Influencing factors on digital noise are the length of an exposure or a high ISO setting.
Prior to this exercise I have looked at the images 'Grey Texture' and 'Turkish Dance' in the Key Resources section of the student web site. This goes to show that distinguishing noise from a real pattern can be difficult as well as subjective.
To carry out this exercise I have set my camera to aperture priority and carried out a little test to make sure my slowest shutter speed is no longer than 1/2 a second. I settled on an aperture of F8. The camera was mounted on a tripod and I set up a house-plant with some white foam board as the background. I made sure the white foam board was in the shadow.
I then started shooting the same setup from my lowest aperture which is 50 ISO to the highest which is 25600. The ISO range I went through is as follows: 50 (L), 100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 5000, 6400, 12800 (marked as H1) and finally 25600 (marked as H2). L, H1 and H2 are classed as ISO expansion in the camera manual (Canon EOS 5D MKII) and are not normally available unless a custom function is set.
Here is the 50 ISO image which will be the least noisy.
|50 ISO F8. The lowest noise image|
|Detail from the 50 ISO image above.|
|Same detail as above but at 25600 ISO.|
This noise is at its most extreme when using ISO expansion. When you look at the same section at 6400 which is the camera's normal top ISO setting, things are considerably better. The coloured noise is still evident in the background but the flower and the stem are a lot cleaner with the noise not so apparent. The main difference between this photo and the first 50 ISO version is that in this image the detail does not come across as being quite so smooth.
|The petal at ISO 6400, the normal top ISO for the Canon EOS 5D MKII.|
|ISO 800. Perfectly usable under normal conditions.|
I think that the noise may have shown up more prominently had I had a darker shadow area judging by the effects of noise on the dark stem of the plant.