In the 16th century, it was noticed that if one placed a small hole to one end of a darkened room, an image would be projected, from the outside, onto the wall opposite the hole. Imagine it this way. You have a room, 10 feet by 10 feet, that you paint black except for the rear wall which is white. You then drill a hole through the front end of the room that gives onto the outdoors. You will see, projected on the white wall, an upside down image of your outdoor scene. No lens is needed. This is the same principle used for the pin-hole camera.
Some theorize that such a darkened room—a camera obscura—was used by such artists as Vermeer to establish the basic outlines of a canvas. In any case, a hand-held camera uses the same principle and typically uses a lens instead of a hole to project the image to the back of the box that is the camera.
Once you understand that this is basically all a camera is, it’s not much harder to understand how it works. The most important functions on a camera, and those you’ll access when you learn to use its manual controls, have to do with controlling the amount of light that hits the rear of the camera where it is turned into an image by a digital sensor or a sheet of film.
There are 3 basic controls for the amount of light hitting the sensor or film. First, is the shutter speed. Most cameras have a shutter that opens up for a relatively short period of time—anywhere from 30 seconds to 1/8000th of a second—allowing varying amounts of light to hit the rear sensor. If you look on the settings of a traditional manual camera, you’ll see that shutter speeds are either half as much or twice as much as the one next to it. In other words, say you start at a 1 second exposure but you want to cut the amount of light down by half (by controlling only the shutter), you’ll set the shutter to ½ second. If, on the other hand, you need twice the light, you’ll set the shutter to 2 seconds. These units, of half or twice as much light, are referred to as “stops” and are the fundamental unit photographers use to describe amounts of light. So, if we’re shooting at a second and I say to close down two stops, we’ll shoot at 1/4th of a second. Keep in mind that stops are not absolute measures of light but are relative. In other words, you can’t say “Oh, this looks like about 3 stops of light.” It’s always a relative measure or a measure of change. One can say “Oh, this looks about 3 stops of light brighter.”
Second, the amount of light can be controlled by the opening in the lens called the “aperture.” If you look at the lens of a traditional camera and turn the dial that sets the aperture, you’ll see the opening open and close as you allow more or less light into the camera. The aperture is measured in “f-stops” which are units of light controlled by the opening. For reasons I can’t explain, traditional f-stops come in units starting at about 1.4 (which is very wide, allowing in lots of light) and, going in stops, to 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64, 90 on to very large numbers (denoting small openings) for pin-hole cameras. As you rotate the dial on a traditional camera, you’ll flip through the standard f-stops while on an electronic camera, you’re liable to end up with exposures that have nothing to do with the traditional f-stops or shutter speeds—readings such as f-8.3 at 1/13th of a second, settings that would have been impossible on a traditional camera. So, on a traditional camera, if I’m shooting at f-2.8 and I want to shut down 2 stops, what is the new aperture setting? Well, 2 stops is f-5.6.
Third, one can control the sensitivity of the sensor. This sensitivity, again based on numbers that make sense only to research scientists, can be adjusted in stops (or, on modern cameras, in any increment) by doubling or halving the number. Originally, this scale of sensitivity—the ISO value—was used to denote the sensitivity of film such that one can buy film at ISO of 50 (very slow, not very sensitive), 100, 400, 800 (very fast and sensitive) and even higher. The higher the ISO, the more sensitive the film or digital back. One of the wonders of modern cameras is that this ISO setting can be adjusted so that the camera can have varying sensitivities.
Once you know that you have access to 3 methods of controlling the amount of light activating a sensor, how do you decide which to use? Keep in mind several factors. Are you trying to freeze action? Do you want the image blurry except in a very small area? Is there a lot of available light or is it dim?
If you’re trying to freeze action, you’ll need a fast shutter speed, perhaps 1000th of a second or even faster. Because such a fast shutter speed lets in so little light, you’ll have to compensate by opening up the aperture or increasing the ISO or both. If you increase the ISO, the sensor will respond more to the amount of light you’re letting in, but the problem with increasing the ISO is that film or sensors using higher ISOs tend to have more grain or noise. In most cases, in an ideal world, ISOs should be kept as low as possible. So the other option is to open the aperture, perhaps all the way to 2.8 or whatever the largest opening on the lens. The only possible drawback to this route is that less becomes in focus as the opening (aperture) increases (the f-stop decreases).
To better understand how aperture opening affects an image, it’s helpful to understand the concept of depth of field. Depth of field describes the amount of the image that’s in focus. Say you’re shooting 3 apples, each varying distances from the camera with the camera focused on the one that’s neither the greatest nor the least distance from the camera. In other words, you’re focused on the one in the middle. Now the question of depth of field rests on how in focus you want the other apples to be. As you close down the aperture, the outer apples will slowly come into focus.
This ability to control depth of field is useful for creating soft-focus effects and bokeh, which is the soft focus that forms around those objects that are in focus.
So, how to adjust your camera settings is sometimes a kind of horse trading—you may have to take on a little noise to make your ISO higher and the camera more sensitive. Or you may decide to open up the aperture instead of tinkering with the ISO, but you’ll loose some of the focus in your scene. If you’re trying to freeze action in a dark place you may have to use a high ISO and a low f-stop.