All OM AUTO aperture lenses have two levers on the rear of the mount. One
shuts down the aperture to what ever setting you have chosen. The other
tells the exposure control system in the body information about the lenses
aperture setting. As the aperture only gets shut down as you are taking a
picture the meter need to know how much bias its reading from the fully open
aperture. If no bias was applied the meter reading would only apply to the
fully open aperture of the lens you are using.
In the OM the aperture information tab defines the zero point for a lens as
being pushed most of the way round its travel. The aperture selection
mechanism on the lens then steps away from this point in single stop values.
Look at the back of a lens off the body and the inner edge of the mount on a
body and you should be able to figure this out. If you have an MANUAL
extension tube mount this without a lens and watch for the tabs action.
In both manual and automatic exposure the control system/meter still needs
to know what the lens aperture is doing before you release the shutter. If
it didn't in auto mode we would not know what the shutter speed was going to
be. In manual we would have to use stop down to get a reading for other
apertures.
All this does not change the way the OTF auto exposure works. The meter is
on the bottom of the mirror box floor looking back at the film plane. Once
you have released the shutter the lens is stopped down and the meter waits
until it has collected enough light for correct exposure.
Remember this only applies to lenses with an AUTO aperture.
Hope this helps you get there.
Dan S.
-----Original Message-----
From: owner-olympus@xxxxxxxxxxxxxxx
[mailto:owner-olympus@xxxxxxxxxxxxxxx]On Behalf Of john@xxxxxxxxxxxxxxxx
Sent: 05 December 2003 13:43
To: olympus@xxxxxxxxxxxxxxx
Subject: Re: [OM] OM Pin-hole photography problem - help!
On Fri, 5 Dec 2003 08:56:55 -0000, "Piers Hemy" <piers@xxxxxxxx>
wrote:
>There is a tab on any OM mount lens which reports the aperture set on the
>lens. If you don't have a pin engaged with the aperture reporting cam on
>the body, you will get up to 6/7 stops overexposure. Try a 7mm manual
>extension tube, which has the fixed pin to do the necessary.
>
Is this really so? I thought the great beauty of real-time OTF
metering was that it simply integrates the light received by the film,
and closes the shutter when it's done (or quenches the flash if one is
attached). It shouldn't matter about coupling pins or anything else in
auto mode. Manual metering - that's another story!
Am I right? Perhaps John H could chip in with his wisdom!
Regards
John Gruffydd (Mold, Wales, UK)
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