Archived posting to the Leica Users Group, 1998/01/28

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Subject: Re: [Leica] Field of curvature good or bad?????
From: Jim Brick <>
Date: Wed, 28 Jan 1998 19:14:18 -0800

Thank you very much Erwin. The effect of FC is more clear now.


At 11:54 PM 1/29/98 +0100, Erwin wrote:
>>After reading all of the following, are Marc & I vindicated? or are we in
>>even more trouble?
>>Erwin... HELP!
>>Even more confused,
>When a lens is corrected for spherical aberration and coma and astigmatism
>all object points regardless of their position in the object field, are
>pictured as sharp image points. These image points however are located on a
>curved surface, in fact a paraboloidal sphere. The center of this sphere is
>located at the focal point of the film plane (sharpness plane). It then
>follows that only image points around the optical axis are located on this
>film plane, all other points will blur progressively as we move farther
>away from the center. FC can be positive and negative and a suitable
>combination of lenses can reduce the amount of FC, which is calculated with
>the Petzval sum. Now in practice FC and astigmatism occur together.
>Astigmatism generates for every object point two different image points,
>one behind the other. The sum total of these two aberrations is an image
>that is sharp in the center and blurred everywhere else. Not only flat
>objects are effected but every object, even extended ones. The FC will be
>more pronounced if the lens is focused closer.
>The effects of FC will be less visible if we stop down, because the radius
>of the blur circle will also decrease.
>In theory a designer could accept a certain amount of FC assuming that no
>film will be perfectly flat in the filmgate. As a film is curved a little,
>a carefully controlled amount of FC would 'follow' this curvature and so
>produce a flatter image.
>In general FC is always bad in every type of lens (wide angle or tele).