Stanley Halpin wrote:
On 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”.
Larry - just to quibble a small bit: how would you set up a testing/calibration
device or facility that allows calibration “exactly to infinity”? Wouldn’t that
require first knowing exactly where infinity is? Are you willing to allow for a
few hundred million light years error variance around the proposed exact value?
In simple terms, the definition of infinity focus would be to have
parallel incident light rays. Divergent rays from a point source are
closer than infinity. One could probably set up a calibration unit with
parallel light rays using conventional optics. I suspect that a low
powered laser would also work.
As to the other part of the question,
http://www.outsight.com/hyperfocal.html
says that a 1000mm f/1.4 lens has a hyperfocal distance of 78119 feet,
or 14.8 miles. Using that as a worst case scenario, calibrating on any
physical target more than 15 miles away would pretty much guarantee that
infinity would be in focus.
If you take a 600mm f/2.8 lens as the worst case you get 14061 feet, or
merely 2.66 miles as the hyperfocal distance. As you can see, in
practical terms you don't even need to shoot the moon (~240,000 miles)
to effectively achieve infinity focus.
Being an engineer, I understand that I don't need a perfect solution, I
just need a solution that is good enough.
--
Larry Colen l...@red4est.com (postbox on min4est) http://red4est.com/lrc
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