"John Miller" <me@privacy.net> wrote in message news:bf66rs$tqq$1@n4vu.com...

> brian wrote:

> > Wide angle lenses in particular need to be flat field because

> > typically most or all of the subject will be within the depth of

> > field. Any lack of flat field correction will simply result in poor

> > corner performance for ordinary photography. Why do you suppose that

> > these lenses are designed and evaluated using flat object and image

> > planes?

> One could make the case that wide lenses in particular should be curved

> field, so when the lens is set to, say, 10 feet, everything at that

> distance will be in focus. With a flat-field design, the corners focus at

> a different distance from the film plane than the center (do the trig if

> you doubt it).

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...;-). Or, with a flat-focus lens, if you focus

on an infinity-distance subject (horizon, stars, etc.), all parts of the image

will be in focus regardless of the orientation of the lens (which is why

I test lenses with very distant subjects...;-). With close focus, the tilting of

the focus plane will change the focus of things off center, even if all

the items of interest remain at the same distance. This is a problem with

rfdr and center-focus-only SLR shooting with wide-angles, and can be

partially compensated for by using a compromise field curvature (but

this is never accurate in all situations, and it *will* compromise edge

performance with infinity focus and flat subjects....). Preferable is

a flat-focus lens, used with experience...;-)

--

David Ruether

d_ruether@hotmail.com

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

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