Back in the days when film photography predominated, film was manufactured with various sensitivities to light and measured with a standard called "ASA" or "American Standards Association".

Fun Fact: Of the three elements of the exposure triangle - shutter speed, aperture, and film speed - only one was determined separately from the camera - film speed. Once the type of film was selected, the photographer could only manipulate exposure with aperture and shutter speed.

Film sensitivity is refereed to as "film speed" and the methods of determining a film's speed have evolved over the history of photography. The most familiar standard to any film user in the 70s - 80s was ASA or ANSI which traces its roots back to the 1940s.

Based on earlier research work by Loyd Ancile Jones (1884–1954) of Kodak and inspired by the systems of Weston film speed ratings and General Electric film values, the American Standards Association (now named ANSI) defined a new method to determine and specify film speeds of black-and-white negative films in 1943.

The current International Standard for measuring the speed of color negative film is ISO or the International Standard. Gaining film speed has always been a trade off. To create faster films, manufactures have had to make the grains of photosensitive elements larger, thus faster films result in grainier negatives.

The important things to know about film speed are:

1. The higher the number, the faster the film speed
2. A doubling of film sensitivity is represented by a doubling of the numerical film speed value. ISO 200 is twice as sensitive as ISO 100.
3. Higher ISO represent a compromise in image quality (more grain in film and more noise in digital)

Film Speed or ISO in Digital Photography

In digital photography, film is replaced by a senor made up of tiny light sensitive circuits called pixels. Think of these pixels as tiny buckets that collect photons from light.

What is light? Einstein called rays of light or energy packets "photons", and these are now recognised as a fundamental particle. Visible light is carried by photons, and so are all the other kinds of electromagnetic radiation like X-rays, microwaves and radio waves. In other words, light is a particle.

In digital photography ISO measures the sensitivity of the image sensor. The same principles apply as in film photography – the lower the number the less sensitive your camera is to light and the finer the "grain". Higher ISO settings are generally used in darker situations to get faster shutter speeds.

A digital camera produces its highest quality (low noise) images at its lowest ISO setting, usually ISO 100 (but some camera might have something like ISO 150 as their lowest setting, others might go as low as ISO 50).

Fun Fact: In digital photography, ISO can be changed with every shot unlike the days of film when ISO was "cooked in" to the whole roll of film.

At ISO settings above 100 the camera's sensor is cranked up and the resulting images will start showing increasing levels of noise or grain. At higher ISOs those little photo buckets start to heat up and the photons spill over to the neighboring pixels creating noise. The higher you crank ISO the harder the camera sensor works to cram in photos and thus creates more heat, more spill over, and more noise.

What to know about ISO in Digital Photography:

• For the best quality use the lowest ISO possible.
• Higher ISOs will allow you to shoot in lower light conditions although you will get more grain in the image.
• ISO/Aperture/Shutter-speed are the three things you change to adjust exposure (besides adding more light via a flash or studio lights)

Article by fine art photographer Edward M. Fielding - http://www.edwardfielding.com