As you’ve previously learned, the primary role of the shutter is to control the length of time that light is permitted to pass through the lens to the image sensor. Beyond that, the shutter controls a critical aesthetic element, the perception of movement in photography.
Shutter Priority auto-exposure mode, Bulb, and Time
Shutter Priority mode is an automatic exposure mode in which the photographer selects the desired shutter speed, and the camera attempts to achieve optimal exposure by varying the aperture. Shutter Priority mode is commonly indicated as S or Tv (for time value) on most cameras’ mode dials. Shutter Priority mode is different from other automatic exposure modes because it allows photographers to control the perception of motion, either by freezing movement, showing movement, or minimizing camera shake.
Your camera may offer two additional shutter-specific functions known as Bulb and Time. Pressing the shutter button in Bulb mode (often labelled B) will activate the shutter and keep it open for as long as the button remains pressed. Releasing the button will terminate the exposure. Pressing the shutter button once in Time mode (often labelled T) will open the shutter, and pressing it again will close the shutter. Both Bulb and Time modes are intended for long exposures. They are often used with wired or wireless remote shutter releases to avoid vibrations induced by touching the camera during exposure.
Focal plane shutters
As the name suggests, the focal plane shutter is positioned just in front of the plane of the image sensor. In modern cameras, focal plane shutters consist of two separate metal “curtains” (better described as overlapping blades) that travel vertically during exposure. When primed for exposure, the top (“second”) curtain is in a retracted position above the frame, and the bottom (“first”) curtain is fully extended upward, blocking light from the image sensor. Upon full depression of the shutter button, the image sensor activates, and the first curtain retracts down to uncover it; after a precise interval, the second curtain extends down to terminate the exposure, and the image sensor deactivates.
The process is slightly different in mirrorless cameras because their electronic viewfinders and displays receive a real-time video feed from the image sensor in place of a DSLR’s optical viewfinder. When taking a picture, the camera momentarily closes, or “primes,” the shutter before engaging as per the description above.
In both styles of cameras, the shutter curtains accelerate to a constant velocity regardless of the shutter speed. At slower shutter speeds, the first curtain retracts to uncover the image sensor fully, and after a precise interval, the second curtain closes. Beyond a certain threshold, faster shutter speeds are achieved by timing the second curtain to start closing before the first curtain has fully uncovered the image sensor. The horizontal slit formed between the two curtains exposes the image sensor as it travels across its surface. Increasing the shutter speed narrows the slit by reducing the interval between the first curtain starting to open and the second curtain starting to close. On modern cameras, the precise engagement and fast movement of the shutter curtains can form very narrow slits that achieve exposures of 1/8000 seconds.
There are three main disadvantages to focal plane shutters. First, they typically have a hard limit for flash synchronization. The fastest shutter speed at which an image sensor is completely uncovered is known as its X-sync or flash synchronization speed. The fastest available X-sync speed on modern cameras is typically 1/250 second. Using a flash at faster shutter speeds results in uneven illumination of the image sensor as it’s being exposed by a travelling slit. Cameras with physical shutter speed dials often mark the X-sync speed with an “X.”
Secondly, focal plane shutters suffer from a phenomenon known as shutter shock, which are the minute high-frequency vibrations created by the shutter curtains as they accelerate into motion and decelerate to a stop. In DSLRs, the effect is exaggerated by the mirror’s slap, which is the vibrations induced by the speedy raising and lowering of the mirror that gives SLRs their distinct sound. While mirrorless cameras don’t experience mirror slap, they suffer from extended shutter shock. As a rule of thumb, if you can feel the vibrations through your hand, they have the potential to affect your image below a specific shutter speed.
Lastly, focal plane shutters may induce an image distortion known as a rolling shutter when photographing fast-moving subjects at shutter speeds faster than X-sync. Imagine shooting a fast-moving subject with a static camera. Exposure of the image sensor is accomplished through a narrow slit. As the narrow slit travels across the focal plane, it exposes minutely different instances of time in the subject’s progression within the frame. The resulting rolling shutter distortions vary depending on your subject’s direction of motion. When the subject’s movement within the frame is perpendicular to the shutter curtains, it may appear slanted; subjects moving in the direction of the shutter curtains will appear elongated or stretched; and, subjects moving in the opposite direction of the shutter curtains will appear foreshortened or compressed. These phenomena occur incredibly rarely in practical photography.
The leaf shutter, or central lens shutter, consists of several metal blades, or leaves, in a circular arrangement that is similar to an iris diaphragm. During exposure, the blades open and close very quickly. When opened, they retract to clear the aperture and allow proper exposure. When closed, the edges overlap to ensure that no light penetrates the assembly. In modern photography, leaf shutters are used predominantly in lenses designed for medium and large format cameras. They also appear in several fixed-lens consumer-oriented enthusiast cameras, such as the Sony DSC-RX1R II and Fujifilm X100F. There are several significant differences between leaf and focal plane shutters.
First, the leaf shutter is built directly into the lens and is located near the iris diaphragm. This adds to the mechanical complexity of a lens; lenses that feature leaf shutters are typically more expensive than similar lenses without them. Furthermore, a leaf shutter does not preclude a camera from having a focal plane shutter; a camera with a focal plane shutter can be fitted with a lens using a leaf shutter.
Two, leaf shutters fully expose the entire recording surface at all available shutter speeds. The practical benefit is that flash synchronization is available throughout the shutter speed range. Since modern medium format leaf shutters can attain exposure durations as fast as 1/2000 second, this gives them a tremendous advantage over camera systems with focal plane shutters with regards to flash photography.
One of the primary drawbacks of the leaf shutter design is that the central portion of the image sensor is exposed for longer than the edges, which causes both vignetting and distorted bokeh. The effect is hard to notice at slower shutter speeds, or when relying exclusively on flash for illumination (because the flash fires at the precise moment the shutter is fully open). It becomes increasingly prominent with faster shutter speeds and larger apertures, which aren’t recommended.
An electronic shutter is a function of an image sensor that allows it to activate and deactivate its reading of light over a set period of exposure without the aid of mechanical curtains or blades. For example, if your shutter speed is set to 1/100 second, the image sensor will read the light values striking its surface for 1/100 second. It activates upon pressing the shutter button and deactivates after the set period. Electronic shutters can attain incredibly fast exposures, with some cameras reaching speeds of 1/32,000 second! In addition, they’re totally silent, and, like leaf shutters, free from the vibrations induced by shutter shock. Unfortunately, currently available electronic shutters have several disadvantages: rolling shutter, absolutely no flash synchronization, and the potential for banding when used with high-frequency lights, such as fluorescent tubes and LEDs.
Electronic shutters exhibit a far more severe form of the rolling shutter effect than focal plane shutters. It’s a consequence of how CMOS image sensors (the most common type in use today) record picture information, and is most commonly experienced as the “Jell-O” effect in quick-panning digital videos. Unlike CCDs, CMOS image sensors cannot record the light values from every pixel on their surface simultaneous. Instead, the sensor activates a single horizontal line of pixels at a time until the entire sensor is recording light. After a precise interval equal to the set exposure duration, the image sensor follows the same sequential process to terminate exposure and record the data. This is a relatively slow process in comparison to the speed of mechanical shutters. The curtains of a focal plane shutter can travel across the span of the image sensor much faster than the sensor can cycle the exposure and capture of successive lines of pixels to completion, even though each individual line of pixels may be exposed for a shorter total duration. For this reason, digital cameras that use mechanical shutters (leaf or focal plane) will always activate the image sensor before the shutter curtains open and deactivate it after the shutter curtains have closed.
The remaining disadvantages of electronic shutters all stem from the rolling shutter effect. Most cameras disable the flash when shot with an electronic shutter because the results would be similar to using a focal plane shutter above the X-sync speed (see above). Lastly, at faster shutter speeds, electronic shutters may capture banding in the light cast by LEDs and fluorescent bulbs and tubes. Lights operate at the utility frequency of the AC current in your country. In North America, the AC frequency is 60 hz, which means that the power to the light cycles on and off sixty times per second. Unlike mid- to high-wattage tungsten bulbs, which retain enough heat through their off cycles so as not to show a noticeable light loss, LED and fluorescent lights lose and gain brightness sixty times per second. When taking a picture under such lighting conditions, the scene’s ambient light levels would fluctuate as the electronic shutter scans sequential lines of pixels. The resulting pattern, a gradual transition between light and dark across portions of the photograph lit by the light source, is known as banding.