Although relay optics are not needed for chip-on-tip digital endoscopes, for completeness we will discuss the main types of relays used in surgical applications. As mentioned previously, many video endoscopes still utilize relay lenses in order to take advantage of the superior electronics available by using larger video sensors. As described previously, the classical endoscope employs a series of achromatic doublets for the relay optics, alternating between field lens and relay lens. Modern endoscopes typically utilize much longer lenses, with length-to-diameter ratios as high as 10 (see Figure below).
These are the so-called rod lenses of the Hopkins design. The advantage of the rod lens designs is that the chief ray bundle can be confined more tightly to the optical axis by reducing the ray divergence in the small air gaps between the lenses. This reduces vignetting, one of the most severe problems in small diameter rigid endoscopes. The added glass also reduces the optical path length for a given real length, reducing the number of air-to-glass interfaces in the system. This helps with glare, transmission, and Petzval curvature, as well as cost.
The mounting of rod lenses is also simpler than thin achromats. The rod lenses tend to tilt less when they are assembled into the inner lens tube. Also, since the spacers are shorter in a rod lens design, they can be made thinner, increasing the clear aperture. Rod lenses are typically fine ground on their outer surface to reduce stray reflection. There are now many variations of the rod lens concept, including designs that combine achromatic lenses with long plane-parallel windows.