helped establish how the film numbering scheme works. Film around the world uses ISO
numbers, so you can buy it anywhere and it’ll all work the same. In the United States,
photographers used to call this system ASA, which stood for the American Standards
Association. That term was essentially abandoned about 20 years ago, so if you want to be
considered a grizzled old geezer, you can refer to ISO numbers as “ASA.”
A fairly typical ISO number for ordinary daylight photography is ISO 100. Increasing the
ISO to 200 doubles the sensitivity of the film, meaning it only takes an exposure half as long
to capture the same picture; dropping back to an ISO of 50 halves the sensitivity of the film,
requiring an exposure twice as long as an ISO of 100.
The ISO number 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.
exactly twice as sensitive to light as the previous roll of film. And that means, all other things
being equal, that you need to leave the shutter open for only half as long (a 500th of a second, or 1/500) to take the same picture.
much light available? You might need to leave the shutter open for 1/30 in that situation to gather enough light with ISO 100 film. 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 halves the shutter speed yet again, to a crisp 1/125.
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, also sometimes
referred to as the f/number of the lens. F/stops are represented by numbers that start with “f/ ”—such as f/2, f/5.6, and f/11. The larger the number, the smaller the opening, so an f/22 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 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.
We’ll talk about this in more detail in Chapter 4 (it’s really important, yet really simple), but
for the moment take a look at Figure below. This diagram shows the relationship between the f/stop and the shutter speed. If you reduce the shutter speed, you need to increase the diameter of the aperture in order to have enough light to take a properly exposed picture. This concept—that there are many equivalent exposure possibilities by varying the shutter speed and aperture—is illustrated in Figure. Here’s how it works: If you know that any given combination adds up to a good exposure (let’s say f/5.6 at 1/60 second), then you can find an equivalent exposure setting by traveling along a diagonal line. In this example, f/4.0 at 1/125
second will yield the exact same exposure.
Of course, there’s a relationship between aperture, shutter speed, and your film’s ISO rating as well. Look at Figure below. At any 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 either the aperture or the shutter speed so that you still get a properly exposed picture.