With regards to a borescope, fiberscope, videoscope or pipe camera the depth of field or focal distance is the distance, measured from the objective tip of the instrument at which a usable image is available. Typically, expressed with a minimum and a maximum distance, i.e. 3 to 9mm. In this example if the tip of the instrument were closer than 2mm the image will become blurry and if further than 9 it will become too grainy to be of use.
Manufacurers do not determine the depth of field in real world conditions. Light output may be a limiting factor that prevents a usable image within the stated depth of field. For example it is not realistic to expect a video borescope to be able to see to infinity in a large and dark tanks or vessel despite manufactures specifications.