The photo uploaded here is shot with a compact dcam at ISO 400, which inevitably shows the grain when enlarged to 100%. The face of my grand son I picked up from the whole image. The details look merged. My point here is that in compacts if we go above ISO 200, the noise appears. The compacts are sold more, as they are aimed at general public photography/ enthusiasts/beginners However, if you use prosumers, even the problem of grain at ISO 200 may be started showing. In DSLRs, this problem will arise at ISO 800 and above. During film days the photographers generally got the film rating at ISO 100 for most purposes and for professionals ISO 200 and 400 were readily available in small towns too. So most of photographers used ISO 100 film and got best results and enlargements in weddings, travels and all functions, with good sunlight and bright shadows or indoor or outdoor with good brands of powerful flashguns....Though a bit difficult in low light, unless tripod is used.
and I can get a good enlargement of the full image shot on the small sensor of a compact camera....The sensor size is the main issue of having noisy results because of the compactness/thinness of the small cameras. SO IF YOU shoot with a compact, shoot at the minimum noise level set by the manufacturer i.e. may be ISO 80 or 100. The pros who use DSLRs are aware of the problem and therefore set and shoot according to their requirements in those cameras almost at ISO 100 or ISO200 FOR FINE DETAILS and sharpness in images...................YES there is cost difference in compacts, prosumers and DSLRs. Therefore interest in photography go accordingly from person to person and their requirements.