[Leica] When a Biogon is no match for THE Biogon

[tgray at 125px.com (Tim Gray)]

On Apr 06, 2012 at 02:25 PM +0930, Marty Deveney wrote:
>Inevitable; the ZM "Biogons" are retrofocus lenses, after all.  But
>they display less of the bad sides of symmetrical wides, including
>vignetting less, and they actually clear the mirror or shutter of most
>cameras you can use them on.

If you guys are interested, check out pages 9-10 of this pdf by Zeiss.  
They talk a bit about this stuff and how the ZM lenses show that there 
are 'degrees' of design between full symmetric lenses and full 
retrofocus.  The ZM biogons (and probably lots of Leica's newer lenses) 
can probably be thought of as a cross between the two, but they still 
share many of the characteristics of symmetric lenses.

http://blogs.zeiss.com/photo/en/wp-content/uploads/2011/12/en_CLB41_Nasse_LensNames_Distagon.pdf>

Also, the bit about entrance and exit pupils on page 11 is interesting.  
I quote:

> With symmetric lenses, the entrance and exit pupils are the same size; 
> this is the case for the old Biogon lenses as well as the Planar types 
> for the rangefinder camera. The Biogon types slightly modified for TTL 
> metering display slight asymmetry of the pupil ratio.

I can confirm that my ZM C-Biogon 21/4.5 exit and entrance pupils are 
pretty much the same size.  It does have a larger back focus distance 
than the original and the lens element diagram doesn't look that 
symmetrical (unlike the original), but it has very little distortion, 
has equal sized entrance/exit pupils, Zeiss calls it a Biogon, a lot of 
vignetting, and it has the poor performance of a symmetrical lens on 
many digitals.  Does it have some retrofocus 'genes' as Zeiss says?  
Probably.  But it if it looks like a duck, quacks like a duck, and walks 
like a duck...

I guess my point is that very few lenses stick with the pure designs of 
the original Biogon, Planar, etc.  Also, too, the Zeiss pdf is an 
interesting read.

P.S. - the bit about entrance/exit pupils might give a clue as to why 
Zeiss named the ZM 85/2 a Sonnar.  Maybe not - I've never seen one in 
real life, much less looked at the pupil sizes.