Shutter Speed

The second part of the tripartite exposure settings (other than sensitivity and aperture) is shutter speed. The shutter opens or shuts to let in light. The longer it remains open, the more light is let in. Shutter speed is generally expressed in seconds or, more often, in fractions of a second. A shutter speed is said to be "faster" than another shutter speed when it lets in less light.

The shutter can be mechanical, electronic, or a combination. Mechanical shutters are often designed as focal-plane or leaf types. Other types are also possible, but these two are the most common types. Most modern DSLRs use focal plane shutters. A focal plane shutter is a shutter that is attached near the focal plane (e.g., film or sensor) of the camera. Focal plane shutters normally have one or more curtains. In one common design, one shutter opens and a second shutter closes just behind it. The gap between the two moves at the selected shutter speed. This design enables camera manufacturers to provide very fast shutter speeds. For example, the fastest shutter speed on one of my first cameras was 1/250 second. Modern day DSLRs can provide shutter speeds of 1/4000 second or faster. Thus, although each shutter curtain may open and close slower than the fastest shutter speed, the gap is only visible by any position in the frame at the selected (e.g., fastest) shutter speed.

Shutter speed can affect how much blur you see in an image. When your subject is moving a faster shutter speed will cause the subject to appear still whereas a slower shutter speed will capture some motion blur. For example, in fast-moving sports, you may want a shutter speed of 1/250 second or faster. A human walker may only require 1/125 second. A stationary (e.g., still life or architctural) subject will let you any shutter speed you want because it isn't going anywhere. A second concern in blur is whether you are using a tripod. With good camera holding technique, most people require a shutter speed of 1/f seconds where f is the effective focal length of your lens. For example, if you are using a cropped frame sensor with a 70-200 mm lens with the lens set at 100 mm, then your effective focal length is approximately 100 mm and so you would want a shutter speed of no slower than 1/100 seconds. If your technique is not very good, you may want a faster shutter speed, such as 1/200 seconds. If your lens or camera has vibration reduction (also known as image stabilization; VR for Nikon and IS for Canon), then you may be able to get away with a slower shutter speed (e.g., 1/100 seconds with poor technique or even 1/50 seconds with good technique). I try to go with 1/f even with vibration reduction. If your camera is on a tripod, you don't need to worry about the 1/f formula at all.

Most cameras have a maximum flash sync speed (also known as x-sync speed). This is the fastest shutter speed the camera can use with a flash. If a faster shutter speed is used, the frame will be partially blocked by the moving shutter curtains. In automatic exposure modes, the camera probably will not let you select a faster shutter speed when the flash is turned on. In manual exposure modes, you need to make sure that you don't select a faster shutter speed unless you want a portion of the frame to be unaffected by the flash.

Sensitivity, aperture, and shutter speed are all equally important. Once we learn about aperture, we can discuss how we can select appropriate combinations based on what we are trying to accomplish.