Hi Ric
Speed is mainly related to the physical size of the “pixel” or “grain” which captures the incoming photon. Small pixels and small grains require a lot of light, but produce nice images with little noise when exposed optimally.
Large pixels and large grains capture photos more efficiently due to their larger surface area but tend to produce noisier or grainy images.
In the good old days of 35mm film SLR photography,
- ISO 100 film was for brightly lit daylight scenes, such as sunny days at the beach, park, etc. This was a Slow Speed film.
- ISO 400 film was used when taking photos at the daytime cricket match to allow fast shutter speeds to “freeze” the action. This was a Medium Speed film.
- ISO 800 to 1600 was used at night events such as circuses, concerts, floodlit sporting events, etc. when flash was not practical. These were Fast Films.
In the 1990’s, advances in film technology meant that ISO 400 films were as good as older ISO 100 films. The “pixels” of film were silver halide grains which reacted (non-linearly) to light.
- ISO 100 had a very fine grain which could produce prints up to say, 20”x24”.
- ISO 400 had a more coarse grain, visible on 10”x8” prints
- ISO 800 grain was quite visible even on 6”x4” prints.
Generally, lower ISO settings such as 100 to 400, produce less noisy images, with smoother tones, but require slower shutter speeds (risk of camera shake or subject movement) or wider apertures in order to grab sufficient incoming photons. These are your typical day at the beach scenes.
Cheers
Dennis