firstly, grains are for film. Noise is for digital. Any noise is bad for digital, however, grains while not always good is not totally bad either. Confusing?It's best u google to learn the difference of grains & noise. I don't have the exact knowhow to explain here.
Meanwhile, lower iso levels will get u better quality pictures meaning finer grains/less noise.
Remember that 3 factors u control on ur cam determines the final pic. Aperture, Shutter spd & ISO. Ideally, to get the best quality pic, u set ur ISO to the lowest eg. iso100. Ur cam will, based on ur ISO setting, measure the aperture and/or shutter speed for the closest accurate metering.
So when do u need to set ur ISO higher? In poor/low light conditions, eg: overcast, indoors, dusk, night, under shade. In these instances, you will notice u r unable to capture scenes in settings u normally use in sunny/bright conditions. Hence, u set ur ISO as high as it is needed to capture ur intended scene.
for fireworks, u dun use a high ISO. 'cos a high ISO will 'take in' more light than u need during a 2sec exposure than vs a low ISO, resulting in overexposure.
hope my explaination is a bit helpful. (and dun mind being corrected
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