Activity

When the S2 launched, Leica announced that the camera would feature a dual shutter system. The rear of the S2 has a power switch with three positions: Off, FPS and CS. FPS stands for focal plane shutter, which is standard in just about every SLR-type camera out there (except the Hasselblad H system). CS stands for central shutter, also commonly referred to as a leaf shutter or a lens shutter. Leica is getting close to launching the CS lenses and I had an opportunity to try them out. Before getting into the tests, let’s take a look at what a dual shutter system offers.

Advantages for focal plane shutters

Found in just about every SLR made, focal plane shutters have the advantage that they can fire at much higher speeds than lens shutters. The S2’s shutter is capable of speeds up to 1/4000th of a second, while leaf shutters are usually limited to 1/500th or 1/800th. This can be an issue if you want to shoot in bright sunny conditions with a very wide aperture. If we suppose that an average daylight exposure is 1/500th @ f/8, ISO 160, then if we want to shoot at f/2.8 we really do need to be able to crank the shutter speed up to 1/4000th. In the case of a Hasselblad H4D, the camera only has a lens shutter that is limited to 1/800th. In order to shoot at f/2.8 in bright sun, you’d have to use a 2 or 3-stop ND filter on your lens. Of course, this also makes the viewfinder very dark and provides a challenge to the autofocus system. Besides being able to shoot at wide apertures for depth of field control, stopping action also becomes an issue if limited to slower shutter speeds. Even the fastest action can be frozen at 1/4000th.

And because the focal plane shutter is in the camera body, lenses from other systems can be adapted given an adequate focal flange distance. The S2 body can easily accept lenses from Pentax 67, Mamiya 645 and Hasselblad V systems by using adapters.

How a focal plane shutter works

A focal plane shutter is comprised of two shutter curtains working in unison. The first curtain opens to expose the sensor, then after a predetermined amount of time, which we call the shutter speed, the rear curtain closes to end the exposure. At slow speeds (slower than 1/125th on the S2) the first curtain is fully open before the second curtain stats to close. At faster speeds (faster than 1/125th on the S2) the second curtain starts to close before the first curtain is fully open. At the fastest speed of 1/400th the space between the curtains is just a horizontal sliver which moves vertically in front of the sensor. The reason for this is that the speed of travel of the curtain is fixed; only the time between first and second curtains affects shutter speed.

Disadvantages for focal plane shutters

The downside of a focal plane shutter is that the flash sync speed is limited. Because a strobe fires a single burst at a very short duration, usually in the neighborhood of 1/3000th of a second, the sensor must be fully exposed when the flash fires. If the second curtain has already started to close in order to get faster shutter speeds, you’ll see a dark line in the resulting image. The fastest speed at which the shutter is fully open is referred to as the X-sync speed. In the case of the S2, this is 1/125th of a second. This isn’t an issue in studio where everything is controlled, but it could be an issue when using battery-powered strobes outside.

A look at central shutters

The central shutter used in Leica CS lenses is comprised of four carbon fiber blades mounted in the center of the lens (kind of obvious looking at the name, I suppose). The optical elements and the light ray bundle passing though them have the smallest diameter at this point, which allows the shutter blades to be as small as possible in order to open and close in the shortest amount of time. As opposed to a focal plane shutter, a central shutter only has one set of blades which open and close in a circular fashion to expose the sensor, mechanically similar to how a lens aperture works. One of the benefits of this approach is that a central shutter is able to sync with a strobe at any shutter speed up to its maximum. In the case of the S2, this is 1/500th of a second, or two stops faster than the focal plane shutter’s X-sync speed.

Besides faster sync speeds, lens shutters also offer lower vibration than focal plane shutters due to less moving mass. In DSLR systems, the mirror is usually the key vibration factor, which is why it is good practice to use mirror lock-up when shooting at slower speeds on a tripod. But, even with mirror lock-up, not all focal plane shutters have the same amount of vibration. The S2’s shutter is extremely well damped. Other cameras like the Pentax 645D…not so much. No amount of mirror lock-up will prevent shutter bounce at speeds like 1/15th.

What about both?

On most camera systems the photographer is usually limited to one shutter system or another. Nikon and Canon DSLRs only offer focal plane shutters. The Hasselblad H system only offers lens shutters. The Leica S2 was designed with the intention of allowing the use of either the built-in focal plane shutter or a central shutter when used with a CS lens. Even more conveniently, there is a quick switch on the back of the camera to select which shutter to fire. And, Leica has added a couple fail-safes to the procedure. If you are set to CS but have no CS lens mounted, the camera will still function in FPS mode with just an on-screen alert to the user. Similarly, if you are shooting in CS mode with a CS lens mounted but set the shutter speed dial faster than 1/500th, the camera will automatically switch into FPS mode. Both are designed to keep the camera from getting in the way of the natural flow of shooting.

S2 product manager Stephan Shulz explained to me how the S2 works with both shutters. He outlined the firing sequence as follows: