In article <1996Apr30.204528@alder.cc.kcl.ac.uk>, udee785@bay.cc.kcl.ac.uk says...

>In article <4m3p67$lmt@newsstand.cit.cornell.edu>,
>d_ruether@hotmail.com (Bob Neuman) writes:
>> I didn't say that a 1/4 stop would break a slide - I
>> said that a consistent 1/4 stop error (as from a misadjusted meter)
>> would show as a trend toward lightness or darkness in overall slide
>> exposures.

>This I agree with. What I have a problem with is
>the original remark by you. It was in the context
>of whether lens aperture control is continuously
>analogue -- thus theoretically allowing fine
>adjustments of exposure via aperture -- or
>discrete in say 1/3 stop steps at the finest --
>thus theoretically not catering to those wanting
>to get their aperture tuning to within 1/4 of a
>stop. My practical question to you is: how could
>you turn that aperture ring accurately enough to
>1/4 of a stop consistently? Many (if not most)
>lenses only have click stops at full stop marks.
>In many, if not most, lenses, using the f number
>to approximate the t number already gives more
>than 1/4 stop of error.

If we are talking about exposures made with a lens
with an infinitely variable aperture setting on a camera
with a TTL meter, there is no more need to identify
particular aperture positions (just the exact one that
centers the meter when the desired range and proportion
of tones are included in the meter's angle of acceptance)
when taking a manual-exposure meter reading than there is
to know the exact fractional shutter speed used when in
aperture-priority auto mode. If you are using a separate
meter, it is not difficult to approximate 1/4 stops (or
less) - I used to do it all the time with Nikkors and a
Weston meter. You are right that theoretically, the marked
aperture tracking of actual aperture is probably not exact,
though in practice, it has never offered a problem (except
with a couple of lenses - found by testing, and between the
widest and next widest aperture, and sometimes near minimum
aperture). The (potential) problem is reduced considerably
with stop-down metering, which is possible on several Nikon
bodies, since the reading is taken using the actual diaphram
set, and not the representative f-number.

>And how could a discretely controlled aperture be
>consistently wrong by 1/4 stop across the board
>in one direction even if the finest differece it
>could handle is only 1/3 stop? As you said only a
>misadjusted meter could manage that but you were
>not talking about meters when you complained
>about discrete apertures.

??? Huh? Of course one references the error to the meter
and not the aperture, unless the aperture permits the
selection only of exposure increments, in which case the
errors would almost always exist, but would be random.

>All things considered, a discrete adjustment in
>1/3 stop steps on aperture has an accuracy of 1/6
>stops in exposure by adjusting aperture alone. So
>your case should have been made with 1/8 or finer
>rather than 1/4, and to be able to turn that
>aperture ring consistently within 1/8 of a stop
>is sort of mind boggling for me.
>Regards, Fei

Again, consistency of turn of the aperture ring is not
the issue, but the ability to center the meter precisely is.
I would favor a continuously variable aperture (and shutter)
over one that is limited to 1/3 or 1/2 stop increments,
which (with an incremental shutter speed selection), would
almost guarantee exposure error of a scale that is detectable
with slide film.
Hope This Helps