There are three basic things that every camera does to take a picture. In other words, there are only three basic things that you need to set on even the most manual of cameras. Anybody can learn to do these three things, and I am going to explain them to you right here.

1. Focus

A camera lens must be focused to create a sharp image at the film plane, just like the lens in your eye must be focused to create a sharp image on your retina. The principle is the same.

Some cameras are fixed focus. That means that the focus is set at the factory for a certain medium distance, usually about 12 feet, and glued in place. This is common in the cheapest cameras.

Other cameras are scale (or zone) focusing. You estimate the distance from the lens to the subject and then turn the lens until the appropriate distance in feet or meters (or symbol in the case of zone focus) lines up with an index mark.

Other manual focus cameras have sophisticated methods of assisting you to focus accurately. Single lens reflex (SLR) cameras and view cameras both let you look through the taking lens itself as you focus for sharpness. Twin lens reflex (TLR) cameras have a special viewing lens, just above and identical to the taking lens, that you look through to focus. Some modern cameras have optical rangefinders built into the viewfinder and coupled to the lens, which allows very precise focusing. This is the most accurate focusing method of all.

Autofocus cameras have electronic contrast comparison systems or active infrared rangefinder systems to allow the camera to focus itself. All you do is aim the camera at the subject and press the shutter release part way down to activate the focusing system. With these cameras, you cannot forget to focus before you take a picture. These are very analogous to the human eye, which also automatically focuses on the subject for you. However it gets done, the lens must be focused to make a sharp image on the film before you take a photograph.

2. Aperture (Lens opening):

The slang term is "f/stop." At one time these "stops" were fixed, as they still are in the cheapest cameras. A hole drilled through a thin metal plate mounted inside of the lens, called the diaphragm, controls how much light can hit the film. A lens like this has only one aperture (f/stop). The volume of light that comes through such a lens cannot be varied.

Inside of most lenses today there is a variable aperture diaphragm. This controls the volume of light that hits the film, and its opening can be made larger or smaller. Look into the lens from the front or back (depending on the type of lens and the camera it is mounted on) and you will see the little interlocking blades of the diaphragm that swivel to create larger and smaller openings. F/stops are calibrated as fractions of the focal length of the lens, so "16" is really 1/16th, and "8" is really 1/8, and "2.8" is really 1/2.8, and so on. They just leave off the numerator of the fraction (the top number) for reasons of space. Since 1/16th is a smaller number that 1/11th, "16" is a smaller opening than "11." Likewise, 1/11th is a smaller number than 1/8th, so "11" is a smaller opening than "8," and so on.

The f/stop numbers, in full stops, that may be seen on lenses run from "128" (1/128) up to "1" (1/1). The sequence, in full f/stops from smallest to largest, is: 128, 90, 64, 45, 32, 22, 16, 11, 8, 5.6, 4, 2.8, 2, 1.4, 1. Each time you open a lens one full f/stop, you have doubled the amount of light that will hit the film. Each time you do down one full f/stop, you have cut the light in half. Thus, f/11 lets through twice as much light as f/16, but only half as much light as f/8. This is an important relationship, so make sure you understand it.

No lens will have this entire range, but most will have a good part of it. A normal 50mm lens for a Leica, for example, has f/stops from 16 up to 2. A normal 80mm lens for a Hasselblad has f/stops from 22 up to 2.8. Some lenses have a maximum f/stop that is not a full stop larger than the f/stop immediately preceding it. Examples would be f/1.7, which is a half stop bigger than f/2; or f/3.5, which is a third of a stop bigger than f/4.

The iris of your eye, which makes the pupil (opening) larger in dim light, and smaller in bright light, lets you see correctly in a wide variety of lighting conditions. The variable diaphragm in a lens basically does the same thing. It lets you take photographs in a wide variety of lighting conditions.

3. Shutter speed

Unlike the human eye, a still camera only lets in light when you tell it to. A part called the "shutter" blocks the light from reaching the film until you press the "shutter release" button to take a picture. Then the shutter pops open for a pre-selected period of time to let light hit the film, and closes again automatically.

What we call "shutter speed" is really the time the shutter is open, usually expressed in fractions of a second. "2" is 1/2 of a second, "30" is 1/30th of a second, "125" is 1/125th of a second, and so on. Again, for space reasons, they left off the numerator of the fractional times. One second is "1" (1/1=1). How long the shutter stays open determines how much time the light has to hit the film. The shutter speed control is usually a dial on the camera body, or a pointer concentric with the lens. In either case, you can set it to point to a series of numbers in this progressions: 1, 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000. Not all of these numbers may appear. Some cameras do not go up to 500 (1/500th second), and some do not go down to 1 (a full second). And some modern cameras with electronically controlled shutters add speeds down to 30 seconds and up 1/8000th second. But you get the idea.

There is something worth noting about this progression of shutter speeds: every full step faster cuts the time the shutter is open in half (thus cutting the amount of light that hits the film in half); every full step slower doubles the amount of time the shutter is open (thus doubling the amount of light that hits the film). Hmmm...that is the same amount of change in the light hitting the film as changing the lens one full f/stop. So the loss in light that occurs if you go to the next faster shutter speed can be compensated for by changing the lens aperture to the next larger full stop. If you speed up the shutter, you compensate by opening up the lens. If you slow down the shutter, you compensate by stopping down the lens. Got it?

For example, the amount of light that hits the film when your camera is set to f/11 and 1/125th second is exactly the same amount of light that hits the film if you change the camera's settings to f/8 and 1/250th second. The lens and shutter settings move in lockstep. Once you know one correct setting (exposure value) for the amount of light you are trying to take a picture with, you really know the whole range of settings (f/stops and shutter speeds) you can use in that amount of light.

Conclusion

That's it, the three basic things you need to do to operate any camera. For most people, focusing is easy to understand and to do. That just leaves exposure to worry about. A correct exposure means getting the right amount of light onto the film. Balancing the aperture setting and the shutter speed is how we accomplish this. Just two things to set. You can use a hand held light meter to tell you what lens and shutter values to set, or you could use the light meter built into your camera, if it has one (line up the needles, or light the correct LED), or your camera may have an automatic exposure mode in which it uses its built in light meter to read the light and set the correct exposure for you. All of these methods work well.

To get an idea where to set the lens opening and the shutter speed, in the absence of any other help, carefully unfold the cardboard box a roll of Kodak film comes in, and inside you should find printed little pictograms of sunny days, cloudy days, etc. And with each pictogram is a suggested lens opening and shutter speed. Just set those on your camera, don't forget to focus the distance, and you are ready to take a photograph!