Hi Walt,
Your question got me thinking....so I found myself Googling to read up on the ins-n-outs of borescopes and stumbled on some helpfull info:
Choosing a Borescope
Field-of-View
Field-of-view may be very wide, wide, medium, or narrow. Think of it as a cone coming from the borescope tip, so that anything within the cone is visible. The field-of-view of the ************ borescope is approximately 37°.
Field-of-View should be dictated by the distance from the distal end of the borescope to the subject, for your application. The wider the field the lower the magnification, and vice versa. If you have plenty of space to move inside the cavity, but want to see both detailed close-ups and big picture views you might choose a 67° moderate wide angle. If the space is more confined but you still need to see most of it at one time, try a 90° extreme wide angle. On the other hand, if you can't get close enough to show the detail you need, a 30° telephoto might be required.
Magnification
Microscopes and loupes have very limited depth of field — they are only in clear focus at a single distance, so the magnification is also fixed. A borescope, however, has a very large depth of field — often from infinity down to an inch or less — that makes them easy to use without constant refocusing. The closer an object is to the lens of a borescope, the greater the magnification. To calculate magnification you must know the distance of the subject from the lens. The same principles apply to rigid or flexible borescopes.
Hope this helps a bit
