In digital camera's, ith higher ISO settings there is more voltage set on the sensor. The higher the voltage, the more noise in your image, that's the downside. The upside is that you need less light to get the same result. If you're compensating the ISO in photoshop by increasing the light setting, you're missing details because they didn't get caught in the first place. dSLR's have less noise because they use bigger sensors which are less susceptable to electronic noise than the smaller ones found in compacts.

What is a friend? A single soul dwelling in two bodies. - Aristotle
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