[rollei_list] Re: Meter Testing
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Sat, 22 Aug 2009 15:27:06 -0700
----- Original Message -----
From: "Elias Roustom" <eroustom@xxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Saturday, August 22, 2009 8:49 AM
Subject: [rollei_list] Meter Testing
Hi All,
My hand held meters, a Gossen Luna Pro, and Sekonic Auto
Lumi L-158 (selenium), read more or less the same, but
they are often off by as much as a stop from my TTL
meters in my SLRs, and don't always match what my Rollei
35 reads. I consider the meter in my Rollei 35 to be one
of the most reliable (and it's still running a mercury
battery that came with the camera, my guess is the
battery is about 15 years old). Any thoughts on meter
testing, battery life, the difference between types of
meters? Not a new topic, I know, but it's something I've
been bogged down with recently since my Gossen started to
give me some strange readings over vacation.
Elias
---
Rollei List
I use my Luna-Pro as my standard. It agrees with
several other meters. One problem is in comparing reflected
light meters including TTL types because you must match what
they are looking at and it is sometimes difficult to tell. I
compare using a large uniformly illuminated matt surface. It
doesn't matter if its gray or white as long as its neutral
colored and large enough to fill the window of the meters
since it is being used for comparison.
The same problem occurs when comparing incident to
reflected light meters. They should read the same when
reading an 18% gray card but not all reflected light meters
are calibrated to read 18%. I found that unless the test
target was carefully used differences in reading would
appear.
Selenium cell meters can be checked by finding a light
value that is in the overlap region of the scales, there is
such a region on nearly all selenium meters. Ideally one
reading should be near the high end of the meter scale and
the other at a substantially lower value. The ranges are
always obtained by using some sort of light attenuator over
the cell. Selenium cells begin to show damage by reading low
for high light intensities. If the reading at the top end of
the meter scale is not the same as the one with the light
attenuator in place the cell is damaged. This error is on
the order of one stop or more. There may be a smaller error
due to meter scale tracking even if the cell is good. If the
cell is bad there is no way to regenerate it, they must be
replaced.
There are ISO standards on the calibration of exposure
meters but they have changed over the years so there can be
some disagreement among meters which are actually working
correctly but which were made at different times. These
should be slight.
I've found that a good replacement for the usual
mercury cells used in many meters and cameras can be made
using zink-air hearing aid batteries. These have the right
voltage and are nearly as constant voltage as mercury cells
but not as long lived. I was able to make replacements for
my Nikon-F and Luna-Pro using type 675 cells with rubber O
rings to compensate for the smaller diameter. I made
comparison readings on both meters using good mercury cells
and got identical readings with either kind of cell in both
meters. For the Nikon I had to wrap some aluminum foil on
the O ring to make contact with the battery contact on the
side of the cell case but this is not necessary for the
Luna-Pro. The zinc-air cells seem to last about a year in
service.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
Rollei List
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