Aircraft carriers and naval air stations typically use Fresnel lenses in their optical landing systems . The "meatball" light aids the pilot in maintaining proper glide slope for the landing. In the center are amber and red lights composed of Fresnel lenses. Although the lights are always on, the angle of the lens from the pilot's point of view determines the color and position of the visible light. If the lights appear above the green horizontal bar, the pilot is too high. If it is below, the pilot is too low, and if the lights are red, the pilot is very low.
A single-element camera lens is as long as its focal length; for example, 500 mm-focal-length lens requires 500 mm from the lens to the image plane. A telephoto lens is made physically shorter than its nominal focal length by pairing a front positive imaging cell with a rear magnifying negative cell. The powerful front group over-refracts the image, the rear restores the focal plane, thereby greatly shortening the back-focus length.  Originally, accessory negative cells were sold to attach to the rear of a regular lens. The Barlow lens, a negative achromat magnifier invented by Peter Barlow in 1833, is still sold to increase the eyepiece magnification of amateur telescopes.  The teleconverter is the modern photographic equivalent.  
A hand-held meter, especially for travel and outside use that I can most strongly recommend is the Gossen Lunalite. It uses 3 LEDs instead of a meter needle. I have even dropped mine and it still works fine. I have now acquire a second one to use with my Leica M3. The 9v PP3 battery is available widely, Avoid the ones that take the Mercury battery, not worth the hassle. Another, more recent variant is the Multisix. This too takes the 9v battery and is an LCD readout. However, despite also now having one, I have to say that it’s quicker to reset the ISO on the older models with the big dial. However, Gossen have been very helpful in providing a large label on the back of the Multisix to guide you in making changes to ISO, shutter priority/ aperture priority/ EV readouts etc. Try to find a meter with the hard type of leather case. More protection.