School me on ISO.

The are different grades of paper, in the 70s most companies, Kodak, GAF, ILford, Agfa offered grades 1 to 6, or maybe 2 to 4 depending on the manufacture along with multi contest papers. In the modern world in hard to find any grades beyond 3 and 4, some normal and hard. Multi contrast paper has become the norm.

KerrKid said:

What can you tell me about ISO? For example, why shoot at a different ISO than box speed? Thanks!

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I may have missed it but what I don't see covered here is the fact that most all color and b&w films I have tested have near endless overexposure latitude.

This from Kodak Tri-X

Kodak TRI-X 400 Exposure by Les DMess, on Flickr

This from Kodak Portra 400

Kodak Portra 400 exposure range by Les DMess, on Flickr

A practical application of this is that a single frame of Kodak Portra 400 can capture on a single frame of film the dark and the otherwise blownout highlights. The film has captured it but you will need to work on it to bring it out.

Kodak Portra 400-04-24A by Les DMess, on Flickr

Another practical application of this is that you can effectively change the ISO to suit the task. For instance I saw this scene and with the aperture I selected my camera's meter recommended 1/60 shutter speed. The water flow wasn't much and I needed at least 2 seconds to achieve the effect but knowing the latitude of the film and my workflow, I was confident that I can work with the +8 overexposure.

This on Fuji 100

Fuji 100-26-26B by Les DMess, on Flickr