ABSTRACT
Modest apertures are common on extreme long focus (ELF) lenses, especially of the catadioptric, simple telephoto and zoom types, but comparatively large values of f/2 to f/4 are available with super telephoto lenses. ELF lenses are commonly used for distant subjects in order to obtain a useful image size within a small field angle of view. It is useful to be able to estimate the smallest object size that may be resolved by the lens. The limiting factor in ELF lenses is that of correction for lateral colour or transverse colour aberration, and elements of low-dispersion glass and fluorite elements are necessary in long-range zooms and super telephoto lenses. The use of ELF lenses is not without considerable practical difficulties. Predominant is that of vibration and camera shake degrading the image, and a massive tripod is needed, preferably two with one at the camera and one beneath the lens, to minimize vibration.
