Hi all, this is specifically for Gary, but I thought
the rest of the group might be interested. On this
manner of curveture of field I seem to remember that
Minolta had a lens back in the late 70's where that
could be adjusted. I think it was a 24mm f 2.8 and
there was a ring on this lens with 3 positions marked
)-l-( ,and by adjusting this ring you could change the
shape if the DOF from concave to flat to convex.
Because of this I'd always thought that field flatness
was something that could be designed into a lens with
little trouble but I suppose there are tradeoffs on
other optical problems to be considered.
--- Gary Reese <pcacala@xxxxxxxxxxxxx> wrote:
> Dirk writes:
>
> << So, are macro lenses "flat field"? >>
>
> Depends. The term "macro" got used a bit fast and
> loose by marketing
> folks. A macro focusing zoom isn't likely to be flat
> field. Perhaps
> cheap macro lenses aren't, either. But there is only
> so much flatness
> that can be incorporated into a macro lens before it
> won't precisely
> focus on a body with focus offset as part of its
> design. Joel W. pointed
> out a posting on the Nikon list where a Micro-Nikkor
> required a bit of
> focus shift to get the focus right on the film (as
> opposed to the
> viewfinder). That sounds like a perfect example of
> making the projected
> image too flat.
>
> << I assume this is because the depth of field under
> macro conditions is
> extremely small, and any significant curvature of
> the projected image
> will seriously degrade the resolution? >>
>
> Yup.
>
> << Are macro lenses typically also "flat field" at
> infinity focus, or
> just
> at a predetermined reproduction ratio? >>
>
> I don't know. I presume that distance focusing
> aberration correction
> (floating elements) keeps the image flatter than it
> otherwise might be
> at infinity. But there are lots of other faults to
> correct and maybe
> that isn't one of them. Curvature of field would be
> less important an
> attribute to correct for when shooting 3D subjects,
> although one could
> argue that at infinity it is very important. I think
> we can assume that
> at the optimum magnification ratio, flatness of
> field is optimized.
>
> Gary Reese
> Las Vegas, NV
>
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