Focal length and lens aberations

[John Dorio]

It seems logical and evident that the amount of aberations present in a lens is proportional to the focal length. Has anybody measured any performance differenc es among lenses of different focal lengths? Would anyone care to share these com parisons with other LF lens users?

[Garry Edwards]

In theory it is proportional and until computers took over the design of lenses it was probably proportional in practice too. I suspect that some LF format lens makers simply don't try too hard. Check out the Canon 14mm lens for their EOS 35mm cameras, the quality and rectilinear properties are outstanding. If they can do it, albeit at UK#2200, surely it could be done for LF too?

[Glenn Kroeger]

John: The answer is sometimes... some aberations, longitudinal chromatic aberation for example, are proportional to focal length... thus the use of low dispersion glass (ED, Fluorite) in long focal length (telephoto in 35mm) lenses. Other aberations, spherical for example, are nearly inversely proportional to focal length... thus the increasing use of aspherical elements in wide angle optics. A good example of the latter is in reflecting telescope mirrors. A spherical mirror makes a pretty good telescope if the focal length is long enough since a low curvature spherical surface is pretty close to the shape of the center of a parabolic surface... but shorter focal lengths require a true parabolic surface to produce good images. I suspect other aberations have other, or perhaps no, relationship to focal length. An overall look at lens designs suggests that both long and short extremes need special approaches to aberations, while "normal" focal lengths balance aberations pretty well without extreme measures. Thus many older LF lenses in the normal range (100-300mm in 4x5) still hold their own with the latest whizbang low-dispersion aspherical powerhouses.