[rollei_list] Re: Back-Focus and Retro-Focus - Flipping lens

  • From: Emmanuel Bigler <Emmanuel.Bigler@xxxxxxxx>
  • To: rollei_list@xxxxxxxxxxxxx
  • Date: Wed, 09 Jul 2008 18:45:55 +0200


    Well, I don't know who has been using terms like
semi-retro-focus, not I said the little red hen.....
>  .......They tend to be more
complex for a given focal length, speed, and level of
performance, than a standard lens but make possible the use
of relatively short focal length lenses on cameras with
limited clearance behind the lens.
> ---
Richard Knoppow


Thanks Richard for this tutorial !

May be we could add that retrofocus designs usually have a pupillar magnification ration greater than unity. I have no general clue however on how the pupillar magnification ratio is related to the back focal distance i.e. the distance between the last lens vertex and the film plane, but it is an "experimental" rule that many if not all well-known retrofocus designss have a pupillar magnification ratio gretater than one. For example the 40, 50 and 60mm Zeiss distagons for 6x6 SLR cameras have a pupillar magnification ratio of above 1.5

This ratio is not directly given in Zeiss datasheet but the diameter of the pupils is listed, so one can re-compute the ratio of diameters easily, so we get the following table for example :

-----------------------

lens - entrance pupil diameter (mm) - exit pupil diameter (mm) - ratio of diameters (exit/entrance)

biogon 38 - 8.6 - 9.1 - 1.06 ==> this is a quasi-symmetrical design covering 90°

distagon 4/40 "old C" - 10.2 - 23 - 2.25
distagon 4/40 "CF FLE" 10.2 - 24.1 - 2.36  ==> typical retrofocus design
distagon 4/40 "IF-CFE" - 10.2 - 27.7 - 2.71
distagon 4/50 "CF old"  - 12.8 - 23 - 1.79
distagon 4/50 "CF FLE" - 13 - 28 - 2.15
distagon 3.5/60  - 17 - 26.6 - 1.57

planar 2.8/80 "CFE 7-element" - 28.8 - 34.5 - 1.19 ==> note > 1 like retrofocus
planar 3.5/100 "CF 5-element" - 28.9 - 32.8  - 1.1

sonnar 4/120 "CF Makro" - 29.7 - 33.5 - 1.13
planar 5.6/135 "CF Makro" - 24.1 - 28.4 - 1.18

sonnar 4/150 "CFi" - 37.4 - 28.0 - 0.77 ==> note < 1 like telephoto designs
sonnar 5.6/250 "CFi" - 43,8 - 25,0 - 0.57
sonnar 5.6/350 "CF" - 59.4 - 24.2 - 0.41

for reference with a classical wide-angle view camera lens with a quasi-symmetrical design
Schneider Super-Angulon 8/90 covers 100°, pupillar ratio = 0.98

--
Emmanuel


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