Focal-Plane Shutters vs. Leaf Shutters
How They Work + Key Differences8 min read by
Without a shutter, a camera is just a box.
In the early days of photography, light-sensitive chemistry required minutes of uninterrupted exposure. A lens cap and a stopwatch were enough to control the light volumes. Some modern cameras, including those you can make at home, need no shutter for the same reason: long exposure times. But for the most part, camera shutters will let the light fall onto film for just fractions of a second — not something you can do with a lens cap.
Early cameras had a large variety of shutter mechanisms though eventually, manufacturers settled on two major designs: focal-plane and leaf (diaphragm).
How focal-plane shutters work.
Focal-plane shutters are a clever mechanism. Their name implies that the shutter is located near the film plane — as opposed to being between the lens elements or in front of the lens. They are generally cheaper to make than leaf shutters, and they can produce much swifter exposure times — up to twelve-thousandth of a second!
But of course, this miraculous exposure speed isn’t promoted by anything as obvious as stronger springs. There’s a trick to it.
✪ Note: Focal-plane shutters are usually implied to be of a rolling-blind design, described below.
Invented in the 1890s, the principle of a plane shutter is a travelling slit behind the lens, close and parallel to the light-sensitive plane.
Leica popularized this design with their dual-curtain system, where the first curtain clears the entire film exposure area at about one-sixtieth of a second, immediately followed by the second curtain — travelling in the same direction. Cocking the shutter would reset the curtains by winding them back — with the gap between the curtains closed.
Shutter speeds slower than one-sixtieth of a second would have the curtains pause in the position of the film plane being completely exposed — for set periods of time.
Shutter speeds faster than one-sixtieth of a second do not increase the actual travel speeds of the curtains. Instead, the size of the slit/gap between the curtains decreases. For example, a slit that’s half the width of the film plane clearing/“scanning” the entire exposure area in one-sixtieth of a second effectively produces a 1/120th of a second exposure.
Faster curtain travel speeds are possible with vertical-travel focal-plane shutters. Designed to look like metal window blinds, they’re made of lightweight, rigid materials that create a horizontal slit. Vertical-travel focal-plane shutters usually move at one-hundred-and-twentieth of a second.
There are other variations to focal-plane shutters, including the rotary focal-plane shutter and revolving drum focal-plane shutter — both of which are also based on a slit travelling across the film plane. The rotary shutter is typically a metal circle mounted parallel to film while the drum shutter is a cylinder with a slit cut on two sides which’s axis is vertical relative to the film travel.
Olympus PEN half-frame SLR cameras employ rotary focal-plane shutters. Panon Widelux and KMZ Horizont panoramic film cameras use revolving drum focal-plane shutters to help produce relatively distortion-free ultra-wide panoramic shots.
Focal-plane shutters’ advantages.
Due to their travelling-sit makeup, focal-plane shutters are generally the fastest type, only yielding to fully-electronic shutters on digital cameras.
Focal-plane shutters are also the predominant mechanism on SLRs and rangefinders with interchangeable lens designs as they allow the glass to sit closer to the film plane. Focal-plane shutters also cover the light-sensitive part of the camera as part of their function. Focal-plane shutters don’t need to be installed in every lens like central leaf shutters — thus making for an overall economical design.
Focal-plane shutters’ disadvantages.
Despite their numerous advantages, focal-plane shutters come with a few significant drawbacks.
Focal-plane shutters have a hard upper limit for the speed they can fire at while using flash. This speed is determined by the shutter’s minimum time the curtains stay open across the entirety of the frame at once, without relying on the narrowing slit. For most film cameras with cloth shutters, this speed is 1/60th of a second, while some modern SLRs may work with 1/125th of a second or faster. This limit is called “flash sync speed.”
If you happen to set your shutter to a speed higher than its sync speed while using flash, you will end up with only part of your frame exposed properly. This is because most flash bulbs fire at extremely fast speeds, around 1/12,000th of a second; the slit of your focal-plane shutter will not be able to cross the entire frame by the time the light is extinguished, leaving a chunk of your frame underexposed.
Photographers that rely on high shutter speeds while using flash with focal-plane shutters sometimes invest in specialized flashes which have a longer overall illumination time or fire multiple times in quick succession.
Another drawback is a “rolling shutter” effect, which is a distortion that cameras with focal-plane shutters produce in certain conditions while photographing fast-moving subjects. You can see this effect in the image taken by Jacques Henri in 1912 below: the wheels of the car are egg-shaped, and the vehicle, along with spectators, are slanted.
As with flash, the “scanning” slit ends up exposing different parts of the film at slightly different times.
Modern focal-plane shutters have curtains that travel vertically, which minimizes the rolling shutter effect as most panning and fast motion happens on the horizontal plane. And if that’s no help, the top advice for those types of cameras is to change the camera’s orientation so that your shutter curtain’s travel direction is perpendicular to the direction of the moving object.
In addition to the above, focal-plane shutters tend to be much louder and produce significantly more camera shake than their leaf counterparts. Often, this is the fault of an SLR design that requires moving a relatively heavy mirror out of the way before taking a photograph. But the shutter construction that involves a set of relatively large curtains moving in a single direction is no help either, especially when compared to the better-balanced movement of leaf shutter blades.
How leaf shutters work.
Leaf shutters are much easier to understand in terms of their basic working principle. A blade or a series of blades open (virtually instantaneously) and remain open for a set time — usually a fraction of a second. After the set delay lapses, the blade(s) shut.
Single-blade leaf shutters can be cheap and relatively simple devices, often used as a part of disposable or cheap plastic cameras. These kinds of shutters are usually very limited in their speed range, often just one, typically around a hundredth of a second.
Diaphragm leaf shutters, on the other hand, are complex mechanical devices. Those shutters consist of multiple blades that work in sync as they swiftly open and close.
Most leaf shutters I come across are central shutters, meaning that they are located between lens elements near the aperture blades. This design ensures that there’s no vignetting at higher speeds.
Werramat by Carl Zeiss Jena is an example of a camera that spots two leaf shutters in its lens assembly. Its central leaf shutter is responsible for the camera’s above-average 1/750 top speed; however, cocking it causes the blades to briefly open and close as you wind the film. Werramat’s second leaf shutter located closer to the film plane remains closed during film advance, preventing unwanted exposures.
Curiously, some cameras use their leaf shutter in place of aperture. For example, Polaroid SX-70 does this by opening its blades to the maximum iris size of the set aperture before shutting again.
✪ Note: A “leaf” shutter is a name based on the key component of the device, while “focal-plane” describes the shutter’s location. You could technically have a leaf shutter that’s close to the focal plane or a rolling-blind shutter near the lens’ centre. However, neither of those designs are common.
Leaf shutters’ advantages.
In contrast to focal-plane shutters, leaf shutters can synchronize with flash at any speed. They tend to be quieter and produce virtually no camera shake, which means that you can take photographs with longer exposures without a tripod. Leaf shutters can also be made more compact, as is the case with the tiny Minolta TC-1.
Leaf shutters’ disadvantages.
Most leaf shutters will not fire faster than 1/500th of a second. While there are some exceptions to this rule, namely the above-mentioned Minolta TC-1, Minolta V2, and a few Hasselblad lenses, there are often drawbacks to their designs that require shooting at smaller apertures (since the shutter blades do not have enough time to fully open).
Cameras that use interchangeable lenses will often have individual leaf shutters in each lens, making the system considerably more expensive to manufacture.
Alternatively, leaf shutters may be placed in the camera body near the film plane; however, that limits the range of possible focal lengths and the maximum aperture. Part of the reason for this limitation is the fact that at higher shutter speeds, the center of the frame is exposed for a relatively longer time than its sides which may cause vignetting. Keeping the shutter close to the aperture within the lens prevents this distortion since a smaller opening at that level only creates a larger depth of field.
The amount to which leaf shutters affect depth of field or image quality at higher speeds is unclear. Although, some suggest that there’s a perceptible and favourable quality over focal-plane shutters. I haven’t had the chance to verify this claim.
Choosing your shutter type.
While there are cases when a certain shutter design performs better than the other, your choice between the two may not be as important as your understanding of the limitations of your system. Available lenses, camera controls, and ergonomics usually play a greater role in the camera’s practicality and appeal.
And if you end up developing a preference for either kind (mine’s the leaf-type), there are clever designs out there that can help you overcome some of the inherent drawbacks. Like the balanced modern focal-plane shutters that prevent camera shake or the ultra-fast leaf shutters.