Canon users: only use an ISO at a full stop

ISO settings usually start at 100 and then increase in stops of light by doubling the ISO number, as follows:

100, 200, 400, 800, 1600

Canon cameras also offer ISO values in between these full stop values. However, it is best to avoid these partial stop ISO values since they are simply achieved by the cameras software intentionally under or overexposing the picture. This can lead to unintentional side effects, include more obvious noise if the image is slightly underexposed. It is better to use the full stop iso settings and be in full control of your shot, preferably exposing to the right.

You can force your 50d to stop using intermediate ISO values by setting the option in the custom function C.fn. I-2.