[Leica] Field of curvature good or bad?????

["BIRKEY, DUANE" <dbirkey@hcjb.org.ec>]

I will politely suggest that I think both Jim and Marc and many others 
are holding to a widely held concept of field of curvature that is a 
bunch of bunk.  My understanding of field of curvature is that is the 
primary cause of why resolution drops off sharply or not so sharply in 
the corners when a lens is wide open.  Stopping down the lens improves 
resolution in the corners because it is correcting that curvature.  A 
lens with a high field of curvature will do a poor job at rendering 
flat objects unless stopped down, it will also do a poor job of 
rendering objects in 3-d  since many of the individual objects you are 
photographing are relatively flat but at different distances.   It's 
depth of field that allows us to shoot 3-D subjects with sharpness, NOT 
CURVATURE OF FIELD!   A lens with high curvature of field will 
certainly not improve sharpness at near distances in the corners and at 
infinity in the center.  

I think before anyone responds to this thread they should do what I 
have just done.  Take a wide angle lens, put it on a SLR and get down 
on the floor, preferably one with carpet.  Focus at the minimum 
distance and look at the corners.  With my Canon FD 20-35L, FD f/2, 
17-35L EOS and possibly the  20mm Nikon (that wasn't sharp in the F3 
finder in the corners period) the extreme corner focus point is several 
inches further away than at the center.  The exact opposite of what 
MARC wrote below as the benefit of lenses with high curvature of field. 
  Perhaps Leica lenses are made to curve the other way.  I doubt it but 
I'm open to that possibility so try it anyway.   Besides that I have 
yet to use a foreground in any of my pictures that is curved to "match" 
the lenses curvature.  But then again I rarely put anything in the 
bottom corner that isn't in the bottom center anyhow as I like to keep 
corners "clean" so they don't distract attention from the subject.   If 
you have a fence as your foreground,  field of curvature will cause the 
center, bottom center, corners and edges to be at different focusing 
points.  If that's good, I fail to see why.  I applaud the new flat 
field designs Leica is putting out.  Besides that, if curved fields 
were really better than flat ones, wouldn't Leica continue to make 
them??????

Duane Birkey
HCJB World Radio
Quito Ecuador

>>At 01:21 PM 1/27/98 EST, Tom Shea wrote:
>>>Fortunately, from my perspective, the newer Leica lenses seem to be
>>addressing
>>>the curvature of field issue.  
>>  
>>There are folks who LIKE wide-angle lenses to have marked curvature 
of
>>field due to the nature of wide-angle photography:  quite often, the
>>picture has close-in sides and far-away centers, so curvature allows 
a
>>picture crisp and sharp from corner to corner.
>>
>>Marc
>>

>I, for one, subscribe to this (what Marc just described). So what do 
other
>people use wide angle lenses for?

>Jim

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