Most lenses go beyond infinity for two reasons: they focus shift and need the extra room to reach infinity or they are designed to deal with temperature fluctuations. A cold lens shrinks.
On Sun, Dec 4, 2016 at 4:50 PM, Larry Colen <l...@red4est.com> wrote: > > > Igor PDML-StR wrote: >> >> >> Larry, >> >> I haven't done much of astro-photography, so, I am just >> curious: why focusing at infinity wouldn't work in this case >> (i.e. turning the focusing ring to the end of the range)? >> Is it just because the particular lens(es) are not designed or >> manufactured well for infinity focus? (E.g. they have "beyond infinity" >> focus at the end of the wring range.) > > > Exactly. I've been told that "to infinity and beyond" is necessary for > autofocus, although that doesn't seem true for my FA77. I suspect that it's > just a lot less expensive to guarantee that a camera will focus past > infinity than to carefully calibrate to go exactly to infinity. I wonder > how much sample variation there is in registration distance between the > mount and the sensor, as that would also affect the "infinity focus point". > > >> >> Igor >> >> >> Larry Colen Sun, 04 Dec 2016 11:49:18 -0800 wrote: >> >> Jostein wrote: >> >> >> >> Your results are on par with what I've got from single exposures with >> the Sigma 500/4.5. Proper focusing seems to be the limiting factor >> rather than actual MTF resolution. >> >> >> Yes, focusing was a real challenge. If I remember to bring my green >> laser, I can aim that at a distant point and have something to focus on. >> I probably should have tried focusing on Sirius, it was a lot brighter. >> >> > > -- > Larry Colen l...@red4est.com (postbox on min4est) http://red4est.com/lrc > > > -- > 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. -- 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.
