Aperture and Shutter Priority

The Missing Pages column is a collection of all of the information that should have been included in your camera’s Owner’s Manual—but somehow got left out. This is a hybrid assortment of short articles that delivers the know-how you need to derive the maximum enjoyment—and creative expression—from your equipment.

It’s sort of a juiced-up User Guide for creative people who are not necessarily technical. Each part will teach you how to use one of the camera features or functions that you previously ignored or left set on Auto. And each will include a Creative Project so that you can try some scripted experimentation.

We will explain complex technical subject matter a way that everyone can understand. And if you happen to be a technical expert yourself, we’re including “Nerds Only” sidebars just for you. That way you can dig in deep—or just straddle the edges—of the technological stuff. It’s your decision.

Installment III: Aperture & Shutter Priority

Definition: exposure modes that provide more control than Program AE but less than Manual mode.

Shutter Priority is an auto exposure system in which the camera achieves correct exposure by selecting the aperture after the user has selected the shutter speed.

Conversely, Aperture Priority is an auto exposure system in which the camera achieves correct exposure by selecting the shutter speed after the user has selected the aperture.

Each system has its own set of distinct advantages and disadvantages. But to fully understand Aperture Priority and Shutter Priority you must understand the fundamentals of exposure. Please refer to the preceding segment (Program AE) if you need a quick review.

Today’s digital SLR cameras (and a few high-end compact models like the Canon PowerShot G10) offer both Aperture Priority and Shutter Priority automatic modes in addition to Program AE. There was a time, however, back during the era of Canon AE-1 popularity, that 35mm film cameras offered one autoexposure system or the other. And each method had its own passionate group of proponents—and detractors.

The Canon (Shutter Priority) crowd argued that it was better to set the camera’s shutter speed and allow the camera to respond by setting the appropriate f/stop. They claimed—and rightfully so—that by choosing a fast shutter speed it was easier to control camera shake and to stop subject movement. Minolta, Nikon and Pentax (Aperture Priority) followers insisted that setting the aperture was the preferred procedure because it gave the user full control over the depth-of-field and allowed the user to preselect the optimum aperture for any lens.

Both sides were correct, of course, each in their own way. In retrospect, this issue was one of the most polarizing disputes among camera enthusiasts, no pun intended. One might have expected a different outcome to this hotly debated argument, but the final result was the development of Program cameras that set BOTH the shutter speed and aperture, essentially leaving the user out of the equation almost entirely. Better cameras offered all three: A mode, S mode and Program AE. The Minolta XD-11, which debuted in 1977, was the first camera that could claim “Dual Mode” by offering both A and S modes.

Use Aperture Priority to control the background.

Aperture Priority

Sometimes called “Aperture Preferred” because that term more accurately describes what’s in the user’s mind when they make decisions about the camera settings; and labeled “Av” (Aperture Value) by Canon and others; Aperture Priority allows the user to control depth-of-field (that zone of sharpness that extends in front of and behind the actual point of focus). Because it’s often indicated by an “A” on the control dial, some people mistake the setting for “Automatic.”

Having control over the DOF is very important in more situations than not. When shooting portraits, for example, it’s desirable to force the background to blur by using a large aperture. This effect separates the subject from what’s behind them and gives the appearance of accentuated sharpness. It also allows you to deemphasize distracting backgrounds by making them fuzzy and indistinguishable. Controlling DOF is similarly important when you want a larger zone in focus (greater DOF), such as when shooting close-ups or certain landscape scenes.

Controlling the aperture allows you to select the “sweet f/stop” that delivers the sharpest images. There are rare exceptions, but most lenses perform better when stopped down two or three f/stops. A 50mm f/1.4 Nikkor, for example, will yield better results at f/4 than it will at f/1.4. And contrary to intuitive logic, using an f/stop like f/22 that’s quite small introduces diffraction, which robs the image of sharpness, despite the extended depth-of-field.

Use Shutter Priority (and a high shutter speed) to stop action.

Shutter Priority

Known as “Tv” to Canon (for Time Value), there’s a lot to be said about the efficacy of this system—particularly in light of the fact that all camera makers recognize the detrimental effects of camera shake and are moving toward some sort of Image Stabilization to reduce it.

In S Mode, you set the shutter speed and the camera does the rest. Well, not quite. The argument that the Aperture Preferred crowd often made was this: if you set a shutter speed that’s too high (1/250s indoors, for example) you’re likely to get no picture at all. As a countermeasure, many cameras will warn you and then shift to a slower speed when it’s impossible to achieve correct exposure at the speed you selected. And some will even adjust ISO (although that’s really a case of relinquishing control over the final outcome to the camera and hoping for the best).

Controlling the shutter speed means you can help prevent camera shake and more importantly, freeze subject movement. If you’re shooting a track and field event, for example, or a little league game, you can use a very fast shutter speed to stop motion.

This theory works well indoors, too. For any given lens you should know the minimum shutter speed you can successfully handhold without shake. I call this the “SBA” or “Shake Begins At” shutter speed (ungrammatical but easy to remember). With a 50mm lens equivalent, for example, most people can shoot at 1/60 of a sec or so. General rule of thumb is “1 over the focal length” as in the fraction 1/focal length. So the equivalent to a 135mm lens would require a shutter speed of about 1/125 or higher for safe, jiggle-free results. (Remember that this old rule came to be before digital, so you must account for the lens multiplier when calculating.) In Shutter Priority you can set the camera at your SBA and blaze away with little fear of camera shake.

A Mode allows you to control Depth of Field.

Creative Project

Do you have a favorite lens? One that always seems to yield the sharpest images? With that go-to lens in mind, what range of apertures delivers the absolute best performance? You really need to know. For the sake of having an example, let’s say you’re a Nikon D90 shooter and your favorite lens is the 50mm f/1.4. (Not a bad choice, by the way, in that it translates to a 75mm f/1.4 on your DSLR, which gives you a nearly perfect portrait lens).

Exercise 1

Get out the tripod, find a subject with detailed texture, and set up ten to fifteen feet away. If you can’t think of anything better, aim your camera at a painting that’s hanging on a neutral wall. Shift your camera into Aperture Priority mode and begin shooting at the largest f/stop, working your way down to the smallest. How do the images in the series compare in terms of sharpness?

You may find that your results resemble these: Wide open at f/1.4 you get good results, but stopped down to f/4 or f/4.5 (the half stop between f/4 and f/5.6) the lens really shines. Sharpness remains until around f/16 where diffraction takes over and the results—while still acceptable—are not as biting-sharp as at f/5.6 or f/8.

Exercise 2

For the second exercise, position a human subject five feet from the painting and ten feet from the camera. Repeat the exercise above, keeping the camera focused on the human. This time when you examine the results, closely look at how the subject becomes more separated from the background as depth-of-field decreases.

Exercise 3

Don’t freeze the action all of the time. Movement can add dramatic emphasis to the dynamic flow of a river, for example. You’ll need the tripod again. If you don’t have a gushing waterfall handy, moving traffic (especially at night), frenetic musicians (especially during the wee hours) or typical preschoolers (after a sugary snack) will provide enough subject movement to make this test worthwhile. In S mode, select a very slow shutter speed—one-half second or longer, unless it’s so bright that you’ll overexpose. Make sequential exposures at progressively faster speeds until you reach the speed that completely stops all subject movement. Then compare the results in your favorite image editor.

What about M mode? M stands for Manual, of course. That’s a great Creative Project but that’s for another episode!

For Nerds Only

What’s Pi got to do with f/stops? And why is f/2 twice as big as f/2.8 and four times larger than f/4? Shouldn’t bigger numbers mean bigger apertures?

Shutter speeds are pretty easy to understand. 1/100 of a second is obviously half as long as 1/50 of a second. But when it comes to f/stops nothing seems to make sense. That’s because an f/stop is a fraction. It’s the ratio between the lens’s focal length and the diameter of the active aperture. For example, a 50mm lens that is set at f/2 has an aperture that is 25mm in diameter. Similarly, a 50mm lens that is set at an aperture that is 8.9mm in diameter is said to be set at f/5.6 (i.e., 50/8.9 = 5.6).

Remember high school algebra? I’m sure many photo.net readers will want to jump in here and untangle my amateur math, but here’s what I remember. To calculate the area of a circle one multiplies the radius times the radius times pi. Or in other words, the radius squared multiplied by the constant 3.14159.

If you’re still with me, look at the following table.

For a 50mm lens:

So, it’s (fairly) easy to see why f/2 is about twice as large as f/2.8. The area is about twice as large. As we go down the scale, each increment of one f/stop equals a 2X change in exposure because the area becomes half as large. That is to say, each subsequent f/stop is one-half as large as the preceding f/stop. Half of a half is a quarter, so the difference between f/4 and f/8 is two stops, or 4X.

Why use such a goofy system? Because it can be applied to a lens of any focal length. For the purposes of calculating exposure, a 135mm lens set at f/5.6 delivers exactly the same amount of light as a 300mm lens at f/5.6. Were it not so, it would be virtually impossible to base camera exposure settings on an external handheld light meter.