Noise is not an issue at ISO 50, with superb image quality resulting, even for long exposures (up to 30 seconds) in very dark areas.
ISO 100 is very good, but dark shadow areas do give up smoothness and a certain clarity.
ISO 200 starts to look grainy; this is a threshold ISO—usable, but really the limit when medium format quality is still retained, and you give up a lot in the shadow areas, with a real loss of clarity and fine detail in dark areas.
At ISO 400, significant detail begins to be lost. For high quality work, it’s too far gone.
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Since 2006, Diglloyd Advanced Photography is the authoritative review and reference for DSLRs and their lenses: Canon and Nikon primarily, but also Pentax and some medium format. Reviews of key DSLRs are included, but the primary focus is on lens performance. Also included is a wealth of technique and workflow approaches.
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ISO 50, 4 second exposure