Digital Camera Master

Some people think that photography is akin to magic. They turn on the camera, snap a picture, and a day or two later they’ve got a mystical re-creation of the scene they saw in the viewfinder. With a digicam, it’s even more magical—the pictures are available instantly! How does it work? Who knows?

The best place to start is often right at the beginning—how on earth does a camera take a picture, anyway?

All cameras, regardless of type, work more or less the same way. They open their shutter for a brief time, allowing light to enter. That light then interacts with a sensitive photo-receptor (like film, or perhaps a computer chip), and an image is recorded. Let’s start by looking at a traditional 35mm camera to give us a little perspective.

Inside a 35mm Camera
Traditional cameras rely on good old-fashioned film. But what is film, really? It’s just a strip of plastic that has been coated with a light-sensitive chemical. The chemical soup on the film is loaded with grains of silver halide. When exposed to light, the silver halide reacts, and that is the essence of photography. The longer the film is exposed to light, the more the silver is affected.

The two kinds of film in common use today are negative and slide film. They work a little differently, but the end result is similar. When you use color negative film, also referred to as reversal film, the film itself becomes a “negative” image of the scene you photographed. After processing—which includes letting the film sit in a chemical bath that coaxes the grains of silver to visually materialize on the film—the negative is used to create positive prints of the scene.

It’s a two-step process, and one that is highly subjective. When creating prints from negatives, photo-finishers often tweak the picture to improve its appearance. Of course, what the corner shop considers an “improvement” may not be what you were trying to achieve, and that explains why your pictures never seem to benefit from filters, exposure changes, or any of the other corrections you try to make when taking pictures.

But I digress. The other kind of film is simple slide film. This is a color positive development process—after fixing the slide film in its chemical bath, the film becomes slides that can be held up to the light to display images.

No matter what kind of film you have, it eventually needs to be exposed to light. When you take a picture, you obviously press the shutter release. The shutter release instructs the camera to open a diaphragm in the lens for a brief period of time and then close again. If all went well, that was just long enough to properly expose the film.

If you want to shoot with 35mm film, scan the results, and then edit and print the results on your PC, you might want to work with slide film. Slides are more exacting—they require you to nail the exposure fairly precisely —but they’ll better represent what you actually photographed instead of the local photo shop’s vision of what you photographed.

How and Why Film Varies

As you no doubt know by shopping for film, not all canisters of 35mm film are alike. Film is differentiated principally by its speed, or ISO number. A film’s ISO number refers to how sensitive it is to light. The lower the number, the less sensitive—requiring long exposures or very bright scenes.

A fairly typical ISO number for ordinary daylight photography is ISO 100. Increasing the ISO to 200 doubles the sensitivity of the film; dropping back to an ISO of 50 halves the sensitivity of the film.

This film sensitivity has a tangible effect on the mechanics of photography. The lens is equipped with a diaphragm—called an aperture—that has a certain diameter and consequently is designed to allow a specific amount of light through to the film. With ISO 100 film in specific lighting conditions (say, at midday) the shutter might need to open for a 250th of a second (1/250) to adequately expose the picture.

But what happens if we instead try to take the same picture with ISO 200 film? The film is exactly twice as sensitive to light as the previous roll of film. And that means, all other things being equal, that we need to leave the shutter open for only half as long (a 500th of a second, or 1/500) to take the same picture.

That’s not all. Suppose you’re trying to take a picture in late afternoon—when there isn’t as much light available? You might need to leave the shutter open for 1/30 in that situation to gather enough light. That shutter speed is a bit on the slow side, though. Not only might you jiggle the camera as you’re taking the picture (it’s hard to hold a camera steady for 1/30), but your subject might move as well, causing a blurry picture. You can probably guess what the solution is—stepping up to ISO 200 film will enable you to grab that picture at a much more reasonable 1/60, and ISO 400 would halve the shutter speed yet again, to a crisp 1/125.

The F/stop Ballet
So far so good—but there’s one other aspect to consider, and that’s the fact that camera lenses can change the diameter of their aperture, thus letting in more or less light as needed.

The size of a camera’s aperture at any given moment is called the f/stop, or sometimes referred to as the f number of the lens. F/stops are represented by numbers that start with f/—like f/2, f/5.6, and f/11. The larger the number, the smaller the opening, so an f22 is very, very small (not much light gets through to the film), while a lens set to f/1.2 is a huge opening that floods the film with light. Changing the camera setting by a “whole” f/stop, such as from f/5.6 to f/8 or from f/11 to f/16, doubles or halves the available light, depending upon which way you’re going. If you adjust a lens from f/8 to f/11, for instance, you’ve reduced the light by half.

Of course, there’s a relationship between aperture, shutter speed, and your film’s ISO rating. At a given film speed, you can take a picture with a specific aperture/shutter combination. If you double the film speed without changing the lighting conditions, though, you have to adjust the aperture and shutter speed so that you still get a properly exposed picture. And perhaps most importantly for us, suppose you are in the situation I describe next.

You want to take a picture of frolicking lions at the zoo near dusk. The aperture is wide open at f/2—it won’t open any farther. Nonetheless, your camera needs to use the relatively slow shutter speed of 1/15 second to take the shot. You know the image would be a blurry mess at that sluggish shutter speed, so what is there to do? Take a look at your film speed. It’s ISO 100 film.

Well, you might be in luck. If you’re willing to pop the film out of your camera and put in film that’s two f/stops (often, just called “stops”) faster, you can keep the aperture at f/2 and change the shutter speed to 1/60. That’s probably good enough to get the shot. Just do it quickly—it isn’t getting any brighter out, and if you dally, you might find you need to increase the speed by three stops by the time you get the film loaded and ready to go.