[Leica] M lens 21mm F2.8 pre ASPH

[henningw at archiphoto.com (Henning Wulff)]

Unfortunately with respect to distortion, the f/2.8 Biogon has little 
to do with the 21 Biogon for RF, and the 38, 43, 53 and 75 Biogons 
for larger formats. Those were all nearly symmetrical designs, like 
the SA.

The new 21/4.5 Biogon is more closely related to those, and again has 
little to do with the f/2.8 Biogon.

For film the 21/4.5 Biogon is outstanding, especially with respect to 
distortion. Unfortunatly, it really doesn't work on digital M's.

When you look at a distortion curve that just keeps on increasing (in 
the negative direction) as it gets closer to the border, you're 
dealing with true barrel distortion, which is quite easy to correct. 
If the distortion heads back to zero again as it gets to the corners, 
you're dealing with 'mustache' distortion, which generally looks 
nastier, and is harder to correct due to the higher order corrections 
required.

Retrofocus lenses tend to mustache distortion. The f/2.8 Biogon has 
this, the Elmarits have this, the Super Elmar has this, the Wide 
Angle Tri-Elmar has this and the Summilux has more than any of those. 
Note: the pre-ASPH Elmarit does not have the most. What it has is 
astigmatism, coma and chromatic aberrations in the corners that is 
worse than the others; ie, it is softer and has more flare in the 
corners.

When you're at f/5.6 all this is largely irrelevant, as usually other 
factors conspire to make your pictures not as good as you'd like, 
rather than the lens faults. But if you want to shoot at f/2.8 to 
f/4, the other candidates are all better than the pre-ASPH Elmarit in 
the corners.

All this doesn't mean you can't take pleasing pictures with it. One 
of my favourite lenses is the thin T-E 90mm, which is acknowledged to 
be one of the poorer performing lenses that Leica made in the last 40 
years, along with the 21 Elmarit. Mine is a fairly good example with 
no flare problems. But I like many of the pictures I've made with the 
T-E. That's good enough for me. I'm not going to compare it in a head 
to head resolution/contrast test with the 90 AA, but here I've sold 
the AA and kept the T-E (and got a late pre-AA Summicron). Use the 
lenses you like.

I would love to have a near zero distortion 21 like the f/4.5 Biogon 
for my M9, but there isn't one available. So I shoot with other 
lenses that give me images I like, and use correction software to fix 
the distortion when that becomes objectionable.

It's extremely hard to make a very high performance retrofocus 21 
(meaning great MTF and other tested parameters) that also has low 
distortion. It takes a very large lens that's relatively slow. 
However, it's possible to correct distortion in software, but most of 
the other optical faults are not easy to fix, so I'm glad Leica chose 
the path they did with the new Super-Elmar. It's a worthy 21st. 
century successor to the 21/3.4 SA.




At 8:10 PM -0700 6/28/11, Frank Filippone wrote:
>Not to add too much to this "discussion"... The Biogon design, from what I
>remember, is one of the most distortion free designs around.  It is made in
>many focal lengths, including the famous 21 Biogon for Contax RF, and the
>really famous 38 Biogon on the Hasselblad SWC.
>
>
>Frank Filippone
>Red735i at earthlink.net
>
>
>
>
>_______________________________________________
>Leica Users Group.
>See http://leica-users.org/mailman/listinfo/lug for more information

-- 

       Henning J. Wulff
  Wulff Photography & Design
mailto:henningw at archiphoto.com
   http://www.archiphoto.com