At 10:51 AM 4/10/98 EDT, you wrote:
>warning: long tricky boring confusing technical question follows.
>
Don't worry. It sounds like everything is working OK. The f stop is
measured geometrically. Different lenses will let in different amounts of
light for a give f stop, and the amount can change with focusing distance.
In this case, I think the extra elements in the zoom, and possibly worse
coatings, are soaking up the extra light. Also, the f number might be
optimistically stated (such things often are, especially on inexpensive 3rd
party equipment).
If the diaphragm couplings are working OK, your pictures should be properly
exposed when you use the internal meter -- you will just be using a
different number on the aperture ring. With an external meter, you will have
to remember that you must set the lenses differently, and variable aperture
zooms are a real pain for this case, because not only does the real aperture
change with focal length, but that change may vary according to setting. f/4
may change to f/5.6, but f/16 will stay f/16!
The tab that moves when you change aperture does not signal f number. It
signals the difference between max aperture (where metering is done) and set
aperture (where the picture is taken). This has the advantage of getting
around all the issues mentioned above, but only for the internal meter.
Press the stopdown button and see if the shutter speed changes one step for
each f stop.
Paul Farrar
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