[pure-silver] Re: Neither My Hassy Nor View Camera Have This Problem
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Thu, 28 Oct 2010 01:28:13 -0700
----- Original Message -----
From: "Elias_Roustom" <elroustom@xxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Wednesday, October 27, 2010 6:28 PM
Subject: [pure-silver] Re: Neither My Hassy Nor View Camera
Have This Problem
This leads to very interesting further reading. Thank you
for sharing it.
I recall Richard mentioned T Stops before (here or in the
Rollei list).
I put my exposure meter under the enlarger the other day to
see if the aperture stops actually corresponded to full
stops in light output, and they did not. Some lenses were
better than others, but a full stop of aperture change is
not necessarily the same as a full stop of exposure change.
Photography like any art or craft is about dealing with
limitations of tools.
I've also experimented with my Gossen Luna Pro, and my
digital camera, the readings from the now ancient hand-held
produce fine exposures on the sensor - but not all the time.
Can't say I don't have that problem with film.
Elias
T stops became popular sometime in the 1950s I think.
Mostly for motion picture use for the reasons Bob Kiss
detailed, namely in order that exposures from different
lenses would intercut. Some lenses, like the Bausch & Lomb
Baltar came T-stoped from the factory but there were
services that would T-stop existing lenses. The reason is
that the transmision of a lens is dependant on things other
than simply the size of the hole in the stop. There are
losses going through the glass and losses from internal
reflections. For an uncoated lens with eight uncoated
glass-air surfaces it can amount to nearly a stop. Modern
lens coatings increase transmission a lot but there is still
some absorption in the glass and some reflection so that in
a complex lens, like a zoom lens, the transmission predicted
by simple geometry may be significantly in error. Usually,
the difference between the T (means Transmission) stop and
the f/stop is not enough to be significant in calculating
depth of field but there will be a difference. T-stops will
always be physically larger than the true f/stop so depth of
field/focus will be a bit less.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
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