Typically represented by a capital A (or sometimes Av, short for Aperture Value) on the camera mode dial, aperture priority allows the photographer to dial in this specific exposure setting—the ƒ-stop—and asks the camera to calculate the correct corresponding shutter speed in the instant before the shutter is released. Before we get ahead of ourselves, let’s back up just a moment for a better understanding of how aperture priority mode works.
A camera has three primary exposure modifiers: aperture, shutter speed and ISO. The aperture (also called ƒ-stop) is the size of the opening in the lens, which modifies the amount of light that’s let into the camera. The shutter speed modifies the duration that light is let into the camera. And the ISO (or film speed, back in the old days) represents how sensitive to light the sensor will be. A higher ISO is more sensitive to light (and produces more noise), while a lower ISO is less light sensitive but produces a cleaner signal and better image quality. These days, though, sensor technology is so good that even high ISOs still look great.
It’s by adjusting these three settings in combination that a proper exposure is established. If you allow in less light with a smaller aperture, you’ll need to balance it with more light from a longer shutter speed. Add to one, take away from the other. Simple, right?
Well, sometimes it’s not so simple, particularly in situations in which the light is changing. This could be when I’m on the move, photographing in full sun then moments later in shade. Or when I’m photographing indoors and then quickly I’m outside again. These changing situations make automatic exposure modes much more convenient than continuously recalculating the correct manual exposure. But rather than turning over all the control to the camera, modes such as Aperture Priority and Shutter Priority allow the photographer to retain manual control over one specific setting. In this case, it’s Aperture Priority, where the photographer sets the ƒ-stop and the camera calculates the correct shutter speed to accompany it.
When is Aperture Priority mode particularly helpful? In situations where the photographer wants to set a specific depth of field and have that take priority over the shutter speed. Or, somewhat counter-intuitively, when the shutter speed is crucial in a slightly different way. More on that in a moment.
When I’m photographing people, I often want a shallow depth of field in order to put the focus on the subject and let the background fall out of focus. In these cases, I dial in a fast ƒ/2 aperture via Aperture Priority, and the camera will choose the appropriate shutter speed to accompany it. (Alternatively, if I wanted maximum depth of field, I could set ƒ/32 in Aperture Priority mode and the camera would choose a slow enough shutter speed to allow for it.)
With ƒ/2 dialed in and the camera set on Aperture Priority mode, I can walk around from light to dark and the camera will compensate by slowing the shutter speed to, say, 1/30th when I’m in low light, and speeding it up to 1/500th if I’m in brighter light. All the while, the ƒ/2 aperture remains constant, and my exposures will be accurate.
Another instance in which Aperture Priority mode makes sense is when a travel or landscape photographer wants to create lots of sharp image area through greater depth of field. They can dial in a tiny aperture such as ƒ/22, and in Aperture Priority the camera will determine which shutter speed will produce the correct exposure. Be warned, though, that in this case, a tripod may be necessary if low light requires a long shutter speed that’s too long to handhold.
What about the counterintuitive approach I mentioned? Here’s how that works.
Let’s say you’re photographing a baseball game and the action is fast-moving. You probably want to use the fastest shutter speed possible, no? Well if the sun is going in and out from behind the clouds, or if part of the field is in shade, you can use Aperture Priority to account for that. By choosing the fastest aperture available on your lens (let’s say it’s ƒ/2) via Aperture Priority mode, you’ve effectively ensured that your camera will choose the fastest shutter speed possible for whatever light you encounter.
Why not just use Shutter Priority in that setting? It would work fine, but you might sometimes sacrifice a bit of speed or inadvertently underexpose. For instance, if you dial in 1/500th as the Shutter Priority shutter speed, the camera might stop down to ƒ/4 in bright sunlight—effectively “wasting” two stops of speed on that ƒ/4 aperture. Had you set ƒ/2 in Aperture Priority, that same lighting would have enabled an even faster 1/2000th shutter speed.
And if you had set the shutter speed to 1/2000th in Shutter Priority, when the light dips a bit the camera might open the lens all the way to ƒ/2, but if it still isn’t enough you’ll end up with an underexposed frame. Thinking outside the box though, you can prioritize the fastest possible shutter speed by thoughtfully using Aperture Priority.
Check back in the coming weeks for a continued look at exposure modes, including Shutter Priority.