> I don't understand why lenses of the same focal length would have > different fields of curvature, unless this is something to do with > chromatic abbreviation. Field curvature relates to the tendency for the image surface of a lens to be spherical (I know this is not totally true - but lets keep it simple). Since we actually want to use a piece of flat film we have to take action to correct this. This can be done as one of the many items which have to be balanced within an optical design. There is some quite nice theory about what you can and can't do with field curvature but that is really something for the optical physicists. As an aside, when some very fast lens designs have been required, it has been necessary to design with a curved film plane. In itself field curvature is not related to what we normally call chromatic aberrations (be they longitudinal or transverse) although as with all aberrations it is influenced by the wavelength (because glass is dispersive). Andy Shaw
