On 11/02/2010, Miserere <miser...@gmail.com> wrote: > 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.
Great theory but there's far more to it, granted the SWA lenses are required to be retrofocus designs but there are virtually an unlimited number of ways to acheive this, just look at the design of the new Nikkor 24/1.4 for example. From a practical perspective I can tell you that my Sigma 14/2.8 has a substancially larger front element than the DA14/2.8. -- Rob Studdert (Digital Image Studio) Tel: +61-418-166-870 UTC +10 Hours Gmail, eBay, Skype, Twitter, Facebook, Picasa: distudio -- 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.
