On 5 Nov 2003 at 16:58, [EMAIL PROTECTED] wrote: > They are still not great compared to 50mm lenses. > Nikon's aren't either, nor are Canon's likely to be. It's just not as > easy to make a good 20mm as it is to make a good 50mm, and certainly not > easy to to it small and inexpensive (nikon's first 20mm took 72mm > filters!). Even the most modern, expensive ultrawides do not perform as > well in terms of distortion and corner sharpness as lenses in the 35-85mm > range.
Well it's to be expected, compare the number of elements and the complexity of the optical faces in the 50/1.4 and 15/3.5 designs. One is near symmetrical the other is extremely asymmetrical and has many more optical elements. The brand of manufacture has little to do with it. > I've always wondered when I read one of those reviews that > says "great at all apertures" or "great lens" whether the reviewer means > that the performance is great in an absolute sense or simply relative to > other similar lenses. Relative, MTF diagrams which are an absolute measure of basic performance will bear this out. > Why does it affect the ultrawides more? It certainly seems to, since the > Nikkor 20-35 and Canon 17-35 have lousy reputations with respect to > chromatic aberration and such. I suspect that it has a lot to do with the angle that the light rays exit the lens. The more acute they exit then the more displaced the colour offsets due to chromatic aberrations will be and hence the less likely the rays will hit the appropriate cluster of pixels. The error will be worse the further from the central axis of the lens. > It does suggest that unless the Foveon technology succeeds in the market > and the Bayer grid disappears the lens-makers are going to have to try > to correct better for chromatic aberration in future lens designs. It would definitely be very educational to see the difference in aberrations produced by one lens design on both Foveon and Bayer sensor equipped bodies. > That 1/6th in each corner however is precisely where the ultrawides tend > to struggle, especially the aforementioned 20/4.5. The extreme edges and > corners of my 18/3.5 Nikkor show some really nasty distortion and lack of > 3-dimensionality that don't show in DSLR images. If you inspect the MTF of most wide lenses you'll find that the deterioration is progressive, it doesn't just get bad in the corners, the only general remark that would be true is that they are most sharp in the centre. > The corner performance of lenses is often tested presumably as a sort of > "worst case scenario" for the lens. Depending on how you shoot there may > not be anything important in those corners anyway. MTF tests show resolution/contrast vs radial distance from the lens centre, much more informative. What it all comes down to however is knowing what the lens is capable of and using it to it's full potential. If the resolution hoovers in the corners then buy a better lens or move up format. Their 'aint much choice at the moment :-( Rob Studdert HURSTVILLE AUSTRALIA Tel +61-2-9554-4110 UTC(GMT) +10 Hours [EMAIL PROTECTED] http://members.ozemail.com.au/~distudio/publications/ Pentax user since 1986, PDMLer since 1998
