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?
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