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".  Telephoto A "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 |
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