One of the major difference between a consumer digital camera and a digital Single Lens Reflex (dSLR) is that the former produces images with a lot of noise when using high ISOs and long exposure times, and the latter is practically noise-free. Noise is apparent by the presence of color speckles where there should be none. For example, instead of a blue sky, you notice faint pink, purple and other color speckles amongst the otherwise blue sky.
Why Are dSLRs Practically Noise-Free?
If it has not yet occurred to you to ask it, you should. Really, why are dSLR images almost noise-free? The answer is simple: a larger image sensor!
See, with a larger image sensor, each pixel can be larger and each photo site can be a bit further away from its neighbor (of course, there is an optimum distance beyond which we’ll have ‘gaps’). This extra distance is often enough to prevent signal leakage from one photo site onto another — hence much less to almost no noise!