On Wed, Jun 13, 2007 at 12:05:46AM -0400, Doug Franklin wrote: > John Francis wrote: > > > But there's no reason for the lens design to be any different. A design > > that works for 16x24mm sensors will work for 24x36 sensors (or film). > > There's more than enough image circle to spare, so there's no need for > > (or benefit to be gained from) a different optical design. And if the > > lens mount is the same (which it is, for a K-mount camera) then there's > > nothing to make two different mechanical designs necessary, either. > > Sure there is. Savings in weight and the materials that compose that > weight. Reduced structural design and strength requirements because the > entire assembly is lighter. Gimme a minute, I'm sure I can come up with > more.
You're arguing in circles. You're assuming that there will be a saving in weight, and then using that to say that therefore there's a reason for a new design. I'm saying that your initial assumption is incorrect. Any lens design that is suitable for use with a 16x24mm sensor would also work with a 24x36mm sensor, so there's no reason for Pentax not to have taken any weight and cost savings years ago, long before the digital era. > By the logic you're using above, it seems to me there'd be no reason for > them to have replaced the FA series withe DA series. I can't imagine > how a 50-200/4.5-5.6 for full frame could be as small and light as the > DA version. That could simply be a failing of imagination on my part, > though. ;-) There is no reason to replace any FA lenses with a focal length greater than somewhere around 100mm with DA equivalents. It's only worthwhile when you get down to focal lengths where the size of the desired image circle is a significant factor in the optical design of the lens. I don't know how badly the 50-200 would vignette if used with an old film body. It may work just fine at all focal lengths, or it may not. Somebody on the list has probably tried it. The big weight saving with a small-sensor camera comes because I don't need to carry my 300mm/f2.8 - for the same field of view I can instead get away with using my 200/f2.8, and that weighs a great deal less. -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
