Exercise: Your tolerance for noise.
For this exercise I made a series of identical images using the full range of the ISO settings on my camera. (from ISO 100 to ISO 6400 in 1/3 stops – 19 in all) The aperture for each image was constant (f6.3) and the shutter speed ranged from 1/250 to 1/4000 sec. Noise reduction was turned off. For comparison, I have included details of only the first, tenth and nineteenth images and for each I have shown the details of the textured and untextured parts of the image to demonstrate the effect of increasing the ISO sensitivity on different parts of the image.
Here is the whole image:
2928 f6.3 1/250s ISO100
Here is a table of the images comparing the results for the textured and untextured areas of the image, in sunlight and shade, at ISO 100, 800 and 6400:
Conclusion: From the images above, it can be seen that noise in the textures of the cloth is not so easy to distinguish. At high ISO the texture seems to be less sharp with some occasional coloured artifacts. at ISO 800 there appears to be no appreciable difference. On the other hand, where there is no texture, at ISO 100 there is little noise, at ISO 800 more granular structure and colour banding and at ISO 6400 there are distinct clumps although the colour banding seems to have gone. Although this exercise was a bit hit and miss as far as control over the lighting is concerned, it clearly demonstrates the relationship between ISO sensitivity and apparent noise in different parts of the image.