Canon users: disable Highlight Tone Priority (HTP) and Auto Light Optimiser (ALO)

HTP and ALO are two settings which you should disable if you are trying to achieve results based on your settings (as opposed to results manipulated by the cameras decision making), if you want your display to show the actual shot you have taken rather than an altered one and if you want to stop your camera from doing pointless work thereby potentially reducing lag.

Tim explains why these settings are not desirable, in particular if you are shooting in RAW, in the following thread:

http://www.talkphotography.co.uk/forums/showpost.php?p=2684230&postcount=385

For the sake of completeness I will point out that a number of people have experimented and found which of these intermediate stop ISO settings are created by software and which are natural ISO settings for the camera. However, it all becomes a bit too much to remember and the reality is you are safe with the full stop ISO settings.

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.