Manuel Viet wrote:
> Le samedi 30 Juillet 2005 13:19, Brian Swale a écrit :
>
>
>>I think that for OMs, vibration has been the unsolved bugbear. Mostly
>>unsolved - and for me it would still be unsolved were it not for the tests
>>done and put on the 'net by Gary Reece; and for the information so freely
>>given in this group - all of which has made a huge difference to my
>>enjoyment of photography.
>
>
> Could someone please expand a bit on that topic ? A while ago, I've been
> responsible for a wet lab activity in a college, and we had a Pentax K1000
> for 15 students, so I generaly let them use my own OM-1 as well to speed up
> the turnover a bit. Clearly, while pentax pictures were "good enough", OM
> pics were always a bit sharper. So I'm wondering under which circumstances
> that vibration "bug" would show ; maybe on landscape pictures, tripod
> mounted, long exposures ? But then, why would so many astrophotographers give
> an arm and a leg to get an OM today ? I'm confused.
>
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I think most of what is known on the subject of OM vibration emanates
from Gary Reese's extensive lens testing here:
<http://members.aol.com/olympusom/lenstests/default.htm>
Gary eventually realized that vibration with certain bodies and lenses
was signfificantly impacting performance. In fact, many lenses, when
retested using methods to reduce body induced vibration, showed much
better performance than the initial testing. This probably happens with
many other brands of camera bodies as well but you won't find it
documented as you do here.
The first surprise to many is that simply locking the mirror up on an
OM-1 will reduce the vibration problem but only part of it. There are
other sources of vibration not affected by mirror lockup... the diaphram
activation lever which gives a suddden kick to close down the lens and
the actions of the shutter curtains as they suddenly fly open and slam
closed. All of this shaking is dramatically reduced in later bodies
such as the 2sp and OM-4 by utilizing "aperture pre-fire" via the
self-timer. When the self-timer is used the mirror is flipped up and
the aperture arm kicks well ahead of the shutter opening.
As to why astronomers want OM-1's and mirror lockup... conventional
wisdom. Better they should remove the aperture arm from the camera
which serves no useful purpose on a scope.
Actually, what's important on a scope depends on what type of
photography you're doing. Two seconds of camera induced vibration on a
one hour long guided exposure will never show. A short exposure on the
moon is a different story. If you can, it's probably best to use the
"hat trick" so there is no vibration at all. Place a "hat" or dark
cover over the end of the scope, fire the camera shutter, wait several
seconds for vibrations to settle, remove the cover from the end of the
scope for the duration of the exposure, replace the cover, close the
shutter.
Chuck Norcutt
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