A large cavity usually requires a large amount of light to successfully conduct a remote visual inspection with a borescope. This simply requires the largest possible borescope. If the borescope can be rigid it will be a simple matter to focus at the required distances.
If the borescope needs to be smaller due to access limitations, multiple access points should be investigated. Tools can be made to deliver a smaller scope to defined points within the cavity. Auxiliary lighting probes can be produced to help deliver light in addition to the light from the borescope probe.
If a semi-rigid borescope is desired for durability, a tip focusing scope can focus at multiple distances. Another advantage of a tip focus scope is the brightness can be better than an equivalent rigid scope. If the access is restricted only in one dimension, probes can be built to take advantage of the space available to maximize illumination. In the case of a slit access, the probe can be built with tubes placed adjacent to carry more lighting fibers and higher resolution image optics.