This is an essential part of every stamp collector's tool kit. There are always details of every stamp that can only be seen with a magnifying glass, no matter how good your eyesight is.
Most collectors have several magnifiers for various views.
The basic magnifier is a traveling one that can be taken to stamp shows to examine material from a dealer's offering before you buy the stamp or cover. This can vary in strengths, but one should be in your pocket at all times. Collectors claim that more than ten power actually shows too much in detail and may be useless in examining stamps. Less than five power simply does not show everything that you will want to see on a stamp. The easiest way to check if the lens is color-corrected is to examine a black line on a white background.