On 9 February 2010 21:17, Rob Studdert <distudio.p...@gmail.com> wrote: > On 10/02/2010, jtainter <jtain...@mindspring.com> wrote: > >> Can anyone calculate how large (diameter) the front element of the DA* 20 F2 >> will be? This lens will have to be larger than the FA 20 F2.8. Sigma's 20 >> F1.8 takes, IIRC, an 82 mm filter. So this will probably take a 77 mm. >> filter. > > I don't think that the front element for the cropped sensor 20mm lens > would need to be nearly as large as the FF 20mm lenses but still that > lens pictured does look kind of small, it would be a 58mm filter at > most.
Rob (and JTainter), The front element is precisely the one that doesn't change (substantially) with format size. A lens that is f/1 will have a front element diameter no smaller than the focal length of the lens. For each stop that you slow down the lens from f/1, the diameter can decrease by square root of 2, or aprox. 1.4 time. So the minimum diameter for the front element of a 20mm f/1 lens is 20/1 = 20mm; for f/1.4 it's 20/1.4 = 14mm; for f/2 it's 20/1.4/1.4 = 20/2 = 10mm. Except this is all wrong, because Pentax's register distance is 45.46mm, and lenses wider than 45mm will start bringing their rear element closer and closer to the film/ccd plane. Because there is a mirror in the way in SLRs, extreme wide angle lenses use a different optical construction than standard and long focal lengths, it's called retrofocus, or inverted telephoto. In this design, the front element is much larger than the aforementioned formula would decree. How large? As far as I know, it depends on the design, although I'd gladly hear about any formulas dictating minimum required size independently of the design. So I suppose I didn't really give you an answer, huh? :-) In practical terms, a Pentax filter size is always smaller (or equal) than that for the equivalent Sigma lens :-D But seriously, once upon a time Pentax designed a 20mm f/1.4 prototype lens and it had a filter diameter of 77mm: http://kmp.bdimitrov.de/lenses/primes/_prototype/K20f1.4.html The very nice (and compact) FA 20mm f/2.8 has a 67mm diameter, which could maybe be retained considering we'd be increasing the entrance pupil by 1 stop, but also reducing the FoV by about 1 stop, for a DA 20mm f/2. Of course, we also have to allow space for weather sealing, and maybe SDM gadgetry (this is just a guess). But I think that if they want to, they could keep the filter size at 67mm. Cheers, --M. -- http://EnticingTheLight.com A Quest for Photographic Enlightenment -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
