Back to Basics: Aperture, Shutter Speed, and Depth of Field
By Tamia Nelson
August 6, 2013
Digital cameras have revolutionized the art of photography, completing a process of democratization that began with the debut of the Kodak Brownie, back in 1900. Now we are all photographers, and very little escapes the gaze of the unblinking eye. This has its downsides, of course. (Just ask former Congressman Anthony Weiner.) But there's good news, too. The Facebook billionaires certainly have reason to rejoice. And so do many former shutterbugs, who started snapping pictures again only after a long hiatus occasioned by film photography's high costs and uncertain rewards. Is there anyone over the age of 30 who can't remember the letdown that often accompanied his first glimpse at some newly processed rolls of film, knowing that each flawed print or spoiled slide was an opportunity lost, and likely lost forever? I doubt it.
But such disappointments are now history. The digital camera has freed amateur photographers from the tyranny of film, and ended their unhappy codependence on indifferent laboratories. It's also given Everyman (and Everywoman) the keys to the darkroom. Expert image editing is no longer the exclusive preserve of affluent professionals. It's available to all. Moreover, and despite the formidable complexity of the underlying technology, as reflected in owner's manuals that run to hundreds of pages, digital cameras need be no harder to use than their film predecessors. Many photographers simply tweak the dial to Automatic and shoot away, leaving all decisions about aperture and exposure to the camera's electronic brain. And more often than not, they're perfectly happy with the results.
That said, the digital revolution has taken its toll. Every revolution produces casualties, after all. Film photographers — at least serious film photographers — were forced to serve a protracted apprenticeship, in which they learned to manipulate shutter speed and aperture to control exposure and depth of field, choose films with appropriate ASAs (a measure of their sensitivity to light), and frame images properly. And they did this knowing they wouldn't see the results of their labors for days or even weeks. Only then would the correctness (or otherwise) of their decisions be apparent. Not surprisingly, therefore, film photographers got to know their cameras and lenses very well indeed. Most also acquired at least a rudimentary understanding of practical optics in the process, knowledge which continues to yield dividends in the new digital age.
Perhaps you escaped this enforced apprenticeship. Or maybe it happened so long ago that the lessons you learned are now lost to memory. In that case, why not join me in exploring some of the optical magic that underlies all photography, beginning with …
The Interplay Between Aperture and Shutter Speed
Photography is all about light. A camera is simply a light‑tight box with a single hole in it. The hole admits light in controlled amounts, and that light paints a picture on film or sensor. And how is the amount of light controlled? In either of two ways: (1) by enlarging or reducing the size of the hole (aperture) or (2) by limiting the length of time that it's left open to admit light (shutter speed). Now let's take a closer look, at the first of these:
Aperture. With the exception of the simplest cameras, every lens incorporates a diaphragm that can be adjusted to limit the effective size of the camera's window on the world. The size of that window — the aperture — is indicated by an ƒ‑number. The higher the ƒ‑number, the smaller the aperture.
Here's a peek inside my Pentax DA 18–55 mm f/3.5–5.6 AL II lens:
The shot on the left has us looking through the lens when its window is wide open (an aperture of ƒ/3.5). Compare this with the right‑hand shot, in which the window is shut down as far as it will go (ƒ/22).
Note that the DA lens is a zoom, allowing me to select any focal length between 18 mm (wide‑angle) and 55 mm ("medium" telephoto) with just a twist of the wrist. In the photos above, I've left the lens at 18 mm. Here the maximum aperture is ƒ/3.5. (At longer focal lengths the maximum aperture is smaller: ƒ/5.6.) The DA is not a particularly "bright" lens, as it happens, and that limits its potential uses somewhat, particularly in low‑light conditions and where effects such as bokeh are desired.
This is a good time to summarize the somewhat counterintuitive nomenclature for ƒ‑stop (another name for aperture):
- Lower ƒ‑numbers = Larger apertures
- Higher ƒ‑numbers = Smaller apertures
Small is large and large is small. What could be clearer than that? Fortunately, you soon get used to this. If you want to understand the logic that underpins the seeming contradiction, however, the Wikipedia entry on the subject is a good place to start.
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OK. So much for ƒ‑numbers. But I said there were two ways to control how much light reaches the sensor, didn't I? And that brings us to …
Shutter Speed. If aperture determines the size of your camera's window on the world, the camera's shutter is a little like the window shade. It doesn't matter how big your window is if the shade is drawn. You're in the dark, regardless. If you want to let the sun shine in, you've got to raise the shade. And that, in effect, is just what the shutter does. It lifts the shade in front of your camera's window, allowing light to enter and strike the sensor. Then, usually in less time than it takes you to blink an eye, it pulls the shade down again. This brief interval, the short time during which your camera's eye is open to the light, is known as the shutter speed.
You have a lot of latitude here. If you choose the so‑called Bulb (B) setting, the shutter stays open indefinitely. That's useful if you're making star‑trail shots, or if you're taking some types of flash photographs. More often, though, you'll want a fixed, definite shutter speed. Cameras (and lenses) differ in their capabilities. My Pentax K200D can deliver shutter speeds ranging from 30 seconds (abbreviated as 30 s or 30 sec) to 1⁄4000th of a second (or 1⁄4000 s). Many newer cameras have even wider ranges.
Not surprisingly, aperture and shutter speed work hand in glove to determine the tonality — the lightness or darkness — of an image. And getting the right balance between the two can be a somewhat tricky business. But let's see if we can make things a little easier, shall we?
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The simplest approach to the problem is to let the camera make the decisions for you. This is the Automatic solution. And it works well for many shots. But if you'd like a little more control — and there's no better way to get to know your camera's capabilities, particularly now that you receive nearly instant feedback from the digital display — you'll need to decide on your priorities. If it's action you're after, if things are moving fast and you hope to capture the mood of the moment, shutter speed is key. If, for instance, you want to stop your buddy in midair as he plunges over the lip of a waterfall, you'll need a fast shutter. On the other hand, if you're hoping to render the spiky turbulence of a rapid as a gauzy blur, you'll need to swing the other way. Long and slow is the way to go here.
But maybe you're more concerned with the big picture. Or with a small slice of a larger scene. Then your focus is on … focus. Do you want the distant hills to stand out in sharp relief, while still showing the grain and texture in the rocks at your feet? Or are you hoping to isolate a butterfly on a nearby flower without the distraction of the tangle of greenery in the background? In either case, you need a way to control the shot's …
Depth of Field. The tool of choice for this delicate operation is aperture. If a photo is acceptably sharp from foreground to infinity, it's said to exhibit deep depth of field. (What's "acceptably sharp"? That's your call. But you'll know it when you see it.) And to get this, you'll need to "stop your lens down." In other words, you want the smallest aperture that will let in enough light to make the picture. That means a high ƒ‑number.
The relationship is simple: the higher the ƒ‑number, the greater the depth of field. And the reverse is also true. If you want to isolate your subject and blur distracting elements in the background or foreground, just open up your lens: go as low as you can safely go on the ƒ‑number scale. Of course, this will mean that you'll need to dial in a faster shutter, too. A wider aperture lets in more light, after all. So you'll want the shutter to stay open for a shorter time. If not, the shot will be overexposed. Conversely, shots made at high ƒ‑numbers will demand longer exposures.
Confused? I'm not surprised. It's time for another handy summary:
- Smaller ƒ‑number + faster shutter speed ⇒ shallower depth of field
- Larger ƒ‑number + slower shutter speed ⇒ deeper depth of field
Enough verbiage. Some real‑world examples follow, beginning with a couple of photos I took in my Home Studio. (That's the room next to my Test Kitchen, if you're a regular reader.) I placed a beaver‑gnawn limb and some rock specimens at varying distances from my camera lens — the same Pentax DA 18–55 mm f/3.5–5.6 AL II we've already encountered, still set at the 18 mm focal length. I also used a tripod to ensure that no hand tremor would compromise the test. Then I shot two photos at different ƒ‑numbers using the camera's Aperture Priority mode. (As the name suggests, this allows the camera to choose a shutter speed appropriate to the aperture that I select. It's a sort of halfway house between full Automatic and Manual modes.)
Here are the results:
The left‑hand shot (ƒ/5.6, 1⁄8 s) illustrates a comparatively shallow depth of field: the most distant rock exhibits noticeable blurring, as does the beaver‑gnawn limb in the foreground. But the rock and limb are both in acceptably sharp focus in the companion image on the right (ƒ/22, 3 s). It's just as we expected, in other words.
Now we'll leave the cramped confines of the studio for the larger world, where …
Practical Applications …
Of the principles we've been discussing abound. Take this shot of a waterfall on The River:
I wanted to freeze the falling water in mid‑plunge. And to do so, I used my camera's Shutter Priority mode. I set the shutter speed (1⁄1000 s). The camera did the rest. Then, by way of experiment, I reshot the scene at a much slower speed (1⁄30 s):
In neither shot was depth of field particularly important. Since I was planning to crop the images, anyway, I could happily let the camera choose the aperture. But this isn't always the case. Often I want to capture the full sweep and majesty of a landscape. That's when I turn the camera dial to Aperture Priority and stop down the lens. I usually start with ƒ/8, though I'll sometimes go further — to ƒ/9.5 or ƒ/16, say — if I want a longer exposure for some reason. That done, I need only focus for the hyperfocal distance, and the resulting photo will be acceptably sharp from foreground to horizon, as in this case:
Here I'm shooting at ƒ/16, using an ultra wide‑angle zoom lens at a 10 mm focal length. The camera exposed the shot for 1⁄10 s, long enough to paint the rushing water as a pleasing blur. It was just the effect I was looking for.
Another example: In this case I wanted my subject (a Dryad's saddle fungus) to be the center of attention, and I didn't want it competing with a busy background. So I set the aperture to ƒ/5.6 and left the rest up to the camera, with this result:
Not much light filtered through the trees, and the shutter stayed open for a comparatively long time (1⁄20 s) in consequence. But this didn't matter. Fungi don't fidget, after all, and I had taken care to assume a stable prone position. The final image shows the subject clearly, while relegating the background to its proper place. Enough detail remains to establish the forest setting, but that's all.
Finally, here are two more examples of the ways in which aperture manipulation can be used for creative purposes. In the first, I opened up my lens as far as it would go, creating circular out‑of‑focus highlights in this photo of moss still damp from the morning dew:
And here's what happens at the other end of scale. In this shot, I reduced the aperture to a pinprick (ƒ/22, to be precise), transforming circular highlights into stars, all without any help from my digital darkroom:
By the way, the chromatic artifacts around the sun star are examples of lens flare, a subject I've already touched on.
What are you waiting for? If you haven't explored the possibilities inherent in your camera's range of shutter speeds and apertures, now is the time!
We all like to take it easy, don't we? And there's no doubt that modern digital cameras do a pretty fair job when left to their own devices. But there's a world of opportunity lying just outside the scope of Automatic mode. Getting a handle on the interplay of aperture, shutter speed, and depth of field was a costly and time‑consuming process back in the Age of Film, but it's gotten a lot simpler (and cheaper) today. And if you've read this far, you have all the tools you need to start. Just pick up your camera and go to work. You won't regret it.
Related Articles From In the Same Boat
- Backcountry Photography. This is the Mother Ship, a topical index of my photography columns, including …
- "Bringing it All Together for Beginners,"
- "Digital Master Class,"
- "You Don't Have to Go With the Flow!" and …
- "The Lure of Falling Water."
And some articles from Wikipedia, too, all of them worth reading:
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