Aside from the aperture, another exposure control is shutter speed. We’ve already covered what shutters are and how they work. Now we’ll discuss how shutter speed affects the photograph.
The image to the left shows a typical shutter speed selector dial. The unit of measure used when talking about shutter speeds is in seconds (or fractions thereof). In this example, the fastest shutter speed is 1/1000 of a second (denoted by the 1000). The white colored numbers (including the red one marked “125“) denote fractions of a second (e.g. 1/500 of a second, 1/250 of a second, 1/125 of a second, and so on). Whereas the orage colored ones are values in seconds (e.g. 1 = 1 second, 2 = 2 seconds, and so on).
Some cameras, like the example here, have a bulb mode (denoted be a letter B) setting on the dial. When the shutter speed is set to bulb mode (B), the shutter remains open as long as the photographer presses the shutter release button. When the photographer releases the shutter release button, the shutter closes, thus ending the exposure. This feature is usually employed in astrophotography.
Digital SLRs today don’t have shutter speed dials (like the one above). They have the display on the LCD back panel, and through the viewfinder. You change shutter speeds using a smaller (unmarked) dial. Below is the back panel of the Pentax K-x showing the adjustment dial and the shutter speed display. For the Pentax K-x, the adjustment dial functions in an ubiquitous role: providing adjustment function for the current active mode (whichever is highlighted on the LCD panel). This offers a simplicity in design. If you’re in Aperture Priority (Av) mode, the adjustment dial functions as an aperture ring. If you’re in Shutter Priority (Tv), it functions as a shutter speed dial. If you’re in Sensitivity Priority (Sv), it functions as an ASA/ISO setting dial. If you’re in preview mode, it functions as a zoom-in/zoom-out dial.
Controlling Image Brightness
Shutter speed, just like aperture, also controls the amount of light falling on the film or image sensor, albeit in a different way. Aperture controls the amount of light by varying the opening of the iris diaphragm; i.e. the same way window blinds allow light to enter through a window–by adjusting the opening, you control the amount of light coming in. On the other hand, the shutter speed controls the amount of light by varying the length of time the shutter remains open to expose the film or image sensor records the image. It’s just like cooking steak: if you cook it too short a time you’ll end up a “rare” steak; cook it just enough, you’ll end up with a “medium rare” steak; if you cook it a little longer wou’ll end up with a steak that’s “well done”; and, if you cook it too much, you’ll end up with a “charred” steak—no good at all (unless that’s what you prefer anyway)!
The following photos show the effect of shutter speed on the brightness of the scene. The ISO setting (400) and the aperture setting (F/32) are constant; only the shutter speed was changed. In the first shot, the shutter speed was 1/750 of a second. In the second shot, the shutter speed was 1/500 of a second. The image was brighter right? At 1/350 of a second, the image was brighter still, right? You can see more of the clouds and more foreground details. In the fourth shot, at 1/250 of a second, again the image was brighter than the previous one; the sky was brighter, and you can see more foreground details. At 1/180 of a second (the 5th shot), the sky is a little washed out, and foreground details are more visible.
In taking a picture, if the light meter displays a negative number (i.e. the image is underexposed), and you retain your ASA/ISO rating and aperture (F-stop) value. You can decrease your shutter speed until the light meter displays either a zero (0) or +0.5, or until you get the image you were hoping for. Some photos look great because they are underexposed (e.g. silhouettes). Some photos look better a little overexposed (shadowed areas are clearer).
The photo above (left) was purposely underexposed to create a silhouette. The other one (above, right) was taken with a 20-second exposure to gather as much of the available light to illuminate the blades of grass all around and the rain clouds above.
The second effect of shutter speed is that it freezes action. The following pictures illustrates how motion is captured in mid-action.
In both instances, the shutter speed was fast enough to freeze the action in mid-stride, and no blurring occurred. Notice that the water splash was also frozen to make individual droplets distinct.
The third effect of shutter speed is that it captures the essense of motion. If the object is moving faster relative to the shutter speed, the moving object registers as a blur. To freeze the action, you need to increase the shutter speed.
If you look at the images below, you’ll notice that as the shutter speed increases, the falling water progresses from blurry to sharp. At 1/10 of a second, the falling water appear like streaks. At 1/40 of a second, the streaks appear less, but more obvious is the difference between how the splash at the bottom looks like. At 1/10 of a second, it’s hardly captured. While on the second photo, at 1/40 of a second, the splash is more pronounced. At 1/160 of a second (third photo), the water rivulets are more apparent. Even the splash at the bottom shows more movement. At 1/320 of a second (fourth photo), the falling rivulets are more discrete (more distinct), and the splash at the bottom show the rippling effect.
In the examples below, a long exposure captured the headlights and tail lights of passing vehicles.
Notice that only the moving objects registered as streaks of light in these long exposures. The stationary objects are clearly defined.