I noticed that sometimes when taking pictures with my D5100, they come out with very little visible noise with ISO 800 and up, for example this picture. It was taken at ISO 1600 if I remember correctly, with noise reduction turned off.
But other times, even at ISO 400, noise is very visible. I would like to ask, is there a reason why high ISO noise is more or less visible in certain situations?
Noise is always there but its visibility is relative to the image content. This is why image noise is measured as a S/N or Signal-to-Noise Ratio. The higher the S/N, the less visible the noise. In practical terms this means that noise is less visible in bright areas.
Noise is not constant. While part of what creates noise is simply randomness, there are factors which cause more noise to be there. Sensor heat which can be a result of long exposures and high ambient temperatures is known to increase noise levels.
Noise is not always perceived the same way. Our brain and visual systems work together so that it picks up patterns and even more when patterns are broken. In terms of image noise, this makes it much more visible in smooth areas than where there are lots of fine details.
Take a look at your image. You see noise in the shadow of the top bread on the cheese (smooth dark area) and on the soft-drink glass (another one). The rest of your image has a lot of texture, including the counter and bread which hides noise better.
