My camera has ISO settings ranging from 100 to 3200, the results are quite staggering when comparing one end to the other. For this exercise I have taken each image and then highlighted the left ball where the plain background wall is partly in shadow. All images were taken on a tripod at f/2.8 with aperture priority so that the depth of field remains constant throughout.
The remaining images are cropped in the area of shadow on the far left at 100% so that the levels of noise can be compared.
In summary the level of noise seen even at ISO 200 is visible when compared to ISO 100 but tolerant up to ISO 400. There then seems to be a significant jump at ISO 800 / 1000 and after that serious deterioration and increase in the level of noise. It would therefore be conclusive that to obtain images with a reasonable level of noise my camera needs to be below ISO 800 where possible.