The Basics of Digital Photography: The Exposure Triangle, Part 3

In Part 1 of this series I introduced the new photographer to the concepts of lens speed, lens aperture, f-stop numbers, and Depth of Field. In Part 2 of this series, I introduce the new photographer to the concept of shutter speed and how it relates to lens aperture when calculating proper exposure. In this part, I will introduce the new photographer to ISO and the concept of camera sensor speed.

For those of you who started out in photography when film was king you'll remember the old ASA (American Standards Association) ratings marked on every film box as well as on the film roll or cartridge themselves. ASA referred to the film speed or sensitivity to light. There have been many different speed rating systems used throughout history but the ASA rating was the most common one used in the United States until film producers converted to ISO (International Standards Organization). In a particular case the film speed numbers remained the same, only the letter preceding the number changed i.e. ASA 400 and ISO 400 film had the same sensitivity to light. The DIN (Deutsche Industre Norm) was another system used to indicate film speed, but not one commonly used in the US. The DIN scale was a logarithmic scale where each step on the scale was equal to a 1/3 EV (Exposure Value). The DIN scale was confusing because the numbers were completely different i.e. a film rated at ASA/ISO 800 has a DIN of 30. Some of us who loved German camas like Leica cameras were forced to master the DIN system because there were no ASA equivalents marked on the film speed setting dials. Today's digital cameras have done away with the dial altogether and the ASA/ISO selection is made through a menu selection.

In the days when we all shot film, there was a direct relationship between film speed and the graininess of the negatives produced. A photograph may appear to be made up of smooth continuous tones but close examination of film or paper with a magnifying glass or microscope reveals a different story. The images on film and paper are recorded as tiny microscopic dots, dot silver halide crystals. The faster the film, the larger the silver halide crystals become and soon become visible to the naked eye as the pictures are made larger. In the digital world, the silver halide crystals are replaced by electronic noise but the end results are the same. As one shoots at higher ISO settings, the individual pixels (Picture Elements) soon become visible to the naked eye, especially in the larger print sizes. Most DSLRs offer you a wide range of ISO speeds to choose from when shooting in one of the manual modes, the Nikon D40 allows you to shoot all the way up to an ISO of 1600. At an ISO of 1600, you can shoot in almost total darkness without using a flash but the pictures will be extremely noisy. I try to never shoot above an ISO of 400 unless I really have no choice. As a rule shooting at ISO of 800 is about as high as you want to go if you plan on printing pictures larger than 5X7s.

The amount of light needed to make a properly exposed picture is inversely proportional to film speed or the camera ISO setting. If you get a properly exposed pictured shooting at f-16 at 1/250^th second and an ISO 200 you can make the same exposure of f-16 at 1/500^th second if you raise the ISO setting to 400. As you can see, all three exposure elements-aperture, shutter speed, and ISO-are interrelated and interact together to affect the exposure.

In part 4, the final part of this series, I will pull all this together for you and show you how to control these three elements of the Exposure Triangle to make great pictures.