In 'normal' photography with automatic diaphragm lenses, a coupling pin in
the lens allows the camera to keep the diaphragm wide open, for brighter
viewfinder and easier focusing -- regardless of aperture setting. When you
press the shutter and the mirror flips up (or when you press the
DOF-preview button) the camera moves that pin to stop down the lens to the
desired aperture.
Safe for the OTF in OM-2 & OM-4, the metering is done when the lens is wide
open. But actual exposure may be stopped down, so the camera needs to know
how many f-stops will the lens stop down, to compensate the exposure --
that's the reason of the other coupling pin, linked to the aperture ring.
That pin tells the camera about the aperture setting _related to the
maximum aperture_, because metering was done when wide open. For instance,
in a f/1.4 lens set at 2.8, the pin is exactly at the same place of that of
a f/4 lens stopped down to 8.
Tha manual extension tubes haven't got the coupling for these pins, so they
have two tabs: one in the lens' side to keep the diaphragm at desired
setting, and the other in the camera's side to tell it the lens "is"
wide-open. Since there's no way for the camera to stop down the lens when
shooting (just like when it's wide open), no exposure correction is needed
-- if the diaphragm will stay at f/11, it doesn't matter if that was the
maximum aperture of the lens, or if it's a f/1.2 lens yet stopped down to
11.
>How then does one control exposure with the manual
>tubes?
I think the most convenient way is as follows:
1. Move the aperture ring towards the wide-open end.
2. Compose and focus.
3. Set the aperture ring to the desired f/stop.
4. If needed, check depth-of-field.
5. Adjust shutter speed for a correct exposure, as usual.
6. Shoot.
IIRC, this is called "stopped-down metering".
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