NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Peter Fogg
Date: 2006 Jan 28, 11:10 +1100
Earlier excerpt of mine:
“So two (polarizing
filters) used together would already have quite a significant minimal darkening
effect, which could be halved by using only one.”
To which George has taken
great exception, leading to this sweet inquiry:
“He doesn't appear to
have withdrawn it since, although it has been questioned, and was contradicted
by Bill's numerical observation. Perhaps he will tell us whether from that, and
from the recent web reading he has told us about, he has since changed his
mind.”
to which the answer can only be:
No. The platform from which George is gloating
prematurely seems to have somewhat shaky legs, to put it kindly. It appears to
be solely based on a single experiment, that Bill himself qualified on
presentation as:
“a quick series of tests”
For George to want to run with this as a fait accompli seems rather naïve, or is it
a case of embracing evidence that supports his preconceived notion, while
ignoring evidence that questions it, as did my own? Hardly very scientific or
rational. Tut tut.
Now that this – in itself fairly unimportant –
idea has been thoroughly worried and chewed at, the only change I would make is
to insert a qualifier; an ‘about’ or ‘roughly’. Here it
is:
“which could be roughly halved by using only one”
Happy now, George?
The ‘about’ is necessary because there are so
many variables in practice, some detailed earlier.
The ‘about’ qualifier can be taken (thus improving
the work of both authors) from a recent passage of George’s otherwise
excellent exposition on f/ stops which seems to have rather too many ‘abouts’:
“About the most
efficient lens you will come across in an ordinary camera has a diameter equal
to ABOUT
0.5 f when wide open (so aperture = f / 2). It can be "stopped down"
to a smaller diameter of say 0.35 f (so aperture is f /2.8 ), which will let
through ABOUT
half as much light.”
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.
If the bear is no longer required he may retreat to the
woods for a nice peaceful sleep …