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

[Ken Wilcox <wilcox@umcc.umcc.umich.edu>]

I realize that this thread is mostly about wide angles, but Leitz made a
very good explanation of why curvature is not a problem and in fact may be
usefull in their explanation of the 400 f6.8 Telyt when it was introduced.

They explained that the edges actually focus closer than the center, thus
allowing the foreground of a picture to be in focus when it may not have
been if the field were flatter. I have found this to be useful when using
this lens.

I haven't applied this to wide angles, but I don't see why it could not be
exploited for these lenses also.

kw

>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
>
>------------------------------


- ----
Ken Wilcox                                Carolyn's Personal Touch Portraits
LHSA, MEA, LAW                         preferred--->
<wilcox@umcc.umich.edu>
                                              <kwilcox@genesee.freenet.org>