A lot of new photographers pick up their cameras, switch the mode dial to fully auto, start shooting and… stop learning. But the ones who want to take their photography skills to the next level soon start investigating the manual exposure modifiers that provide them with complete control over every frame. That’s all a camera really is, after all: a light-tight box with three modifiers. Those modifiers are shutter speed, aperture and ISO, and they are quite literally the tools that provide a photographer with creative control over what’s happening in the image. And while we may understand the changes those controls impart in theory, it can be difficult to visualize the results if we haven’t seen them before. To that end, here’s a look at precisely what those modifiers do to an image. Their differences are easy to see.
Also called f-stop or iris, aperture is a measurement of the size of the opening of a lens. A smaller opening is measured by a larger number, while a larger opening equates to a smaller number. And the larger the opening, the “faster” a lens is considered to be. This is because large openings permit faster shutter speeds—particularly in the days of slow-speed film that made it difficult to achieve the fast shutter speeds that freeze motion. (More on that in a minute.) But fast lenses are desirable these days largely because they allow for working in low light conditions and because they do a tremendous job of isolating the center of interest by throwing the background out of focus. In the examples shown here, with the lens wide open the background is almost indecipherable. As the aperture decreases through f/5.6, f/8 and all the way to f/16, the depth of field increases dramatically and all of that background information becomes sharp. If your goal is isolating the subject—as is often the case with portraiture—a shallow depth of field and wide aperture are helpful specifically because of the shallow depth of field. But if you’re in need of showing context, however, a sharper background is necessary. So that necessitates stopping down to increase depth of field and background sharpness. As the examples show, the same shot at different apertures will have a very different appearance.
The ISO setting is one of the most interesting changes that has occurred in the modern history of digital cameras. In the days of film, a typical “low ISO” film speed may have been ISO 64 or 100—chosen by discerning photographers because of the finer grain those films produced. In digital image sensors, however, there’s no such thing as grain. Instead, sensors produce noise— interference in the light-gathering capabilities of a pixel. And this noise is increased as ISO is increased.
The great news, however, is that you’ve got to crank the ISO sensitivity very high before you start seeing destructive noise in practice. Look at the enlarged examples shown here. The ISO 100, ISO 800 and even the ISO 1600 close-ups reveal very little difference in noise. Sure, it’s there, and under close scrutiny you can see that the ISO 100 image file is cleaner than the ISO 1600 file. However, that ISO 1600 image file is also incredibly cleaner than it would have been just a decade ago. And compared with high-speed film, the ISO 1600 image file is light years better. (Back in the old days, ISO 400 film produced grain much closer to what’s seen in the ISO 6400 example shown here.) The improvements to signal-to-noise ratio that make high-ISO photography possible are perhaps the biggest improvement to photography in the digital age. Because increased ISO sensitivity makes it easier to work with ambient light without the need to use strobes or other high-power light sources, everything from candid street photography to Oscar-winning movies can be shot better, faster and with a more authentic look because of that capability to produce low noise even in low light.
The camera’s shutter is like a curtain in a window; when it’s open, light (and the view of the world outside) are allowed in. How long that curtain is open—its duration—is measured in seconds or fractions thereof. And that’s the shutter speed. There are no hard and fast rules about what constitutes a “fast” shutter speed or what constitutes a “slow” one, but rather, it’s a continuum that accomplishes a few different things as illustrated here.
First and foremost, a faster shutter speed is necessary when handholding a camera because a too-slow shutter speed will introduce blur from the camera moving during the duration of the exposure. Typically, photographers think of 1/60th as a minimum handholding speed, but a good rule of thumb is to use a shutter speed no slower than the focal length of a lens. So, a 100mm lens will handhold well at 1/100th or faster. A wider lens, such as a 35mm lens, could handhold down to 1/30th of a second with much better results.
Image stabilization built into a lens or camera also buys the photographer additional handholdability. Look at the backgrounds of the examples here. Even at the slowest shutter speed, where the hand is very blurry, the non-moving background is still pretty sharp, even though the camera was being handheld for 1/15th of a second with an 85mm lens.
The other major thing shutter speed affects is motion blur from a moving subject. If it isn’t nailed down, in a practical sense, it’s moving. So even a smiling subject sitting still for the camera will be moving a bit. Still, that subject doesn’t need nearly as fast a shutter speed as something moving quickly—such as the waving hand in the example here, or a running athlete or rushing river. That motion blur might be desirable; there’s no rule that sharpness is a requirement. In fact, such blur can introduce a sense of motion in a still image. The key, however, is to be in control of that choice, and by understanding how faster shutter speeds freeze motion and slower shutter speeds create blur, the photographer is fully able to make such deliberate creative decisions.