"T P" <tp@nospam.com> wrote in message news:fc9qhv4jf06umvl7n4i1gatvtvtie8tbvd@4ax.com...

> John Miller <me@privacy.net> wrote:

> >brian wrote:

> >> Optically, a distant

> >> horizon is most certainly a flat field subject,

> >Optically, it may be at infinity, but physically, it's an arc of a circle

> >with the observer in the center.

> The visible horizon is exactly what you stated here, John. It is

> curved and at a constant distance from the observer.

> It is *MOST CERTAINLY NOT* a flat field subject! Except maybe to

> people like Brian who design junk plastic lenses for CCTV cameras, and

> think "infinity" is defined as focal length x 40.

While JM is correct (and I think his post was ;^), since

"infinity" and "near infinity" are nearly identical concepts

(and are identical in optical terms), a horizon line is most

certainly a "flat-field subject" in practice - and shooting

it with a flat-field lens (when it occupies nearly the whole

image width) produces better results than when shooting

it with a curved field lens... Two examples: the Nikkor

28-50mm f3.5 at 28mm would shoot a subject in the

center sharp at 6' or so, but show the horizon behind

out of focus in the center, but sharp at the edges; the

Rollei 3.5 twin-lens with the f3.5 Zeiss Planar (poorly

named! ;-) would get the horizon line in focus in the

center and corners of the frame, but not inbetween

until around f8-11 (a Minolta Autocord would get the

horizon line sharp from corner to corner at a much

wider stop, as will most Nikkor lenses that include the

28mm FL - but the AF-D wouldn't...). Landscapes are

a common subject type, and included distant material is

common to other types of photography (architecture,

group photos, even portraiture, etc.), and having the

"plane" of focus wander about is generally not useful.

at least to some of us...;-)

David Ruether