Diameter
The borescope must fit through the smallest hole required. When choosing a borescope diameter, consider more than clearing the sides of the hole. A slightly smaller scope may allow "wiggle room" so that you can see more by tilting the scope. The best thing is to try it. If you need to see into very small holes and relatively large ones, it is usually best to optimize for each, by using two borescopes.

Length
Choose a borescope to penetrate to the greatest depth required, but not so long as to be unwieldy outside the hole.

Illuminators
Before you can see anything in a dark cavity, you need some light on the subject. Prior to 1960, borescopes used hot and often dangerous incandescent lamps at the distal (working) end. Today, quality borescopes usually uses fiber optic illuminator, where glass fibers carry light from an external light source through a flexible light guide, then through the borescope, to the distal end. Some inexpensive borescopes still use a bulb at the tip of the scope for illumination, which can lead to illumination, heating and contamination problems. All Epsilon borescopes use fiber optic illuminators in the scope body.