Archived posting to the Leica Users Group, 1999/05/12

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Subject: [Leica] WG: How to trim a CL's lightmeter
From: Michael Dienert <MDienert@foq.de>
Date: Wed, 12 May 1999 10:42:51 +0100

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> -----Urspr=FCngliche Nachricht-----
> Von:	Michael Dienert=20
> Gesendet am:	Dienstag, 11. Mai 1999 19:54
> An:	leica-users@mejac.palo-alto.ca.us
> Betreff:	How to trim a CL's lightmeter
>=20
> dear luggers,
>=20
> below is my first attempt to send the procedure to the mail-server.
> but to my surprise it didn't show up on the list. so i think the
> server rejected it because of a 70k attachment. this attachment was a
> ms-word document which i omit this time. if you look at the size of
> the html-file you will see how many resrouces on your harddisk and on
> the net are wasted by using billy's rubbish.
>=20
> hello my leica-(CL)-friends,
>=20
> as promised i want to post a procedure how one can calibrate the
> lightmeter circuitry of a CL. it's attached in different text =
formats.
> i hope everyone can open them. i know about the problems with
> differing versions of ms-word.
>=20
> =20
> any comments and questions to the procedure are appreciated.=20
> i calibrated my own CL exactly this way and am very happy with the
> results. i used a gossen-metrawatt lightmeter as a reference.
>=20
> so good luck to all CL-users,
>=20
> yours
>=20
> michael

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<TITLE>Trimming of the LEICA-CL Lightmeter</TITLE>
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<B><P ALIGN=3D"CENTER">Trimming of the LEICA-CL Lightmeter</P>
</B><P>&nbsp;</P>
<P>&nbsp;</P><DIR>
<DIR>

<B><P>Some basics on CdS-lightmetering</P>
</B><P>&nbsp;</P>
<P>The CL=91s lightmeter uses a CdS-photoresistor (LDR =3D light =
dependent  resistor). The LDR=91s resistance is strongly dependent on =
the intensity of light which hits it=91s surface. The relationship is: =
the more intensive the light is, the lower the resistance becomes. In a =
double logarithmic scale the LDR=91s characteristic is rather a =
straight line.  <BR>
<BR>
With the help of a battery and some additional resistors with constant =
values the LDR=91s changing resistance is converted into a changing =
voltage which is measured with a voltmeter. The voltmeter=91s needle is =
what you see in your finder. </P>
<P>&nbsp;</P>
<P>The energy to operate the voltmeter is delivered by the battery. =
Therefore the deflection of the voltmeter is dependent on the voltage =
of the battery. Mercury and silver-oxide batteries have slightly =
different voltages. </P>
<P>&nbsp;</P>
<P>LDR=91s have some unwanted features: <BR>
<BR>
They are not only light dependent but also temperature dependent. This =
effect is compensated in the electronic circuit described above.<BR>
<BR>
Their response to a change of the intensity of light is very slow. =
Especially for very low light it may take seconds until the LDR settles =
to it=91s final value. So be patient measuring in dim light.<BR>
<BR>
And last: LDRs change their characteristic over time. And that=91s the =
reason why older lightmeters must be calibrated from time to time.</P>
<P>&nbsp;</P>
<B><P> Finding the circuit inside the CL</B><BR>
<BR>
The circuitry which converts the LDR=91s signal to the proper setting =
of the lightmeter=91s needle is right to the film cartridge. Take the =
backcover of the CL away and hold the camera in a way that you would =
look on the eyelets of the straps. Then you should see a black =
aluminium foil at the side of the groove where the film cartridge =
normally fits in. This foil is self adhesive and sticks to the case. =
Remove it. <BR>
<BR>
Beneath this foil you will see 4 little holes. With a little =
(electronic) screwdiver one can turn trimming resistors through this =
holes.<BR>
<BR>
The holes and the corresponding trimmers are numbered by Leica as =
follows:<BR>
<BR>
The one nearest to the top of the camera is R3. If you still hold the =
camera as described above it is the most left one.<BR>
<BR>
The next ones are R4, R5 and R6. So from top to bottom (or left to =
right) you have the order R3, R4, R5, R6. By the way: R comes from =
Resistor.</P>
<P>&nbsp;</P>
<B><P>Trimming procedure</P>
<P>&nbsp;</P></DIR>
</DIR>

<OL>

</B><LI>Put in a <B>new</B> battery. After calibration you must always =
use this type of battery (I don=92t mean the maker but wether it=92s =
mercury or silver-oxide)</LI>
<LI>Set the film sensitivity dial to 17 DIN (approx. 50 ASA)</LI>
<LI>Set the shutter dial to =961000-.</LI>
<LI>Point the camera to you slide screen and light it with a very =
bright light source. Use a good, calibrated lightmeter or another =
camera you trust, point it to the screen, read the aperture at 17DIN, =
1/1000s and set the CL=91s aperture to the same value. You can use =
whatever lens you want. Now the needle in the finder must correspond =
with the notch of the lightmeter. If not, turn R3 clock- or =
counterclockwise until it does. Of course you must turn on the CL=92s =
lightmeter by pulling the transport lever.</LI>
<LI>Set the shutter dial to =9630-. Measure again with your second =
lightmeter and transfer the aperture. Turn R4 until the needle is =
OK.</LI>
<LI>The setting of R4 has an influence on the setting of R3. So you =
must repeat the last to steps until the needle corresponds with the =
notch at 1/1000 and 1/30.</LI>
<LI>Set the shutter dial to -2-. Again, estimate the aperture at 17DIN, =
1/2s, transfer it to the CL=92s lens and now turn R5 until the needle =
is in coincidence with the notch. Turning R5 has an influence on the =
setting of R3 and R4. So you must repeat the preceding procedures until =
you have coincidence in all of the 3 cases.</LI>
<LI>Leica announces that this calibration will give you a tolerance of =
2 DIN (film sensitivity)</LI>
<LI>A hint: in the original Leica procedure a special calibrated light =
source with variable intensity recommended. In my procedure this is =
substituted by a screen, any lightsource and a second lightmeter. So =
your calibration will not be better than the lightmeter which you use =
as a reference. It may happen that from 1/1000s to 1/2s you may not =
obtain apertures which you can set on the CL=92s lens. In that case you =
can change the intensity of your lightsource. E.g. at 1/1000 use your =
slide projector, at 1/2s use a 25W lamp at an appropriate distance to =
light the screen. Just take care that the screen is lit well-balanced. =
Or at least use the same spot on the screen to measure with the =
reference lightmeter and the CL.</LI>
<LI>Last step: Set the shutter dial as to measure battery power, press =
the battery measuring button and set R6 until the needle is in the =
middle of the notch. This trimming has no effect on the lightmeter, it =
just calibrates the battery measurement.</LI></OL>
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