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[OM] Re: Aperture/Mirror Prefire

Subject: [OM] Re: Aperture/Mirror Prefire
From: Joel Wilcox <jowilcox@xxxxxxxxxxxxxxxxxxx>
Date: Wed, 25 Aug 2004 22:03:18 -0500
At 09:39 PM 8/25/2004 -0400, you wrote:
>Joel once did a test with the OM and 200/4 lens with an attached laser
>pointer, and I think photographed the vibration of the projected laser
>spot. (I think my memory serves me here, but I have poor memory.)
>(Joel?)

If someone wants to check the archives, I think the thread was "Shutter 
shudders."  I taped a laser pointer to the 200/4 on an OM-1 and projected 
it onto a mirror or flat surface and watched it jump when I tripped the 
shutter.  This was regardless of MLU or not.  MLU had no effect on 
improving this whatsoever.  It was clearly not mirror slap but aperture 
stopdown.

The bucking broncho effect was lessen by the use of a Bogen telephoto 
support.  I should have tested it using Oly's preferred method of handheld 
dampening.

>The 200/4 lens seemed to accentuate the vibration with the
>OM's, or resonate particular badly, and was speculated to be the cause
>of poor performance attributed to that lens. You'll even notice with
>Gary Reese's test data, that lens performance improves a lot with
>aperture prefire OM-4t versus the OM-1 data. The resonance effect is
>most pronounced in the 1/4-1/30 speed range. If you are near that
>range, consider aperture pre-fire for reduced vibration. 1 second
>exposures and longer, you may get away without it.

My subjective impression is that the OM-2S and OM-4(x) shutter/aperture 
stopdown vibration is less a factor than the OM-1's.  In my less exacting 
testing, I have never really noted results using an OM-1 and cable release, 
with or without MLU, that I could say were really and truly spoiled as a 
result of internal vibration.  But Gary Reese I think pretty much proved it 
to the "satisfaction" of all.

Since Gary did not use the hand dampening method, it's hard to say whether 
his tests really indicate how good or bad the OM-1 is, just how much it 
falls short using his particular methodology, which I think he had to 
follow.  If he'd used hand dampening it would have introduced a subjective 
factor that would have ruined every one of his tests (even if it might have 
improved the performance, ironically).

Joel W.


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