ksmattfish said:
ISO stands for International Standards Organization.
Film emulsion is made up of silver halide crystals. The larger the crystals, the less light is needed to expose the film properly. Large crystals make larger grains of silver during development.
Tabular grain films (Tmax and Delta) have a flat crystal that exposes more of the surface area of the crystal to the light. This allows for the use of smaller crystals at the same ISO.
VERY good explanation, Matt. I just want to add to the description of "proper exposure."
With all the variables in photography (like film speed, exposure time, development time, development temperature, etc.) it is necessary to establish a benchmark... a fixed point to which all this relates. Think of it as asking the question, "Is the water too warm?" Too warm for what? Cooking? Bathing? Drinking? Sooo.... in photography, the question, "Is the exposure too long?" begs additional questions. And now, the payoff... All the variables are gauged to tell us how to achieve GRAY. More specifically, 18% of the light.
So when reading light, the meter tells us how to achieve gray with the film we use, in the lighting we us, at the development we use, at the temperatures we use.
That's where you come in. You have to consider all the qualities that result from different films, ISO ratings, processing, etc. when you make an image.
Pete