In the good old days of 35mm film photography, a "slow speed" film was 25 ASA, a "medium speed" film was 100 ASA and a "fast film" was 400 ASA. So, when there was lots of light available, you would normally select a slow to medium speed film and in dimmer conditions, select a faster film, right up to the very grainy Konica 3200 ASA film. Examples of film I used to use were:
- Kodachrome 25 - a 25 ASA slide film.
- Kodacolor 100 - a 100 ASA print film.
- Fujicolor 400 – a 400 ASA print film.
- Ilford FP4 - a 125 ASA B&W film.
- Ilford HP4 - a 400 ASA B&W film.
The grain (film structure) was very fine at 25 ASA and much coarser at 400 ASA, giving a 400 ASA photo a gritty look, much like noise in short exposure CCD image. You could enlarge a 100ASA film to produce a 16x12 inches print before the grain became noticeable, whereas grain would show at 10x8 inches from a 400ASA film negative.
Films improved over the years and a 1990’s era 400 ASA film had grain as fine as a 1980’s 100 ASA film. Generally, CCD’s with small pixels are “slow” whereas CCD’s with large pixels are “fast”; that is, they can record in dim light more efficiently.
Cheers
Dennis