Fixed focal length lenses are entocentric lenses, meaning that they collect rays diverging from the optical axis. Fixed focal length lenses are commonly used optics in machine vision, being affordable products that are well suited for standard applications.
Knowing the basic parameters - focal length and sensor size - it is easy to calculate the field of view and working distance; the focus can be adjusted from a minimum working distance to infinity; usually also the iris is controlled mechanically, allowing you to manually adjust the lens F/# and consequently the light intensity, field depth and resolution.
Fixed focal length lenses are inexpensive and versatile, but they are not suitable for all applications. They usually introduce significant perspective errors and geometric distortion that are incompatible with precision measurement applications. Also, the manually adjustable iris and focus introduce some mechanical play, which makes these lenses not ideal for applications requiring very consistent and repeatable settings.