Archived posting to the Leica Users Group, 1998/01/29
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]>After reading all of the following, are Marc & I vindicated? or are we in >even more trouble? > >Erwin... HELP! > >Even more confused, > >Jim > 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). Erwin