Last week, I mentioned that the right exposure is all about the intensity, amount, and duration of light. We’ve already discussed how Aperture and ISO affect your shooting, so this week, we’ll take up Shutter Priority.
There are several types of shutter, but regardless of how they work, they’re all meant to do the same thing. The shutter’s a bit like a set of little doors built into your camera body. If the aperture is like your iris, the shutter is your camera’s eyelid; keep it open longer, and more light is reaching your retina. Similarly, the shutter will remain open for lengths of time ranging from 1/8000th of a second to several minutes or hours (as is commonly done in astrophotography) controlling how much light will hit your camera’s film or sensor.
If you’re using Shutter Priority (usually denoted by an “S” on your control knob), you’ve chosen to control the shutter, and let the camera meter the scene, guess at aperture, and possibly also adjust your ISO, depending on your settings.
When might it be better to shoot in Shutter priority rather than Aperture? For one thing, if you’re not particular about your depth of field (deep, shallow, it’s all the same to you as long as you’ve got a decent exposure at the end of it). For another, shutter speed comes very much into play if you’re photographing in extreme lighting conditions. If you’re shooting in bright, direct sunlight, for instance, you may want or need a faster shutter speed than your camera’s Auto setting would choose under those same circumstances, so as not to overexpose your picture. On the other hand, if you’re shooting in low light, using a longer shutter speed means not having to use a higher ISO, thereby preserving details in your photo and keeping noise levels down.*
Shutter speed can also have artistic uses, sometimes creating a sense of motion (for instance, showing light trails from fireworks, doing a light painting, blurring a moving object against a sharp background, or vice versa), and other times stopping it dead in its tracks. These can be very individual decisions, and can also vary based on what, and when, you’re shooting.
Shutter priority is useful for far more practical reasons as well. While you can always get more light to the sensor by opening your aperture, no matter what lens you’re using, at some point you’ll have opened it as far as it can go. Sometimes you can’t go any farther for artistic reasons; a lens that opens to f/1.4 will usually give you good bokeh and a nicely blurred background, but will also give you a razor-thin focal plane when sometimes you need, or want, either the focal distance or the depth of field. Other times you just can’t go any further for technical reasons; a lens with a maximum aperture of f/3.5 isn’t going to open to 2.8 simply because we’d like it to. You can, however, control the duration of light, so what you’ve gained or lost in f/stops, you can easily balance by changing your shutter speed.
As we did last week with Aperture Priority, let’s take a look at what the camera’s doing when you shoot in Shutter Priority.
In Figure 1, I wanted to show the motion of the cars as they passed me, so I used a shutter speed of 1/10. I’m not too worried about camera shake, since the subject’s not even going to be in the same ZIP code as “sharp.” It’s a bright, sunny day, so the camera should stop the aperture down… and it does, to f/25. Notice that even at ISO 100 (my camera’s base ISO), the photo comes out washed out. This is one instance in which you’d generally want to stop down, resort to exposure compensation, or use a polarizing filter if you have one.** In Figure 2, I wanted a similar feeling of motion, but this time, I’m shooting in the shade, so the camera should stop the aperture up. This time I’m using a 1/8 shutter speed, and the camera’s giving me an aperture of f/20 with a much more even exposure.
Figures 3 and 4 are a bit different, in that I didn’t particularly care about capturing, or stopping, motion. I just wanted a reasonably well-exposed photo. So in Figure 3, I’ve used a comparatively speedy 1/200 shutter speed, and the aperture the camera’s chosen is f/10. If you recall last week (Aperture Priority), you’ll notice that the camera tries to balance the settings off one another as you change them; if you want to exercise a bit more control when shooting in A or S, you almost have to think like the camera. So if I want a bit more depth of field (at this distance f/10 really should suffice, but play along for a minute), all I need to do is slow the shutter speed; the camera will stop the aperture down to compensate.*** If I want less, I do the opposite. But since I want more in this instance, I’ve chosen a slower 1/30, and the camera has obliged me with f/14, which gets me a bit more DOF.
Not so bad, was it? As always, check your photos as you shoot so you can make adjustments on the fly as you need to.
*Long shutter speeds can also be a source of image noise, which is why some cameras have long exposure noise reduction. However, comparing the noise from a long exposure to that from shooting at a very high ISO generally shows that longer exposures are by far the lesser of two evils.
**A polarizing filter, besides being useful for cutting glare and giving you lovely blue skies, can also be adjusted to vary the amount of light that’s reaching your camera’s sensor by a stop or more.
***There are exceptions. Because any medium, whether it’s film or a digital sensor, can only capture images within a certain range of light, at some point, you run into what’s called “reciprocity failure.” In plain English, the amount of light you’re letting hit the medium is either too high or too low for that medium’s dynamic range, so all you’re going to see is something terribly under- or over-exposed, if you’re lucky to see anything besides just a solid black or white square. If you’re operating around the camera’s limits, it will generally let you know; you’ll need to read the manual to see how and where it will show you that warning.
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