Film Speed?

[selmerdave]

I'm wondering what the explanation is for the various conventions of film speeds available today. Why 160 for portrait film? Why 125 B+W? Just wondering.

Dave

 

[Hertz van Rental]

Why do printers use 'points' to measure type? A point is almost but not quite 1/72nd of an inch. They use en and em spaces too. And why do they call the space between lines 'leading'?*
In the early days of film every film manufacturer had there own way of measuring film speed. Thus you had BSA, Hurter & Driffield, Gosht, Weston, Kodak .... there were dozens and each was different. BSA (British Standard) was itself an amalgam of a number of different speed systems. Eventually things got organised with British Standard (BSA), American Standard (ASA) and the German standard (Deutsche Industries Normal) prevailing. BSA was virtually the same as ASA (Kodak mainly) so they amalgamated and ASA was used into the 80's, alongside DIN (Being German DIN is a logarithmic scale so no one understands it) when another World Standards meeting changed the name to ISO - International Standards Organisation.
How film speed is worked out is a little complicated to go into here so I won't.
Why those speeds for different films? Historical convention. That's just what they were when they standardised so they just stuck. Moves are being made to streamline - 25, 50, 100, 200, 400 being the ISO of choice for amateur film. Pro films - being more critical - have the speeds given more accurately. It's all a little accademic actually because you tend to adjust the film speed in use depending upon conditions and the intended developer. Some developers give an increase and others give a decrease in speed.
The bottom line is: those speeds are used in the same way that photographers still talk about large format lens focal lengths in inches, paper and film size as 'whole plate', 'half plate' et al. And printers use their own measurement system. That's the way it has always been. Photographers are creatures of habit.
(May as well ask 'why 35mm, 120 etc - same answer)


*Don't bother to tell me - I know the answer.

 

[ksmattfish]

Those are just the manufacturer's recommended starting points anyway. If you did a personal film speed test you may find that the film speed you should rate it at is different.