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Peter Wrote:

> My understanding is that there is nothing imprecise about f/ stops; they need
> to be and are quite precise in order to function, ie; to permit an accurate
> exposure. Just try telling Schneider or Rodenstock or any other lens maker
> that its finely calculated then engraved f/stops are only ?about? right and I
> suspect you would be met with withering scorn.

The above statement regarding f-stops is *mostly* true these days.  Ansel
Adams, in book # 1, "The Camera" speaks to that. (Revised 1980.)  First, by
definition, the "relative" aperture of a lens, as George correctly stated,
is focal length over diameter (of lens opening).

To quote Ansel, "The f-stop relates exposure to the effective diameter of
the lens, but disregards other factors, primarily the efficiency of the lens
in its actual transmission of light.  Since lenses of many elements are less
efficient than those of few elements--because of reflection of light at each
surface and the optical density of the glasses--attempts have been made to
develop the actual transmission of a lens.  A scale of "t-stops" has
sometimes been been substituted for f-stops to indicate light transmission.
These values are seldom seen today, except in some lenses for
cinematography, primarily because the efficiency of lenses has been greatly
increased by lens coating techniques.  The t-stop values, while fine for
determining exposure, also distort other mathematical values that relate
directly to the true f-stop, such as depth of field and hyperfocal
distance."

Really, given the modern lens, even if t-stops were treated as f-stops on
the barrel, the depth of field and hyperfocal distance are also on the
barrel of small and medium format lenses, so big deal.  Who do we know that
calculates exposure by using the square root of the aperture related to foot
candles today?  This is in anticipation of (IMHO)) a valid argument; they
can make the hole in the adjustable aperture any size they want as long as
it passes through the prescribed amount of light. Bingo, perhaps a t-stop in
f-stop clothing?

If you are serious about your photography, I would wager you have lighted up
a wall or some such thing, used film from the same batch and photographed it
(bracketing exposures) with the same body and all your lenses to determine
the actual light transmission of each lens, as well as its contrast.

As this is way off topic, I replied to George off list, and would suggest if
we wish to continue, we do the same.  My sphere of ignorance is not limited
to navigation.

> If the bear is no longer required he may retreat to the woods for a nice
> peaceful sleep

Not so fast my furry friend. I feel you, George and I have all gotten to
walk a mile in the bear costume in this discussion if we equate honest
disagreement with sticks and stones ;-)  The lower lower limit of my stacked
guess was inline with measured.  Objectively however, my guess was based on
doubling.  I was wrong.  George guessed 10% loss when it was 35-40%.  George
was, in practice, wrong.  I would be hard pressed to say he or I won a
"bet."  As a sports analogy, I would say George's team won the game but he
lost the bet on the points spread.

Since it is a discussion forum, not a scored debate society, I feel most
interested parties came away with a better understanding of polarization
than they came in with.  The goal.  It is understandable that your fur was
initially raised by George's criticism of you percentages, and a reminder
that we *all* may need to try practicing a bit more diplomacy--telling the
other fellow to go to h-ll in such a way as he will look forward to the
trip.  Or, in  words of Will Rogers, "Diplomacy is the art of saying 'nice
doggie" until you can find a rock."

Enough stonings for one day ;-)

Pax vobiscum

Bill

   

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