Quote Originally Posted by jks_photo
is it usable?

Sometimes yes, sometimes no. It depends on the circumstance. There are many, many factors that affect noise, and the ISO setting is just one of them.
  • Color termperature of the light and White balance
  • Black level
  • Contrast and dynamic range
  • Saturation
  • Display size
  • Raw converter
  • Noise reduction
  • Downsampling filter



Probably the most important factor is display size. Let's say you normally make prints that are 30x20 inches. At this size, it's very easy to see noise, and let's say you never go higher than ISO 400. Now let's sayyou are making a new image for your facebook profile picture. That's only 200 pixels wide, which corresponds to a print size of only 1 inch (at 200 ppi). You can increase the ISO to ISO 6400 (!!) and it will still have the same noise visibility as ISO 400 in the large picture. That's why you can't take web images as proof of noise performance -- they correspond to very tiny print sizes. When you go to make a large print, the noise will be far more visible.