Machine vision lenses come with a fixed focal length.  Sizes typically range from 8mm to 50mm.  The focal length is the distance between the CCD and the back of the lens. 

To determine what lens you want, you need to determine your standoff distance (how far away the camera will be from the target), and, more importantly, your required field of view, or FOV.   Most machine vision and lens manufacturers provide a lens selection chart to help determine what lens you want.  The aspect ratio of the CCD will also have an impact.  A typical 640x480 vision system has an aspect ratio of 2/3.  Most charts are developed with this aspect ratio.

The larger the focal length, the smaller the FOV, and the shorter the standoff distance will be.  A 50mm lens only has a focal depth of a few inches from a standoff of 6-12 inches.  Of course, you can still put a 1" target in a 12" FOV, but you are wasting 10x your resolution.  So it is better to try and fill the FOV up with your target of interest, so you have as many pixels-per-inch as possible.