In a message dated 5/12/99 4:01:35 AM Pacific Daylight Time, norcutt@xxxxxxx
replying to a question about whether a teleconvertor automatically passed
aperture information commented:
<<
Due to the wonders of TTL metering though you don't have to do any of
that. The teleconverter doesn't pass any information to the camera
since the camera doesn't need to know anything about the lens aperture.
It measures the light it sees and combines that with film speed to give
a recommended shutter speed. It doesn't care if the light was a bright
light coming through a small hole or a dimmer light coming through a
large hole. >>
This is at best only partly true since the meter needs to know the maximum
aperture of the lens and what the lens will be stopped down to at exposure
time. A modern (!) manual camera like the OM1 does the metering at max
aperture not stopped down. Thus the teleconverter passes through a corrected
maximum aperture by offsetting the pin indicating the maximum aperture by the
right amount through to the OM body. It also needs to pass through the
aperture setting to be used. In the case of OTF metering (OM2 etc) it would
not matter whether there was a coupling pin or even whether it is corrected
as the metering gets done stopped down during the exposure. The convertors
make all the corrections necessary mechanically so you don't have to worry
about correcting for them manually.
Tim Hughes
Hi100@xxxxxxx
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