--- Acer/Siddiq <waltherppk@xxxxxxxxxxxxx> wrote:
> 12/26/01 12:06:01 AM, "Brian Swale"
> <bj@xxxxxxxxxxxxxxx> wrote:
> >>
> >> Date: Tue, 25 Dec 2001 12:43:02 -0500
> >> From: "roger skully" <robinsnes@xxxxxxxx>
> >> Subject: Re: [OM] (Fwd) DOF button.
> >>
> >> Why disable it or hinder your focusing abilities?
> I cannot think of any
> >> reason (not that there may not be one) for doing
> this.
>
> >Well, the logic behind it goes like this, as I
> understand it.
> >
> >The 35mm system has a small negative. So for best
> results a bit more care
> >is needed to eliminate anything that might
> contribute to unsharpness.
> >
> >There are at least three 'in camera' sources of
> vibration that actually do result
> >i lowered contrast and definition (see Gary Reese's
> lens tests - he
> >comments on this frequently). They are:-
> >
> >Mirror operation
> >Diaphragm operation
> >Focal-plane shutter operation.
> >
> >Mirror vibration is dealt with by raising the
> mirror (OM1) and pre-fire in self-
> >timer operation (OM4), and the delay that allows
> the vibration to die away.
> >
> >Diaphragm vibration is dealt with in the OM4 by
> pre-fire, and not at all in any
> >other OM body. Hence the wish to close down the OM1
> diaphragm once the
> >picture is composed and when the mirror is up. Then
> all that is necessary is
> >for the subject to quit moving.
> >
> >Shutter vibration is dealt with by attaching as
> much mass as possible to the
> >camera. Clamp it to a mountain if possible.
> Otherwise, use a heavy tripod,
> >something heavy at the tripod head or attached to
> the camera, push down
> >on it to anchor it to mother earth, and maybe
> hand-hold it as well to dampen
> >the residual vibrations.
>
> <snip>
>
> I believe I read a test on the same, by Gary Reese,
> in which it was better to leave the DOF button as
> is.
> That's what my memory serves, i may be wrong. the
> theory went thusly:
>
> the snap of the lever to stop down the lens right
> prior shutter opening acts as a damper on the lever
> mechanisms that stops down the diafram. that is, if
> you pre-stop the lens, the stop down lever will snap
> up,
> with no resistance, and make for some awful
> kickback. if you leave the DOF button as it, the
> stop down
> mechanism will enounter the resistance of the
> diafram/spring to close, reducing the kickback.
>
> think of it this way: you are hammering a nail. as
> your hammer hits the nail, it encounters resistance
> to slow
> down the hammer. with no nail, you bang the surface.
> the nail going into the surface serves to absorb
> some
> of the force.
>
> the theory appears to be sound, and make perfect
> sense to me. anyone else? or do i have it all wrong?
Why so many work-arounds? If maximum detail is
required, add a TLR to your kit. A Yashica Mat124 or D
(if a D make sure it has the 4 element Yashinon lens)
can be had for about $150US, and if interchangeable
lenses are peferable the Mamiya C system could be
purchased for about 7 to 8 hundred USD for a body,
65,80,180mm setup. I'm not saying give up your 35mm OM
system, just use the strenghts of each type to get the
results you hoped for. John Robison
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