Using the term "astrophotography" is sort of like using the term "Dr." or "PhD". It implies something that can be rather broad without revealing the particulars. Frankly, I think that calling Milky Way shots, (particularly with landscape sillouettes or light painting )"astrophotography" is a stretch. But to each, their own.
In any event fast lenses are really only important if you must limit your shutter speed, as we often need to do in terrestial photography. In astrophotography, with a motorized, polar-aligned platform, your shutter speed is limited only by your motorized/guided accuracy or periodic error built into the gears. It makes little difference if you are shooting at f2.8 or f/10 (as with using many of the popular Schmidt-Cassegrain telescopes for lenses). Most good apochromats for astrophotography are around f/5.5 or f/6. Most photographic lenses need to be stopped down 1 or two stops for best performance, particularly off-axis. So the wide open "speed" of the lens only determines at what focal ratio that one or two stops down is going to BE. A *scope* made for imaging has no such aperture iris and is made to shoot "wide open" because that is your only choice. Stacking images in post-processing allows you to take your longest guided/polar-aligned images and reduce noise, effectively pulling out more detail. A lens that performs well when stopped down to f5.6 is plenty good for astrophotography, though a lens that is fine at wider apertures might be even better. The DA* 200mm f2.8 is, by all accounts, one of the Good Ones, for deep space objects like nebula, galaxies & star clusters. On Mon, Oct 6, 2014 at 3:26 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. > > >> >> > > -- > Larry Colen l...@red4est.com (postbox on min4est) > > > -- > 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. -- Photographers must learn not to be ashamed to have their photographs look like photographs. ~ Alfred Stieglitz -- 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.
