Ive never completely understood why some photographers do ISO tests with pictures of normal subjects like people. High ISOs cause the most issues on dark shades and shadows. ISO works by artificially amplifying light signals, so for me, the easiest way to test it is to take a picture in a completely dark room, like a closet.

The following images were taken on JPEG Smooth, stacked on top of each other in Photoshop cropped to be a partial image at 100%, although they are all from the same part of the sensor. ISO clean up was on “standard” for this first set:
I wasnt able to tell any differences after 3200 (the all appear completely black).

Then I turned off ISO Clean up completely:I would imagine the above results would also apply to RAW files as ISO clean up would come after RAW processing to JPG.

The take home message is this:

It appears that you can shoot at ISO 3200 with very little if any noise artifacts in JPG (and 6400 with very minimal). However, this clean high ISO capability appears to be the result of the software used to clean up the images, and not as much the sensor. This means that if you are in a tough situation, where you may have to shoot at 3200 or higher, you will probably get cleaner results if you shoot in JPG, with at least Standard ISO clean up.

Lets test it in RAW just to be sure:

(BTW- The RAW files are freaking enormous: 35MB!!)

I just found out I do not seem to have the RAW update for the 5DII. Come back to the post later if you are interested. 🙂

Update- It appears only Photoshop CS4 will be able to access RAW files straight from the 5DII, otherwise you have to use a DNG converter! 🙁