Trimming of the LEICA CL Lightmeter

Here's something from the official Leica Forum. I'm putting it here

because PN has a much more usable archive. <br><br>

 

<b>Some basics on CdS-lightmetering</b><br><br>

 

The CL's lightmeter uses a CdS-photoresistor (LDR 3D light dependent

resistor). The LDR's resistance is strongly dependent on the intensity

of light which hits it's surface. The relationship is: the more

intensive the light is, the lower the resistance becomes. In a double

logarithmic scale the LDR's characteristic is rather a straight line.

<br><br>

With the help of a battery and some additional resistors with constant

values the LDR's changing resistance is converted into a changing

voltage which is measured with a voltmeter. The voltmeter's needle is

what you see in your finder.

<br><br>

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.

<br><br>

Mercury and silver-oxide batteries have slightly different voltages.

<br><br>

LDR's have some unwanted features:

<br>

<ul>

<li>They are not only light dependent but also temperature dependent.

This effect is compensated in the electronic circuit described above.

</li>

<li>

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's final value. So be patient measuring in dim light.

</li>

<li>

And last: LDRs change their characteristic over time. And that's the

reason why older lightmeters must be calibrated from time to time.

</li>

</ul>

 

<b>Finding the circuit inside the CL</b>

<br><br>

The circuitry which converts the LDR's signal to the proper setting of

the lightmeter's 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>

<ul>

<li>

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.

</li>

<li>

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.

</li>

</ul>

<b>Trimming procedure</b>

<br><br>

<ol>

<li>

Put in a new battery. After calibration you must always use this type

of battery (I don't mean the maker but whether it's mercury or

silver-oxide).

</li>

<li>

Set the film sensitivity dial to 17 DIN (approx. 50 ASA).

</li>

<li>

Set the shutter dial to 1/1000s.

</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's 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's lightmeter by pulling the

transport lever.

</li>

<li>

Set the shutter dial to 1/30s. 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 1/1000s and 1/30s.

</li>

<li>

Set the shutter dial to 1/2s. Again, estimate the aperture at 17DIN,

1/2s, transfer it to the CL's 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's 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>

--------------------------

<br>

Tada!<div></div>

Thanks, Jeremy. Great info. I'll have to print this page.

Thanks Jeremy. Couple of months ago I bought some S625PX, 1.5VDC Silver Oxide batteries from batteriesinaflash<dot>com. Batteries are made by a company named Excell Photo Battery in China. I've been using it in my MR4 and will try one with this re-calibration on my CL. Ok gang, now somebody tell us how to install the newer and better photocell (LDR) in the CL.

Re: "...now somebody tell us how to install the newer and better photocell (LDR) in the CL."

 

Never heard of it. More info, please.

 

Does anyone use a battery adaptor with their CL, to take down the voltage?

I have been told that the older LDR is the one pictured. The cell's background is a light brown. The newer or replacement cells I saw are light green in color. I think they also had a few more "lines" in the element. Would appreciate more info.<div></div>

I don't have a CL, but it can't be all too difficult to replace the LDR. I have an electronic parts catalogue that lists several at 1 Euros each. The round one measures 5mm in diameter and 2.2 mm thick. At a light temperature of 2700 K and luminance of 1000 lux (outdoor light on a cloudy day) it has a resistance that varies between, get this, 75 ohm TILL 300 ohm. So these things are hardly precision elements, and after replacement you calibrate with the abovementioned procedure.

 

This LDR in the Consors catalogue has a meniscus lens with a radius of 2 mm. If you'll sand this down a little and fit a collar around the element, you can even make a spotmeter out of it!

 

The advantage of a new LDR is of course that it'll be 30 years younger than the original.

<p>Great info!</p>

I recalibrated my dad's CL's light meter by adjusting the trimming resistors as described. The instructions are great and the process works quite nicely. I needed about 30 minutes. 
Two comments: (a) take it easy when turning the screws - a little goes a long way (specifically R5; 1/8th of a rotation was already too much). (b) the cover of the resistors wasn't made of aluminum in my dad's camera's case - it was a plastic strip. I glued it back on with styrofoam glue, that stuff is not very aggressive and hopefully still a bit elastic in 10 years' time when I'll need to recalibrate again. 
Thank you very much, jeremy_tok, your description is great! Have a great day, Karl