### Telephoto Ratios

 Overview"Telephoto" denotes that positive and negative elements are used to adjust the focal length / package length ratio.Telephoto ratio is defined as the L/F = P, where L is the distance from the front lens to the image and F is the equivalent focal length (insert graphic). "Applied Photographic Optics" refers to this ratio as "Telephoto Power" or "Telefactor". Put figure similar to 5.5 from Kingslake Optical system design. And put expected ratios on that figure. Also put principle planes as in figure 2.14 to 2.15 from practical optical system layout?TelephotoA "telephoto" has a telephoto ratio < 1 or the focal length is longer than the length of the system. It is recommended to not have telephoto ratios < 0.6. Reference Optical System Design by Fischer et al... Actually they claim >0.6 telephoto for 'fast' optical systems (page 137). Kingslake's A History of the Photographic Lens, states telephotos are between 0.8 and 0.6 (page 132). Laiken's Lens Design has a telephoto of 0.5 (page 93) but it is very slow F/11 (and ugly). This rule of thumb applies to an infinite conjugate system (derive and include equations / rules of thumb for finite conjugate in terms of magnification).Example:Reverse Telephoto (Retrofocus ore Inverse Telephoto)When the telephoto ratio is > 1 this is called a reverse telephoto lens. Large ratios create "fisheye" lenses. This is a common configuration when the focal length is shorter than the back focal length, such as a "C-Mount" (BFL ~ 15.5 mm). Most texts do not really discuss Telephoto ratio on these. Looking into Laikin it shows P>2 and can be as large as 9.3. In general the larger the FOV the larger the Telephoto ratio becomes. F/#, image chief ray angle and distortion also play into the telephoto ratio.Example:Future:There is more work that can be done to recommend rules of thumb or starting lenses for a graph of F/# vs Telephoto ratio (vs FoV?). Need to brainstorm a good graphic for telephoto lenses.... F/# or FoV or Telefactor or Pupil ratio (see 14.1 in applied photographic optics). Need a catalog of lens designs for starting points upon the graph