warning: long tricky boring confusing technical question follows.
i bought recently [it all came in yesterday] from paul the olympus guy,
a new om 2000, and a 50mm/1.8 olympus lens.
the om2000 is all-manual, essentially match-needle metering
[leds instead of meter but still the same idea.]
also bought a tokina 70-200mm/f/4.0-5.6 zoom [not from paul].
in doing some elementary checks and tests, i am concerned that the
zoom might cause overexposures.
in any given metering situation where all else is equal, the
aperture you have to set for correct indicated
exposure is 1 stop wider with the zoom [at 70 mm, where i would
expect it to be the same] or 3 stops wider at 210 mm than with a 50mm lens.
example:
outside, sunny day, film speed at asa 125, shutter speed 1/125
aperture where meter indicates correct:
50mm fixed lens : f/16
zoom on 70mm : f/11 [+1 stop]
zoom on 210 mm : f/5.6 [+3 stops]
i trust the 50mm because:
a. it does the "sunny 16" test correctly and
b. it matches my other camera's readings, and i know the
other one works correctly, in spite of being a 20 year old konica :).
getting technical, i looked at the little "output" tab on each lens that
that [i assume] tells the camera where the lens is set.
the comparison reveals something i do not understand:
it is in the same position on the 50mm lens set at f/1.8 as it is
on the zoom set at f/4. [these happen to be the widest settings for each lens.]
this poses a question: how come the f/4 "output" from the zoom
lens is not in the same place as the "output" from the 50mm lens
on the same setting? and how come the difference at 70mm is 1 stop,
not 2? [since the camera should not be able to tell the difference between
f/1.8 from the 50mm, and f/4 from the zoom.]
i took the zoom apart and looked for an adjustment, there was not one that
could be moved this far, and i still was not sure anything was wrong,
so i did not change anything.
[i do understand the reason for the difference between 70mm and 210mm
settings - the zoom is dimmer at long lengths.]
is the zoom going to work right?
is there something i don't understand about this setup?
isn't f/16 supposed to be the same, regardless of the lens it comes from?
is this possibly the answer?: the "output" from the lens is not really the
absolute f/stop, but the amount of stopping down from the max
[which is what is going to the meter when you are viewing the image]
that you are going to use. it's the only one that makes any sense.
but then how come the indicated apertures are different at same light
conditions?
how could i ever use this lens with a separate light meter that
indicates f/stop and shutter speed?
setting the zoom on a given f/stop seems to mean something
different than setting same f/stop on the 50mm..
thanks for reading this far and thinking about it.
??,
wle.
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