iso speeds and grainy

[grooski]

I know that the higher iso has more grain and is more sensitive to light and the lower the opposite. And the higher iso they just add less of the cemical or something to make it more sensitive, but what does the number actually mean? Does it like have to do with 100 (100 iso) of something or 800 (800 iso) of something or rather?

 

[grooski]

anybody know why?

 

[molested_cow]

The ISO number represents the size of the photo cells if I am correct. So, ISO800 film has larger photo cells than ISO 100, thus reacts to light quicker but you also see larger grain.

 

[ksmattfish]

You stumped me. I don't know what the number itself represents.

ISO stands for "International Standards Organization", and used to be called ASA for "American Standards Association". The old European film rating system was DIN; I don't know what that stands for, but the scale is different than ISO/ASA.

Doubling or halfing ISO numbers is one stop; 25, 50, 100, 200, 400, 800, 1600, 3200

Remember, the ISO on the box is just a general recommendation, and personal testing will be necessary to determine your actual film speed.

 

[drlynn]

I have read somewhere how the numbers are actually calculated, but I couldn't remember to save my life. The important thing to remember, as Matt has pointed out, is that doubling the film speed (or halving it) changes the exposure by 1 stop.

The reason for the increased grain is this:
Film is a sheet or strip of acetate or polyester which is covered with light-sensitive crystals embedded in various layers of a chemical emulsion.
The larger those crystals, the more sensitive they are to light. Larger crystals make larger grain on the final image.