A shutter is a mechanical device inside of a camera that exposes film (or an electronic sensor) to the light passing through the lens for a controlled period of time. Shutter speed primarily refers to a focal-plane shutter, found in professional-grade photo cameras, while shutter angle refers to a rotary shutter, found in motion picture cameras. No matter the type of shutter, the speed or angle of the shutter is the way in which the user controls the exposure time. The shutter speed / angle is not the same thing as frame rate, which is how many frames are exposed per second. The frame rate is the speed of the film. The shutter speed / angle is the exposure time.
A focal-plane shutter is used in film & digital SLR cameras
Photo cameras use a variety of shutter types. However cameras with interchangeable lenses primarily have focal-plane shutters. A focal-plane shutter is a shutter that is located directly in front of the film or sensor, in the focal plane. A focal-plane shutter is comprised of a front curtain and rear curtain. The curtains are separate from the mirror, which is commonly mistaken as the shutter. The mirror, which reflects the lens image to the viewfinder, must move out of the way before the shutter can expose the film.
To expose the film or sensor to light, the front curtain moves up and the rear curtain moves down. Once the image has been properly exposed, the front curtain closes, followed by the rear curtain. While one curtain can do the job for most exposures, two curtains are required for fast exposures, as the mechanics aren’t fast enough with just one curtain. So, for fast exposures, as the rear curtain is beginning to open, the front curtain is already beginning to close; which leaves only a small, horizontal “slit” for light to pass through.
Shutter speed is measured in fractions of a second
How fast a focal-plane shutter moves is measured by shutter speed. 1/60 shutter speed means the shutter exposed the film for 1/60th of a second. Slower shutter speeds, like 1/30, 1/20 or 1/10 expose the film for longer stretches of time. This allows for more light, but produces an image with motion blur if the subject is moving. Faster shutter speeds, like 1/120, 1/240 and upwards of 1/2000 allow less light, but effectively freeze motion. DSLRs that film video, measure exposure time by shutter speed, even though video-mode doesn’t utilize the physical shutter to expose the frames. For more on that, scroll down to the electronic shutter section.
Motion picture cameras use a rotary shutter
In a motion picture camera, each frame is exposed individually, just like a photo camera. This sequence of frames, shown at a fast enough rate, creates the illusion of a motion. For a motion picture (film) camera, the frame is exposed, then the film must advance through the gate to expose the next frame. In order to advanced the film, the shutter must be “closed”, allowing the film to advance without getting exposed to additional light. If the film advanced while exposed, the light would streak vertically along the film and blur the image.
Unlike a focal-plane shutter in photo cameras, a motion picture camera has a rotary shutter, which is a semi-circular mirror spinning in front of the film gate. A 180 degree (or half-circular) shutter, exposes the frame for half the time. As the shutter spins, it blocks the light from the lens, and the film advances. When the shutter spins out of the way, a new frame is exposed. Then the process repeats itself.
Shutter angle is measured in degrees
When we measure shutter angle, we’re measuring the slice of the rotary disc that exposes the film to light (like a slice of pie). “Normal” shutter speed is half the frame rate. Since a circle is 360 degrees, a “normal” shutter is 180 degrees (half a pie). So, if the film advances every 1/24th of a second, but is only exposed to light for half that time, it’s only being exposed for 1/48th of a second. This means that a 180 degree shutter angle is equivalent to 1/48 shutter speed. That’s why 180 degrees is the de-facto standard shutter angle for film shot at 24 fps. Below are a few conversions of shutter angle to shutter speed:
- 360 angle = 1/24 speed (not possible with film @24 fps)
- 180 angle = 1/48 speed
- 144 angle = 1/60 speed
- 90 angle = 1/96 speed
- 72 angle = 1/120 speed
- 45 angle = 1/198 speed
Unlike photography, it’s not common practice to change the “speed” of the shutter to compensate for exposure. Changing the angle of the shutter will drastically change the aesthetic of the film. The D-Day scene of Saving Private Ryan was shot with a narrow shutter angle to replicate the look of newsreel footage from the era. Additionally, cinematographers will film with a narrow shutter angle during action sequences to reduce motion blur.
Many digital cameras have an electronic shutter
Most digital cameras with CCD or CMOS sensors do not require a physical shutter to expose an image. The exposure time is still referred to as shutter speed or shutter angle, and the camera is said to have an “electronic shutter”, but there’s no actual shutter to speak of. Instead, the sensor converts an optical image to an electronic signal. Because there’s no film to advance, and the sensor can expose on or off at will, there’s no reason to block the light from the sensor with a physical shutter. Some CCD sensors still require a shutter, but CCDs are quickly becoming obsolete due to advancements in CMOS sensor technology. Sensors that do not require a shutter allow for exposure times equal to frame rates, something not possible if a shutter must close and open in-between frames.
While many DSLRs still have a mechanical, focal-plane shutter to assist in exposing for photos, the shutter opens during video-mode and an “electronic shutter” take over. Since a DSLR is primarily a photo camera, video-mode still refers to exposure time as shutter speed. Other shutter-less CMOS cameras refer to exposure time as shutter angle (which, as we know, is more appropriate for motion pictures). Either way, shutter angle and shutter speed mean the same thing in the digital realm, they’re just different language.