Archived posting to the Leica Users Group, 1998/01/28
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Thank you very much Erwin. The effect of FC is more clear now. Jim 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, >> >>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 >