as others have said above…it is the films sesitivity to light. film emulsions react to light. so it is simple time versus light. the longer it is exposed the more light is let in. now with ISO. it is the speed in which film reacts to light. if you have a low ISO, say 50, then you will need a great amount of light…say the beach. and the photo quality is very clear and no grain(noise). and the subject needs to be very still unless you use i fast shutter speed. now when you go to the other end…say 1600 ISO…then you could be in a dark dark room, say just a candle burning, or the glow of your microwave. and if you set your camera on a tripod and set shudder speed for a long exposure…then you will pull images out of the dark(almost)…but the flip side is you have a lot of grain or noise. now the faster the ISO and shutter speed, you can stop motion with out a flash, such as in sports..say a ball being dunked and a tight end catching a ball.
digital cameras simulate film ISO. thats why they have it on them. so when you are in manual mode or Ap mode you can set you own speed and ISO.
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