[Leica] Biogon SWC (lack of) distortion

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

On Mar 04, 2012 at 04:04 AM -0800, Richard Man wrote:
>Yes, but my question is, how come the SWC Biogon doesn't seem to have the
>distortion of circles becoming oblong on the edges, or am I incorrect in
>saying that?

You are incorrect in saying that.  What you are referring to is often 
called perspective distortion, and any lens that has a rectangular 
projection has it.

What the SWC doesn't have is barrel, pincushion, wave, mustache, or 
other types of distortion that makes straight lines not straight.

Personally I think the SWC downplays perspective distortion a bit 
(relative to 35mm lenses) because of its square format.  The diagonal 
field of view (FOV) is 91 degrees, similar to the diagonal FOV of 21mm 
on a 35mm camera.  But the horizontal/vertical FOV is only 72 degrees, 
which is slightly less than the horizontal FOV of a 24mm lens for 35mm 
format.  Lastly, it's vertical FOV is similar to a 15-16mm lens's 
vertical FOV on 35mm.  So in some senses (diagonally), it's as wide as a 
21mm lens, in other senses (horizontally), it's narrower, and 
vertically, it's a good deal wider.  Compared to a 35mm lens, it's much 
more balanced in the FOV it provides in different directions on the 
basis of it being a square format.

When I had an SWC, I wasn't putting peoples heads in the corners of the 
frame; if they were near the edge, they were much more likely near the 
top or the side, so you get a bit less perspective distortion there.