Film speed and digital ISO...

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bdesj Junior Member • Posts: 26
Film speed and digital ISO...
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I think I understand at a very basic level how film speed works, no clue as to the principle behind ISO on a digital sensor and files.  I'm talking about how it works more than the practical effects.  Just out of curiosity, maybe somebody has a simple explanation?  I should say from the start that while I appreciate anybody taking the time to answer, that answer might very well go completely over my head!

I'm pretty sure (correct me if I'm wrong) that film speed is based on the size of photo sensitive crystals in the emulsion.  Bigger crystals pick up more light, and are therefore more sensitive.  But as they get bigger they are also more visible individually, showing up as "grain" on the developed positive or negative transparency, then on the paper if printed.

If that correlated directly to digital photos, it would make sense that individual pixels on the sensor somehow grew with higher ISO and shrank with lower ISO, but I really doubt that is the case.  What actually changes inside the magical box when we turn the ISO dial that lets us trade sensitivity for grain?

On a related note (maybe explained by the answer to my first question), I know that camera designers get the need for longer shutter speeds, or they would not offer built in ND filters on many models.  There must be some reason they can't just offer lower ISO options.  Though I don't have much experience with a lot of different models, my own modern pixel grabber is fairly well spec'd, and it just barely gets down into the ISO ranges that I used to buy film in, but it goes staggeringly high on the upper end.  Is low ISO governed by some related law of physics like those that make it impossible to build long and fast lens for a large sensor or frame in a compact shell?  Or maybe it's an issue of tradeoffs that make lower sensitivity in the sensor less desirable than simply blocking out some of the light?  Just wondering, thanks!

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