Common photography myths (8)

Myth #7: The shorter the exposure time, the faster the shutter

September 5th, 2009 - 08:37:56 PM:

Photographers can get a bit sloppy with their terminology. When they say “faster shutter” they actually mean “shorter exposure time”. The two are not really the same. Here's how shutters work:

Focal plane shutters (i. e. located in the camera body between the mirror box and the film, and opening and closing vertically or horizontally) are two sets of shutter blades. Before the shot, one set is packed away at one side of the frame, and the other set is unfolded to cover the frame. When you take a shot with a long (let's say 1/30 second or longer) exposure time, the following happens: the unfolded set is opening in the direction opposite the other set, uncovering the film beneath it and letting light fall onto film. It moves with a constant speed until the entire frame is uncovered. This is called the “first curtain”. After some time, the other set starts to unfold and cover the film, starting with the part that was uncovered first. Again, the shutter blades move at a constant speed until the frame is completely covered. This is called the “second curtain”. Every point of the film is uncovered for a certain time, determined by the speed of the shutter blades and the duration the shutter is fully open. This is called the “exposure time”.

To get shorter exposure times the first step is to reduce the duration the shutter is fully open. By doing this you reach a point where the second curtain starts to close just as the first curtain becomes fully open. This speed is called the “x-sync speed”. It's important for flash exposure (I won't go into detail, here).

If you want to have even shorter exposure times, you have two options: you can make the shutter blades move faster. The first curtain takes less time from fully closed to fully open, and you can close the second curtain earlier and faster. There are obviously limits for this strategy. The shutter blades are not weightless, and they have to keep their shape while they move to ensure even exposure across the frame. You can not accelerate them beyond a certain rate. Otherwise the forces of acceleration and the mass of the shutter blades would tear them apart.

The second option is to use a trick: it works not by making the shutter blades move faster, but by starting to close the second curtain while the first curtain is not yet fully open. The edges of the first and second curtain move across the film plane in parallel. Light falls onto the film through a moving slit between the two curtains. This way each point of the film is uncovered for a shorter amount of time, and we get the shorter exposure time that we want. For even shorter exposure times you simply make the slit between first and second curtain narrower. You don't have to move the shutter blades any faster.

Shutter movement below x-sync speedShutter movement above x-sync speed

All modern cameras with focal plane shutters work that way. The shutter blades move at the same speed for all exposure times. The camera only varies the time the shutter is fully open (for times longer than the x-sync speed) and the width of the moving slit (for times shorter than the x-sync speed). This way the shutter mechanism can be kept simple, cheap and durable. Shorter exposure times can be achieved by more precise timing instead of faster shutter blades.

The shortest exposure time of a camera is not a good indicator for the physical speed of its shutter. The x-sync speed, however, is. As explained above, the “trick” only works for exposure times shorter than the x-sync speed. If you want to make the x-sync speed itself faster, there is no other way than to make the shutter blades move faster.

So how fast do shutter blades move? Let's look at a typical modern camera. It has a vertically moving shutter, an x-sync speed of 1/90 s, and 1/4000 s shortest exposure time. In other words, the edge of the shutter has to move a distance of 24 mm in 1/90 of a second. The speed of such a shutter is 2.16 m/s. That's 7.776 km/h or under 5 mph. If you walk briskly, you're moving faster than the typical shutter. Even the fastest focal plane shutters with an x-sync time of 1/300 s move only at about 7.2 m/s. If we didn't use the “trick” the shutter had to move these 24 mm in 1/4000 of a second. That would be a speed of 345.6 km/h or roughly 215 mph. What a difference!

Oddly, if you take a photo with an exposure time of 1/4000 s with the above camera, it will take a lot longer than 1/4000 s to take the shot. It will take a bit longer than 1/90 s (x-sync time plus delay between first and second curtain, which is the exposure time). There are some cameras (mostly panoramic cameras with swing lenses) that use the same “trick” as focal plane shutters, but for all exposure times. A rotating barrel with a narrow slit is exposing the frame from one side to the other. The exposure time is adjusted by making the barrel rotate faster or slower. Just like with focal plane shutters, it takes a lot longer to actually make the shot than the exposure time suggests.