Understanding Exposure, Part 3: Shutter Speed

Shutter speed is a measurement of time that a camera's shutter is open—allowing light, usually after it has passed through a lens and through the aperture diaphragm, to strike a photosensitive surface, like film or a digital sensor.

This article is part of a multi-part series of about photographic Exposure.
1. Introduction: The Exposure Triangle
2. Aperture
3. Shutter Speed
4. ISO

Unlike the nuances of aperture and its relation to light and optics, shutter speed is much more of a straightforward affair. Because of this, permit me to take you off on a bit of a tangent so that we can talk about how camera shutters work.

Photographs © Todd Vorenkamp

The challenge of the mechanics of the shutter is in designing a device that exposes the entire sensor or film plane to an equal number of photons so that exposure is consistent through the image. Many people think that the shutter works like a miniature garage door. If you can visualize a garage door opening and then closing in front of a photosensitive surface, you can see that the problem with that design is that the bottom of the image will be exposed to more photons of light as it is the first portion of the image to see light as the door begins to open and it is also the last part to be blocked by the door as the door closes. Therefore, a photo taken with a shutter that functions like a door will have an unbalanced exposure.

There have been several different designs of camera shutters over the years. The very first cameras had lens-cap-type shutters where the lens was exposed to light by removing the cap and then, after a calculated amount of time, the cover was placed back over the lens. Exposures back then took several minutes, and even hours, so the relatively slow opening and closing of the "shutter" was not problematic.

At the inexpensive end of the spectrum, the simple leaf shutter appears on many disposable and point-and-shoot cameras of yesterday. This is a mechanized version of the old lens cap "shutter" where a leaf, or two leaves, are mechanically pulled aside to let light through an opening. They generally only operate at one speed.

The leaf shutter, also known as a diaphragm shutter, functions very much like the aperture diaphragm of the lens in that a group of metal blades is mechanically opened and closed in rapid fashion. Unlike the blades of the aperture diaphragm that just constrict to a small opening, these blades close all the way until there is no light coming through the shutter. The shutters are designed to open and close extremely fast so that the center of the image does not see an appreciably greater amount of light than the edges. Because of their design, leaf shutters work very well when synchronized with flash strobes, but cannot operate at speeds as fast as the shutter type that we will discuss next.

Almost all modern SLR and DSLR cameras employ the focal-plane shutter. The focal-plane shutter works more like a garage door, but with a second door, called a curtain, that follows behind the first curtain to close the image sensor or film off from light after the first curtain opens. This two-curtain design allows a balance of light across the sensor or film so that exposure is even. This design allows for extremely fast operation, up to 1/8000 of a second, on today's SLR and DSLR cameras. However, focal-plane shutters, due to their complexity and exotic construction, are relatively expensive.

Like most things, the shutter has gone digital. Many modern digital cameras operate an electronic shutter that simply powers the digital sensor on for a selected amount of time. Because there is no mechanical function, electronic shutter speeds can be extremely fast. A global shutter turns the entire sensor on and off at once, while a rolling shutter activates one row of pixels at a time across its width.

Both the focal-plane shutter and rolling electronic shutter, because of their design and function, can cause interesting distortion through an image when there is fast motion across the picture plane.

OK, back to shutter speeds. As I wrote earlier, shutter speed is simply a measure of time that the shutter is open, or, in the case of the electronic shutter, the sensor is powered. The longer the shutter is open, the more light comes through. No inverse square law here (thank goodness!); if you double the amount of time that the shutter is open, you will double the amount of light coming in. Shutter speed is simple, right?

How does this relate to exposure and the exposure values (EVs) that we discussed in the first lesson? Because of the linear nature of the relationship of photons entering the camera to shutter speed, we can use shutter speed to easily and precisely change the amount of light hitting our photosensitive surface. By slowing the shutter speed, from 1/30th of a second to 1/15th of a second, for instance, we will double the amount of light passing through the shutter. This doubling of light is identical to the doubling of light accomplished by opening aperture, albeit by a different mechanical function, and represents a +1 EV shift. Changing the shutter speed from 1/2000th of a second to 1/4000th of a second then halves the amount of light coming through the shutter and represents a -1 EV shift.

Shutter speeds are listed as whole seconds or as fractions of a second. The maximum shutter speed for most SLR cameras is 30 seconds. If you want to take a photo longer than 30 seconds, you can use the bulb (B) or time (T) functions of the camera, if it is equipped with a specially built shutter release that includes a timer. The bulb function opens the shutter as long as the shutter release is depressed. The time function opens at the first press and closes the shutter at the second press of the shutter release.

Just like with aperture and ISO, there are some "side effects" of shutter speed.

Slow shutter speeds allow movement to be imaged across the photograph. This is not just movement of the subject or subjects, but also of the camera too. It is nearly impossible to hold a camera perfectly steady for any length of time, especially for a few seconds. Therefore, an image taken with a handheld camera that covers any length of time will have blur from camera shake. Definitely keep this in mind when shooting with slow(er) shutter speeds. The other motion the camera sees is movement in the frame. A slow shutter speed will allow moving cars, runners, animals, etc., to change position inside the duration of the image. This movement will show on the photograph as motion blur and can be a really great creative element in a photograph. Another creative technique for slow shutter speeds is intentional panning of the camera, or even random camera movements, while the shutter is open.

Low-light and nighttime photography dictate a slow shutter speed to allow enough photons to reach the sensor or film. The camera is usually stabilized by a tripod, and shutter speeds of great lengths can capture images in very dark places or even trace the stars streaking across the night sky as the Earth rotates.

Fast shutter speeds are used in the opposite fashion—to freeze action versus letting it blur across a photograph. Speeding cars, diving swimmers, racing animals, mischievous kids, and more can all be frozen in time with a fast shutter speed.

Ready for more math? The two biggest factors in how movement is "seen" by the camera are: speed and distance.

If you stand on a sidewalk and stare straight ahead and a car goes past you at 40 miles per hour, a few feet in front of you, it will go through your field of view in a very short amount of time—probably a fraction of a second. If you look out to the horizon and another car going 40 miles per hour is passing you, 4 miles into the distance, you will see that car, going the same speed as the one before it, for a much longer amount of time.

The camera's limited field of view works the same way if the subject is moving or if the camera is moving. If you have ever taken a photo of a distant object from a speeding vehicle, you have seen the same type of effect in your image; the distant snow-capped mountain is clear and sharp, but the fences and fields and Armco next to the road are blurry from the motion. So, applying mathematics once again, we can crunch numbers to help us get the shutter speed effects we want in our images. When you double the distance from the camera to the moving subject, you will half its speed through the frame. Therefore, to get the same blur, you can use half the shutter speed. Conversely, if your moving subject is the same distance away each time you photograph it, but you double the speed of the subject, you will have to halve the speed of your shutter to get the same amount of blur.

Our discussion on shutter speed rounds out the physical camera adjustments that control the amount of light that gets to the sensor or film. In the final segment, Understanding ISO, we will discuss ISO and then bring all three adjustments together to show how they work interchangeably to control exposure.