Hi--

We are in such close agreement on all of this........

>===========================================================
>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.
>===========================================================
>
>I have one problem with the above while accepting
>that it's theoretically sound. My problem: I went
>home and tried a total of 3 camera bodies (2
>borrowed from a friend) and did exactly what you
>described above (I suspected you were talking
>about such an action from your post). And with
>all three the body confirms "correct exposure"
>while the aperture ring was turned a small amount.
>Let's call this "correct aperture range (car)" and
>I can confidently say that for all three bodies
>car is larger than 1/4 stop (or 1/4 the dist
>between click stops on the aperture ting).

The older match-needle cameras, of course provided
pin-point analogue accuracy, but the LED/LCD newer
ones do, as you say, have a range - BUT, when I meter,
I don't stop the movement of the aperture ring just
as the indicator appears, but try to center the ring
position within the range of "OK", somewhat as I do with
manual focus.

>===========================================================
>??? 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.
>===========================================================
>
>Agreed and see below.
>
>===========================================================
>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.
>===========================================================
>
>With a discrete control on both the aperture and
>the shutter to 1/3 stops, the error of exposure
>resulting from this discreteness is always less
>than 1/6 of a stop and has a mean value of 1/12 of
>a stop -- assuming everything else such as the
>meter and the capability of operation on the part
>of the user is perfect (this is only fair as you
>also assume these to be perfect for the analogue
>system in comparison). So consistently one would
>expect an error of 1/12 of a stop *on the average*
>and 1/6 of a stop in the worst case. I am not
>convinced that it is important. Also, if the
>shutter and aperture accuracy is not to the same
>1/3 stop but one of them 1/3 and the other 1/2,
>then the mean error becomes 1/24 and the max 1/12.
>This is probably outside the speed accuracy
>tollerances of film producing factories.

I cannot argue with the above - but it is unnerving to
me, who really likes precision in transparency exposure
(and while sufficient precision is possible with the
systems described, I still prefer analogue control of
one of the variables to satisfy myself that bracketing
is unnecessary.....).

>Also, I'd have thought even for critical accuracy,
>only one of aperture or shutter needs to be
>analogue and continuous.

Yes.

>Best regards and thanks for the message.
>Fei

"Likewise, I'm sure." ;-)
David Ruether