[Scythels]

 

The retrofocus construction is an inverted telephoto.  I wonder what the equivalent (size) f/4 telephoto lens would be for an 11mm retrofocus?

front element ~5mm rear element ~10mm sensor clearance 3-4mm, length ~20mm including image clearance.  This is only for one design that I quickly scaled down.

 

The performance quickly deteriorates into oblivion away from the central 10 or so degrees of half field of view.  The telephoto formation is not very compatible with very wide fields of view.

 

Basic symmetry theory here;

 

Elements have two radii of curvature.  If R2=-R1, the petzval sum of the element is 0 (that is, no field curvature). 

 

Unfortunately this contributes to strong astigmatism and spherical aberration.  A retrofocus needs to have net negative power in the front.  Meniscus elements do not contribute strongly to the petzval sum since R1 and R2 are in some range near each other.  By making R2 slightly smaller (=stronger curvature) than R1 the element has net negative power and contributes a positive-signed petzval sum (=forward bending field).  These elements unfortunately focus light, which is contrary to what is desirable.  What is widely agreed to be the best basic retrofocus form is to use a pseudo-relay system like this; http://i.gyazo.com/5ccfe17ce4bcea9fa3f86783a44be89a.png

 

To correct it is an easy thing to just keep splitting the front two elements into many many elements so you gain many degrees of freedom to correct with as the rear portion (which in this case is a cooke triplet with a split rear element) is decently corected itself.  The split split split split split split split method results in the stacked/nested meniscus front formation of most retrofocus lenses we see today.  Large bulbous front elements are created by pushing the lens into a design space where the radius of curvature is very small.  This practice will create strong field curvature used to balance the tendency of the design form.