On Oct 6, 2014, at 10:38 PM, P.J. Alling <webstertwenty...@gmail.com> wrote:
> On 10/6/2014 9:33 PM, Bruce Walker wrote: >> On Mon, Oct 6, 2014 at 9:23 PM, Stanley Halpin >> <s...@stans-photography.info> wrote: >>> On Oct 6, 2014, at 4:26 PM, Larry Colen <l...@red4est.com> wrote: >>>> P.J. Alling wrote: >>>>> and wonder of wonders it's got some interesting information for free. >>>>> >>>>> http://www.luminous-landscape.com/techniques/pentax_645z_astrophotography.shtml >>>> Excellent link, though I'm afraid that if I were to spend $10K on a 645Z, >>>> I wouldn't have any money left over for the $500 astro mount. >>>> >>>> I was surprised to find that there don't seem any lenses faster than f/2.8 >>>> available for the 645. Doing some quick web search, there don't even seem >>>> to be any manual focus lenses faster than f/2.8 available. >>>> >>> There are adapters that support the use of Hassleblad etc. on the 645. >>> Maybe you can find the faster glass you need by going that route? >> I always assumed that there isn't faster glass because there doesn't >> need to be. The DoF on medium format is already razor thin compared to >> 35mm and APS-C and perhaps a 1.4 on a 645z would create a serious >> focusing problem? Or ridiculously OOF portraits? >> >> In other words, we have what's practical to sell, as with other >> formats. Or am I way off base? >> > Not just DOF, but an f2.0 135mm would be quite large and heavy if built to > cover the 645 format, yet it would be the equivalent of a Portrait lens say > 85mm on 35mm, (75mm actually). Fast glass makes in any focal length on 645 > need a tripod, whereas Pentax build a system to be equally good as a hand > held field camera, as well as at home on a tripod in a studio. Traditionally > medium format lenses have been fairly slow. There are exceptions, but they > are exceptions. > Bruce, not to disagree with your point at all, but FYI a 645 135mm lens on the 645z would have an effective field of view equivalent to a 110mm lens on a 35mm film camera. The “crop factor” is 0.8. So taking (many of) the actual lenses available, the 645z has: X 645 lens => equivalent to Ymm focal length on 35mm 25 => 20mm 35 => 28mm 45 => 35mm 55 => 44mm 75 => 60mm 90 => 72mm 120 => 96mm 150 => 120mm 200 => 160mm I think you were basing your comparison on actual 6x4.5 film vs. 35mm film. One other point about lens speed: the importance of wide apertures has seriously diminished (except for very specialized niche applications that call for shallow DOF) - the ability to shoot clean shots at ISO6400 or 12800 really makes “fast” lenses a relic of the good old days when we had a choice of either Kodachrome as God intended it to be at ISO 25 or of that new Kodachrome 64 that was a serious compromise in quality. Though I must admit that I prefer a fast lens to a slower one for the simple reason that I have a brighter image to focus and compose. Stan -- 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.
