter is open completely at that shutter speed. The strobes last for about 1/10000 of a second or even less. Say if the shutter is open completely for 1/125 of a second then the short duration of the strobe’s firing occurs easily within the relatively large time frame while the shutter is open. What if the flash is fired at a speed when the shutter does not open completely and forms a moving slit? The flash lasts for that same minuscule duration lighting up whatever part of the image the moving slit is covering at that time. This causes a well demarcated band of well-lit scene in the photograph, while the area outside the band appears dark. All focal-plane shutter cameras therefore specify the flash-sync speed. The fastest shutter speed at which the shutter is completely open and flash if fired will cover the full image is called the flash-sync speed. Faster than this speed, the shutter starts opening like a slit and the flash ends up lighting only a part of the scene. Recently companies have come out with high speed flashes which are capable of lighting the scene completely beyond the flash sync speed. They do so by firing multiple flashes or a constant long duration flash during the whole time while the slit moves across the frame (more about it and other flash modes – Flash Modes)

Another problem that happens with the focal-plane shutters is with subjects moving at high speed. When a high shutter speed is used, the shutter starts to work like a slit as described above. However in a really fast movement, the location of the subject or a part of it might change in relation to the frame. The moving shutter slit captures this information at different parts of the film or sensor, leading to a distorted image. The distorted tire of the racing car is the best and most famous example of this distortion. Now a days the shutters have been made very effective and so they start to work like a slit at very high speeds. This kind of distortion is therefore not as common as it used to be some decades back.

Jacques-Henri Lartigue's "Car Trip"

 (Jacques-Henri Lartigue’s famous 1913 “Car Trip”, taken with an ICA camera on a 4×5 inch glass plate with an f4.5 lens. The focal-plane shutter’s slit moved horizontally while the camera was panned. This led to the distortion seen here. The spectators appeared to be tilted to the left whereas the car towards the right. Interestingly this same distortion is used in cartoons to show speed.)

 

Leaf Shutter

Leaf shutters open and close like the aperture blades. As soon as the shutter release button is pressed, the leaves open up completely and then close down after the set time (as determined by the shutter speed). Leaf shutters have their own advantages. These are very quiet when compared to the focal-plane shutter. The leaves open and close completely at all shutter speeds so the standard flashes also work at all the shutter speeds. The disadvantages are that it is difficult to achieve very high shutter speeds with these and secondly, these are expensive. Photographers also complain of vignetting with leaf-shutters though I have not found this in my limited use. There are just a few handful of companies that manufacture leaf-shutters. Interestingly, leaf shutters are quite commonly located within the lens itself. In some cameras, the aperture blades double up as a secondary leaf shutter combined with an actual electronic shutter (explained below)

 

Electronic shutter

This is the latest shutter kind. There used to be electronic shutters in old cameras too but those were actually electronically driven mechanical shutters. Now when I speak of electronic shutter, it means shutters with no mechanical device involved. With the use of sensors in the new cameras, the shutter became redundant to some extent and so many camera manufacturers came out with an electronic version of the conventional shutter. All they did was to connect the image capturing electronically to the shutter release. When the shutter release is pressed, the sensor captures the scene in front of it for the time duration that is set as shutter speed. There are no mechanical blades or curtains involved. This kind of shutter first appeared in compact digital cameras with live-view function. Now all the DSLRs have this along with the conventional focal-plane shutters (that is how the DSLRs are able to capture images in live-view mode). Mirror-less cameras also use the electronic shutter.

The biggest advantage with electronic shutters is the instantaneous response. The start and stop the image capture almost instantaneously and so have very accurate shutter speed timings.

(In some inexpensive electronic shutters like those found on mobile phone cameras, the sensor captures information at the rate of one line at a time, effectively working as a mechanical focal-plane shutter and therefore is prone to the high speed distortion discussed above with focal-plane shutters. The slow processing also explains the delayed image capturing after clicking on the ‘shutter release’ in mobile phone cameras.)

 

Train

(Train – D200 with Nikkor 18-35mm)

 

If you want to understand shutter speed further, take your camera out and experiment. To make life easier, the shutter speed also changes in complete stops  in the same manner as Aperture or ISO. Though the present day cameras also have multiple small steps between any two EV stops, still it makes sense to know the values of the full EV stops. Learn the relationships between shutter speed, aperture, ISO and available light, if you haven’t till now, and then you would have mastered the basics. (Basics of Exposure)

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