[rollei_list] Re: Adventures with light meters
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Sun, 6 Jun 2010 17:07:51 -0700
----- Original Message -----
From: "Allan Derickson" <alland435@xxxxxxxxxxx>
To: "Rollei List" <rollei_list@xxxxxxxxxxxxx>
Sent: Sunday, June 06, 2010 4:16 PM
Subject: [rollei_list] Adventures with light meters
When I dusted off my old but only slightly used Rollei
3.5F all functions
seems to work properly except the light meter. I shrugged
and put it off to
old age and figured I would just use a hand-held meter.
After thinking
about it for awhile, however, it didn't make much sense.
The camera had not
seen much use and had been stored in a cool dark place so
it didn't seem
right that the selenium cell would have deteriorated. I
nor anyone else had
ever done any service on this camera but reading my manual
the meter is
user-serviceable to the extent of setting the zero and it
shows how to
remove it. The zero was okay but I decided to take a look
to see what I
could see. The back of the meter has two gold plated
posts which fit into
sockets which also have gold plated contacts. I took a
sharpened pencil
eraser and polished each one and cleaned them with alcohol
for good measure.
I also removed the selenium cell itself which has two gold
plated contacts.
I cleaned and polished these also. After putting
everything back together,
guess what? It reads spot on with my faithful old Gossen
Sixtar (same as
Super Pilot).
As a side note, I'm done buying meters on Ebay. A couple
months ago I
decided I should really start to learn and use the Zone
System so I bought a
Minolta spotmeter F. It bears a sticker that it was
calibrated in March of
this year by Quality Light-metric in Hollywood. After
extensive testing I
find that it is a stop off. At least it seems to be
consistent over the
range of light levels. I'll keep it but put my own
sticker on it to remind
myself to double the ISO value. Now my old Gossen uses
those nasty mercury
cells and I'm down to my last couple of my Canadian stash.
For the first
time in 40 years I dropped the darrn thing on concrete.
It cracked the
plastic housing and the cds cell window. I figured well
40 years ain't bad
and I sniped a Sekonic L-208 in the last seconds of an
Ebay auction. It
just came and it's two stops off in bright sunlight and
pretty close in low
light. In other words, not linearly off. So it goes back.
The Sixtar is
still working so I'll use it til it expires.
Live and learn.
I'm not sure how QLM calibrates meters. I know that some
years ago, when I visited, they had a lamp in a box that was
their standard. I have no idea how it was calibrated. They
were putting neutral density filters into old selenium
meters, which I think is not correct.
Selenium cells can deteriorate with time even when
stored in favorable conditions. The enemies are heat and
moisture. The cell edges are sealed to prevent entry of
moisture but the seals often break down with time. The
symptom is that the meter reads low for bright light on the
cell. Its fairly easy to test since most meters have some
region of overlap between their high and low ranges. A
reading of a fixed level of light in the overlap range
should be very close on both ranges. If the cell has
deteriorated it will read low on the range where the light
reaching it is most intense (low range), typically by about
one stop. Since the error is dependant on the brightness of
the light striking the cell it is not linear and can not be
corrected for by a simple fixed level adjustment such as
changing the film speed. A lot of people _think_ a meter is
OK because it isn't dead but it can still be off.
Bad contacts are a problem especially with all the
polution in the air these days. Cleaning contacts can do
wonders. A very good contact cleaner, available from
electronic and computer stores, is Caig De-Oxit Gold. It
comes in spray cans but apply it with a cotton swab or
toothpick.
BTW, I found a trick for Weston Master meters that read
low or intermittant. Push in on the cell window and twist it
bit back and forth, that will clean the pressure contacts
and often bring it back to life.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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