anand@utastro.UUCP (Anand Sivaramakrishnan) (02/25/85)
# I read somewhere that
# large format lenses are not as sharp as those for, say, 35mm cameras.
My reply
35m lenses are far superior to almost all other format lenses...
the bigger lenses are harder to make (physics... larger surfaces
to figure to amazing accuracy, etc). Perhaps the Zeiss Planar for
120 film cameras is as good as a decent Nikor or Leitz-made
lens.... but that's the exception.
Anand
# As long as I'm asking, does anyone know of a negative densitometer
# (for black and white) that doesn't cost an arm and a leg?
#
# Eric Lundblad
In response I quote a reply to one of my letters suggesting a spot meter
can be used as an adequate transmission densitometer....this article
is very informative, one of the few that I saved (and even printed
on hardcopy!). Bits have been chopped out, but not much (apologies
to Brian Diehm)
All the rest of this posting is reproduced without permission
from Brian Diehm's article ...
From briand@tekig1.UUCP (Brian Diehm) Sun Feb 6 00:28:16 206
Each 1/3 f-stop change represents 0.10 density.
For example, you have a spot meter and a set of 4x5 negatives, each
exposed for zone I at various EIs. 1) Set up light source and meter pointing
to read it, 2) read the light source without the negative, and 3) read the
light source through each negative. The negative that gives a reading of 1/3
stop difference from the reading without any negative has a density of 0.1.
This is a happy accident and is "only" precise to 3 or 4 decimal places.
You should be careful with the lab setup, however. Make sure that the light
source is an evenly illuminated source (flourescent behind drafting tissue or
perhaps a slide viewer light table), make sure the field of view of the meter
is completely filled by the light source, and make sure that the light entering
the meter is ALL coming through the negative being measured - hold it right up
to the meter cell entrance, and finally, make sure that the negative density is
even across what the meter will measure. Specular transmission density, which
is measured by a spot light source, is probably too difficult to do accurately
with this method.
A spotmeter makes this a little easier, and will allow you to measure the
negative held at the light source. With a spotmeter, 35mm can easily be meas-
ured in this way for zone system calibration.
This has been a fairly quiet mathematical/photographic "secret of the
trade" until Parry Yob described (with much more effort than needed) how to use
a spotmeter as a densitometer in Petersen's Photographic several years ago. I
notice the last Ansel Adams series mentions the technique in Book 2, The Nega-
tive. Densitometer manufacturers haven't been known to spread the word.
Note that the results of all this are plenty good for "users" (i.e. Zone
System photographers), but would hardly fall into the ballpark of the accuracy
of real densitometers, which are used primarily by labs for process control.
I don't think working photographers really have much need of a $4,000 Macbeth.
For a good sensitometry reference which describes the reasons for all this
see "Photographic Sensitometry" by Zakia & Todd, the seminal work in the field.
I believe it is published by Morgan & Morgan around 1968.
-Brian Diehm
Tektronix, Inc.
Pedantic P.S. - Since N.D. filters are notated in density, this relationship is
an easy way to tell the number of f-stops reduction of such a
filter.