ISO Sensitivity and Noise
„Noise“ in digital photography means that the pixels of an image have a random variation in brightness and color, please see the sample images on the right.
It varies from camera to camera and mainly with the size of the image sensor at which ISO sensitivities noise get visible and disturbing.
In this respect small compact cameras have their biggest disadvantage compared to bigger ones, especially DSLR cameras.
It is common that compact cameras try to hide this defect by applying a noise reduction after taking a picture but this results in loss of details. Pictures appear somehow cloudy and smeared – not really better. Please see the second sample picture that shows a comparison of low and high ISO with the effect of noise reduction on the second image.
And with low noise especially at high ISO sensitivities the expensive DSLR cameras with a full frame sensor have a visible advantage over the smaller members of this species.
So for ISO sensitivity it is easy to give a recommendation: Just use low ISO sensitivities wherever you can and choose higher ones only if you really have to to get smaller aperture and especially shorter exposure times.