How to calibrate light meters?

[Stephan Goldstein]

A recent bad experience with a Pentax Spotmeter V that suddenly
decided to make me overexpose by 2 stops has prompted me to
wonder about the specifics of lightmeter calibration. Not as in
"send the meter off for calibration", but more like "build myself an
instrument to provide standardized light intensities". As an
electrical engineer with 30 years' analog circuit experience this is
not as far-fetched as it might sound.

So presuming I have a widget that provides stable light intensity
and is linearly variable over a wide range, does anyone have any
specifics of calibrating the Pentax Spotmeter V? This is the last
version of their analog spotmeter, the one that takes three button
cells.

Thanks!

Steve

[Richard Knoppow]

Post by Stephan Goldstein
A recent bad experience with a Pentax Spotmeter V that
suddenly
decided to make me overexpose by 2 stops has prompted me
to
wonder about the specifics of lightmeter calibration. Not
as in
"send the meter off for calibration", but more like "build
myself an
instrument to provide standardized light intensities". As
an
electrical engineer with 30 years' analog circuit
experience this is
not as far-fetched as it might sound.
So presuming I have a widget that provides stable light
intensity
and is linearly variable over a wide range, does anyone
have any
specifics of calibrating the Pentax Spotmeter V? This is
the last
version of their analog spotmeter, the one that takes
three button
cells.
Thanks!
Steve

I think the most practical way is to compare readings
with a known good meter. For reflected light meters a large,
diffuse, target is best. This can be a simple sheet of matte
paper. It does not have to be 18% gray or anything else for
comparison use.
A simple source for scale tracking can be made by using
a small bare filement lamp on a track and calibrating using
inverse square law. This can be used for either incident or
reflected light meters. For absolute calibration you need a
calibrated photometer of some sort.

[Jean-David Beyer]

A recent bad experience with a Pentax Spotmeter V that suddenly decided
to make me overexpose by 2 stops has prompted me to wonder about the
specifics of lightmeter calibration. Not as in "send the meter off for
calibration", but more like "build myself an instrument to provide
standardized light intensities". As an electrical engineer with 30
years' analog circuit experience this is not as far-fetched as it might
sound.
So presuming I have a widget that provides stable light intensity and is
linearly variable over a wide range, does anyone have any specifics of
calibrating the Pentax Spotmeter V? This is the last version of their
analog spotmeter, the one that takes three button cells.

Dr. Richard J. Henry has a procedure in Chapter 6 of the Second Edition of
"Controls in Black and White Photography" Focal Press 1986, pages 167 - 178.
You need some GE Longlife White Lamp Bulbs, 100 Watt, 1585 Lumens, a
lighting stand, a tripod, and you calibrate a 1 degree spot meter. The bulbs
are a bit hard to find these days.

[stefan patric]

A recent bad experience with a Pentax Spotmeter V that suddenly decided to
make me overexpose by 2 stops has prompted me to wonder about the
specifics of lightmeter calibration. Not as in "send the meter off for
calibration", but more like "build myself an instrument to provide
standardized light intensities". As an electrical engineer with 30 years'
analog circuit experience this is not as far-fetched as it might sound.
So presuming I have a widget that provides stable light intensity and is
linearly variable over a wide range, does anyone have any specifics of
calibrating the Pentax Spotmeter V? This is the last version of their
analog spotmeter, the one that takes three button cells.

If your meter is suddenly off by 2 stops, then it may be more than
calibration. It may be seriously sick. Do this: take an 18% grey card,
place it in full, noon time sunlight, held vertically, set your meter at
ISO 125 and take a reading of the card, filling the viewfinder with the
card. It should read f16 at 1/125 or within a 1/3 stop. Now, check
that it zeroes. Does the needle rest on zero after taking a reading? If
it doesn't, turn the zero adjust to zero it. And repeat the grey card
reading above. If it's dead on or close, you're done. If it's not, but
is only off a little, try adjusting the potentiometers inside until it
reads correctly. Test it against another meter you know to be accurate.
If, however, the meter is way off as you indicate it is, it may be well
beyond calibration. A component or the light sensitive cell itself may
faulty.

Did you put new batteries in it? It doesn't use mercury type batteries
does it? If it does, you're screwed. They're no longer available in the
US. And the official replacement cells are the wrong voltage and makes
the meter unusable. I have an old Gossen meter that's perfect working
order, but without those mercury batteries, it gives bogus readings. It's
been retired to the junk drawer.

Good Luck...

Stefan

[Jean-David Beyer]

Post by stefan patric

A recent bad experience with a Pentax Spotmeter V that suddenly decided to
make me overexpose by 2 stops has prompted me to wonder about the
specifics of lightmeter calibration. Not as in "send the meter off for
calibration", but more like "build myself an instrument to provide
standardized light intensities". As an electrical engineer with 30 years'
analog circuit experience this is not as far-fetched as it might sound.
So presuming I have a widget that provides stable light intensity and is
linearly variable over a wide range, does anyone have any specifics of
calibrating the Pentax Spotmeter V? This is the last version of their
analog spotmeter, the one that takes three button cells.

If your meter is suddenly off by 2 stops, then it may be more than
calibration. It may be seriously sick.

Pentax Spotmeter V, if it is the same as the Pentax Spotmeter 1/21, had
serious reliability problems. Fred Picker had several and he found them all
unsatisfactory. I had one that was equally unsatisfactory. For mine, the
problem was that it usually gave correct readings, but occasionally gave no
readings, or readings that were obviously very low (leading one to
overexpose if relying on the readings). I took mine apart to check for loose
contact, broken wires, etc., and found nothing. I finally got rid of it and
got a Zone VI modified Pentax digital that has worked perfectly ever since,
though after too many airplane rides, it got slowly out of calibration and I
had Zone VI recalibrate it.

Post by stefan patric
Do this: take an 18% grey card,
place it in full, noon time sunlight, held vertically, set your meter at
ISO 125 and take a reading of the card, filling the viewfinder with the
card. It should read f16 at 1/125 or within a 1/3 stop. Now, check
that it zeroes. Does the needle rest on zero after taking a reading? If
it doesn't, turn the zero adjust to zero it. And repeat the grey card
reading above. If it's dead on or close, you're done. If it's not, but
is only off a little, try adjusting the potentiometers inside until it
reads correctly. Test it against another meter you know to be accurate.
If, however, the meter is way off as you indicate it is, it may be well
beyond calibration. A component or the light sensitive cell itself may
faulty.
Did you put new batteries in it? It doesn't use mercury type batteries
does it? If it does, you're screwed. They're no longer available in the
US. And the official replacement cells are the wrong voltage and makes
the meter unusable. I have an old Gossen meter that's perfect working
order, but without those mercury batteries, it gives bogus readings. It's
been retired to the junk drawer.
Good Luck...
Stefan

[Heinz Grau]

Post by stefan patric

A recent bad experience with a Pentax Spotmeter V that suddenly decided to
make me overexpose by 2 stops has prompted me to wonder about the
specifics of lightmeter calibration. Not as in "send the meter off for
calibration", but more like "build myself an instrument to provide
standardized light intensities". As an electrical engineer with 30 years'
analog circuit experience this is not as far-fetched as it might sound.
So presuming I have a widget that provides stable light intensity and is
linearly variable over a wide range, does anyone have any specifics of
calibrating the Pentax Spotmeter V? This is the last version of their
analog spotmeter, the one that takes three button cells.

If your meter is suddenly off by 2 stops, then it may be more than
calibration. It may be seriously sick. Do this: take an 18% grey card,
place it in full, noon time sunlight, held vertically, set your meter at
ISO 125 and take a reading of the card, filling the viewfinder with the
card. It should read f16 at 1/125 or within a 1/3 stop. Now, check
that it zeroes. Does the needle rest on zero after taking a reading? If
it doesn't, turn the zero adjust to zero it. And repeat the grey card
reading above. If it's dead on or close, you're done. If it's not, but
is only off a little, try adjusting the potentiometers inside until it
reads correctly. Test it against another meter you know to be accurate.
If, however, the meter is way off as you indicate it is, it may be well
beyond calibration. A component or the light sensitive cell itself may
faulty.
Did you put new batteries in it? It doesn't use mercury type batteries
does it? If it does, you're screwed. They're no longer available in the
US. And the official replacement cells are the wrong voltage and makes
the meter unusable. I have an old Gossen meter that's perfect working
order, but without those mercury batteries, it gives bogus readings. It's
been retired to the junk drawer.
Good Luck...
Stefan

Stefan, do not throw your old Gossen meters away! The MRB625 and MRB675
cells are a suitable mercury-free replacement for the PX625 or PX675 cells.
They are not alkaline nor silver oxide (1.5 Volts), but zinc-air batteries
which deliver 1.35 Volts, same as mercury cells used to deliver, and
accurately enough for lightmeters. I use them in 35mm cameras as well as in
lightmeters. They are marketed by micro-tools.com. Zinc-air batteries are
widely used in hearing aids. They are activated by removing a seal that
prevents air entering while stored. Don't know which type of battery the
Pentax Spotmeter uses.
Heinz

[stefan patric]

Post by Heinz Grau
Stefan, do not throw your old Gossen meters away! The MRB625 and MRB675
cells are a suitable mercury-free replacement for the PX625 or PX675
cells. They are not alkaline nor silver oxide (1.5 Volts), but zinc-air
batteries which deliver 1.35 Volts, same as mercury cells used to deliver,
and accurately enough for lightmeters. I use them in 35mm cameras as well
as in lightmeters. They are marketed by micro-tools.com. Zinc-air
batteries are widely used in hearing aids. They are activated by removing
a seal that prevents air entering while stored. Don't know which type of
battery the Pentax Spotmeter uses.
Heinz

I don't remember what replacement batteries I used, but they were the
"official" replacement type. However, they certainly didn't require
removing any thing to activate them.

Anyway, now I've got to find the meter to check. Then get a couple of
those zinc-airs to see if they work.

Thanks for the advice.

Stefan

[Michael Gudzinowicz]

Post by Stephan Goldstein
A recent bad experience with a Pentax Spotmeter V that suddenly
decided to make me overexpose by 2 stops has prompted me to
wonder about the specifics of lightmeter calibration. Not as in
"send the meter off for calibration", but more like "build myself an
instrument to provide standardized light intensities". As an
electrical engineer with 30 years' analog circuit experience this is
not as far-fetched as it might sound.
So presuming I have a widget that provides stable light intensity
and is linearly variable over a wide range, does anyone have any
specifics of calibrating the Pentax Spotmeter V? This is the last
version of their analog spotmeter, the one that takes three button
cells.

Take a reading of CLEAR northern skylight at noon while shading the
meter from direct sunlight. In the US lower 48, the reading should
be 1/EI at f/16... the "sunny 16 rule".

[Stephan Goldstein]

Thanks all for your replies.

I'll track down the Richard Henry book, that sounds like a good
addition to my library. And I'll certainly remember the northern
sky trick! The meter still sits on zero, the battery check is fine,
it's just 2 stops off but quite linearly so.

Tthis is a Zone-VI-modified Spotmeter V. It's the late version
that uses 3 silver-oxide cells. I spoke with Richard Ritter today
(he did the original mods for Fred Picker, and still services these).
He said they're rather sensitive to vibration. Perhaps my recent
trip to Colorado and all the driving around on dirt roads, not to
mention the airplane trips, had something to do with my problem.

Anyway, the meter is off to Richard tomorrow for recalibration.
Once it's back I'll see about setting up my own light meter
cal lab.

Steve