The clarity of the image on the monitor is determined by the number of pixels in the screen. More pixels give improved resolution. However, bigger screens need more pixels than small screens to give similar resolution, so the number of pixels alone does not give any guide to the resolution of the screen on your camera.
The monitors continually evolve. They are now larger, brighter, and have a better angle-of-view than on the earliest models. The angle-of-view describes how far away from the central axis of the screen you can view your image without it suffering from distortion in colour or brightness. The viewing angle of the EOS 5D, for example, is 170° both horizontally and vertically. This compares to a viewing angle of only 80° on the EOS 20D.
Display brightness is something you may need to change from time to time, depending on the shooting conditions. Low screen brightness is good indoors when the ambient light is low, but will be washed out by bright sunlight. There is no correct screen brightness – just set the level that is comfortable for you. The early cameras – the EOS D30 and EOS D60 – had only two brightness levels: standard and bright. In the subsequent models, this was increased to five levels, allowing you to see the screen more easily in a wider range of lighting conditions. With the latest models, the EOS-1D Mark IV and 1Ds Mark III and the EOS 40D and 50D, technology has improved and they now offer 7 levels of brightness adjustment.
It is not uncommon for a TFT-LCD screen to display one or two 'dead' pixels. These pixels have a slight malfunction and appear as a single solid colour, usually either red, green or blue, but sometimes white or black. If you detect a dead pixel on your screen, don't worry about it as it will not affect the image saved to your memory card.