Exactly! Not that you can not push the design but then you lose more guality. The fact is that the 35mm based Pentax bodies have a backfocus distance of about 46mm. Now the physical nodal point of the lens can be somewhat (a few millimeters) behind that so maybe you can produce a standard lens of say 40mm.
That means that any lens shorter than 40mm has to be a retrofocus design. A retrofocus lens is a lens whose optical focal length is shorter than its physical focal length. You design a 15mm/3.5 that adequately covers 43mm (the circle you need to cover 35mm film) it is certainly going to cover an APS-C sized sensor and the light rays coming from it will be at the same angle as those from a 40 mm normal lens in either case. Now if you have no problem with a 40mm lens on the digital sensor, you will obviously have none with the 15mm either. Claiming that it is the short focal length and the acute angle is causing a problem simply implies that the person making the statement does not understand optics very well. Now by restricting the angle of the back focus (smaller circle of coverage) one can make a lens that has slightly better edge definition, simply because one is willing to give up resolution farther out. That has nothing whatsoever with the angle the light is stiking the sensor however. Of course I guess one could design a 15mm lens that actually has a back focus of 100mm or so, but the compromise in quality would be rather extreme. -- graywolf http://www.graywolfphoto.com http://webpages.charter.net/graywolf "Idiot Proof" <==> "Expert Proof" ----------------------------------- Toralf Lund wrote: >>> Also, don't you get the same kind of problems with a e.g. DX-size (!) >>> sensor and a lens that's sufficiently wider to give an equivalent >>> field-of-view? >>> >> Not if your lens design for the digital sensor is formulated to >> correct the ray trace so as to make the edge/corner rays more >> perpendicular to the sensor plane. This is done with a couple of >> correcting elements well behind the primary lens groups. > Graywolf seemed to suggest that the lens designers are (were) in fact > doing this already on "traditional" SLR wide-angles, and that it would > be hard to correct the rays even further... > > - Toralf > > -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
