Compared to...

Studio scene comparison (higher sensitivities)

On the crops below you can see how the 50D images get softer as you increase ISO and noise reduction becomes more aggressive. You can preserve more detail by switching noise reduction off but, as you would expect, you'll pay for the extra detail with some extra noise. Despite the noise reduction set to 'Standard' you can also see chroma noise creeping in pretty early on. While ISO6400 might be usable for small size low-res prints ISO 12800 is firmly based in the emergency camp.

ISO 800

Canon EOS 50D	Nikon D300
Sony DSLR-A700	Pentax K20D
Canon EOS 40D

ISO 1600

Canon EOS 50D	Nikon D300
Sony DSLR-A700	Pentax K20D
Canon EOS 40D

ISO 3200

Canon EOS 50D	Nikon D300
Sony DSLR-A700	Pentax K20D
Canon EOS 40D (Equiv*)

ISO 6400

Canon EOS 50D (Equiv*)	Nikon D300 (Equiv*)
Sony DSLR-A700	Pentax K20D (Equiv*)

ISO 12800

Canon EOS 50D (Equiv*)

* Additional sensitivity settings denoted by the manufacturer as boosted or equivalent to that ISO setting.