Exposure: The Meat and Potatoes

ISO: Sensitivity Training

ISO controls the sensitivity of your camera (and by sensitivity, I don’t mean that it cries during the ending of Marley & Me). The more sensitive your camera, the less light is required to properly expose an image.

The setting takes its name from the way they rated film for its light sensitivity. A “100 speed” film was good for outdoors, while 400 and up was necessary for indoors, especially if you weren’t using a flash.

You might hear some gray-bearded photographer refer to this as the “ASA” setting. Back in the Triassic period, what we now call the ISO rating was only an American system, and was known as ASA, while there was once an international system called “DIN.” The D stood for “Deutsche,” which as you probably know is German for German. Like many things invented by Germans, the DIN rating was needlessly complicated. Eventually, everyone agreed that DIN was stupidly confusing and the whole world settled on the American system, which become the international standard, hence ISO. For the sake of the more stubborn Germans, the DIN system still exists as sort of an alternate ISO system.

The ISO numbers are easy enough to understand. An ISO setting of 200 is twice as sensitive as 100, requiring half as much light to capture the same image.

You may wonder why we simply don’t crank the ISO setting up as high as possible. For one thing, a high ISO setting may make it impossible to have the desired shutter speed or aperture, especially in bright sunlight.

There is another trade-off. In the old days of film, in order to make a film stock more light-sensitive, they had to make the chunks of silver halide in the emulsion bigger, which made the film grainy. For fine-grained images, like you might print in a magazine, you needed the slowest film you could get away with. Those pictures were often shot with ASA/ISO ratings of 25.

With digital cameras, there is a similar but subtly different problem. Higher ISO settings force the camera sensor to work harder, resulting in digital noise in the image, which is similar to but visually much less appealing than film grain. Digital tools like Lightroom have tools to take noise out, but other tools that put grain in. The tools that remove the noise can also destroy some of the detail in the image. While grain can be a good creative choice, noise is always bad.

As a result, you should use the lowest ISO setting you can get away with. The good news is that as new cameras come on the market, they get better and better at higher ISO settings. In the old days, films with ISO ratings of 1600 or higher were very rare and very, very grainy. Today’s top-of-the-line cameras produce very good results at ISO settings of 6400, and can go all the way to 102,400. This means that you can capture more photos in natural light without a whole lot of that natural light to work with. This is big for photojournalists and street photographers.

As a general rule, however, lower ISO is better for your photos under normal conditions, while high ISO is a useful tool for photos you couldn’t get without it.

Speed is Life

Shutter speed is not really an accurate word for what it is, since we’re really talking about the length of time the shutter is open when you push the shutter button, not how fast the shutter itself movies. It’s expressed in fractions of a second (or seconds), making shutter speed relatively easy to understand. You do have to deal with fractions, but it’s not too bad. All you have to know is that 1/50 of a second is twice as long as 1/100 of a second, and leaving the shutter open for twice as long will let in twice as much light. Simple, right?