Hi, Chuck, Moose, Fernando and all.
Moose wrote:
>OK, I 've had a question for some time for you astrophotogs. In
>exposures measuring from 10s of minutes to hours, what difference does
>mirror lock-up make?
Basically nothing! In fact, the real need for mirror lock-up is for
*planetary* astrophotography, which use much shorter exposure times (around
1 sec.) and higher magnifications (equivalent focal lenghts about 15000
mm!!!) than 'deep-sky' shots -- nebulae, galaxies, etc.
>Chuck Norcutt wrote:
>
>> and a mechanical shutter (prevents battery drain during
>> long esposures).
Another good thing of mechanical shutters is their higher reliability in
*very cold* places, something usual in most astrophoto nights.
I once used a Ricoh GR1 (one of the few P&S with 'T' shutter setting) for a
meteor shower. Apparently, everything went well, but when I developed the
film I saw that only the first two or three frames of that night were
really exposed... The rest came absolutely blank -- even a close-up picture
of a terrestrial subject taken that night with the built-in flash (which
fired OK) was blank too. Later daylight pics taken on the very same roll
were fine.
Back to the original question, the double-digit OM bodies need batteries
for the 'B' setting. On the other hand, the 'B' works mechanically in
*every* single-digit OM: the OM-1 & -3 are purely mechanical at all speeds,
the OM-2(n) only at 'B', and the OM-2S & -4 series have 'B' and the 'red
60'. BTW, at the mechanical shutter speeds, the self-timer (and mirror
pre-fire) of the 2S and 4 *won't* work.
My "best" (or "least bad" ;-) astropics so far were taken with an OM-2n
(don't forget to take batteries out!)
>Hope someone teaches me how to calculate exact exposure time if I measure
>the degrees of the star trails.
>
>Fernando
15 degrees = 1 hour. (something about 10 seconds less, really ;-)
All the best,
...
Carlos J. Santisteban
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<http://cjss.galeon.com>
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