On 29 May 2003 05:45:30 -0700, scott@wsrphoto.com (Scott M. Knowles) wrote:

>brianc1959@aol.com (brian) wrote in message news:

<3c459ba.0305281934.33d3d73b@posting.google.com>...

>> B:

>> I've got a Nikon 16mm f/3.5, and when I try your experiment (shooting

>> a wall from a few feet away *wide-open*) everything is sharp.

>>

>> The film plane is supposed to coincide with the image plane...

>It's my understanding that the image plane of any lens is parallel

>with the shape of the outer element and not flat field. The DOF takes

>care of much of the perimeter out-of-focus areas of the lens. When

>the lens shape is projected out to the image, often approaching

>infinity, it's reasonable to expect the image plane is almost flat

>since the curvature is so slight. Only macro, copy and specialized

>lenses are designed for flat image planes.

>

>If this wasn't the case, then why does my VFC lenses, and Minolta's

>literature, show an out-of-focus areas when using the "normal" image

>plane, and why the VFC lens can actually change this plane? You can

>test this with one object at a reasonably close distance or a building

>as some distance in the full frame at wide-open apertures, and you'll

>see that not all things are in focus corner to corner.

?????????????????????

Some comments:

1) Most camera lenses are generally designed to have

reasonably flat fields of focus, for several obvious

reasons (like being able to shoot a landscape or a wall

at a wide stop, with good focus everywhere in the frame).

2) Most camera lenses fail to be perfectly flat-field,

especially with focus or FL change, if complex in

construction ("floating elements" are used in some

lenses to compensate for this). (A notable example is

the f3.5 75mm Planar on the Rollei twin-lens camera - it

needs to be stopped down to about f11 to cover the

problem with field flatness...)

3) The above does not preclude the design of some

lenses with specialized functions, like adjustable

field curvature...

4) With focus systems that are centered in the frame

(only), it can be an advantage for focus accuracy

to have field curvature that favors a mid-distance

focus point so that with rotation of the camera focus

is not lost (with rfdr cameras, for instance...).

5)I base my lens testing on the fact that I desire

good field flatness, and that infinity-focus subjects

are relatively insensitive to camera-turning effects

(permitting me to compare parts of the image area

with the same subject details without disturbing the

lens focus when reframing).