"John Miller" <me@privacy.net> wrote in message news:bf6bjs$ula$1@n4vu.com...

> David Ruether wrote:

> > Uhhhhhh..............! ;-)

> > I think "flat field" assumes a flat film plane, and is defined as having

> > a flat field of focus - which results in the film and subject *planes*

> > being parallel, and also *the same distance apart* at all points between

> > them (at minimum measured distances between them - or, as measured

> > perpendicularly between them...;-).

> Except that the light path in question is from the edges of the subject to

> the lens aperture, rather than to the extension of the film plane, which

> means we're dealing with hypotenuses instead of parallel lines.

Yes, of course, but looking at it this way defines curved field if you

propose that the focus distance should remain constant with rotation...;-)

You can actually observe (with a good, sharp VF) that as you rotate

the camera with a good flat-field WA on it that the focus distance

will change as an object is moved away from the center of the screen

(this effect diminishes with greater distance, becoming "0" with

infinity-focus subjects). A different result can be seen with curved-field

lenses and with *some* particular focus distance other than infinity,

and rotation of the camera does not change the focus quality at that

distance. The problem with designing lenses intentionally curved-field

to compensate for off-axis shooting with on-axis focus is that you can

design correctly for only *one* focus distance...;-) And flat subjects

that occupy much of the field of view, and are parallel with the film

plane, cannot be focused with one distance setting. Flat-field designs

are preferable - and focus compensations with rotations can be made

when using SLRs with good VFs (as they can be made for curved-field

lenses when used at non-optimal focus distances, except when the

subject is large in the view, and flat...).

> Please note that I wasn't arguing in favor of curved field lenses as much as

> I was pointing out that our assumptions about the necessity for a flat

> field may not be as valid as we think, except for copy stands and

> astronomy.

Yes - but I think we disagree on this, and on what defines a flat-field

lens...;-) I'm a nut about edge/corner sharpness - and a curved-field

lens, no matter how good, will not show a horizon line equally sharp

across the frame unless well stopped down (probably into diffraction

limiting for enough DOF, spoiling sharpness overall).

--

David Ruether

d_ruether@hotmail.com

http://www.David-Ruether-Photography.com

Hey, take a gander at www.visitithaca.com, too...!