dreilly
Chillin' in Geneva
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- 10:14 PM
- Joined
- Dec 25, 2004
- Messages
- 1,045
I found a thread on another forum detailing how to adjust the CL's meter to either a new battery type or the changing response of an aging cds cell. It's a bit tedious (lots of turning, checking, turning again) but I have to report it does work.
I got a bargain basement Leitz Minolta CL with a wonky, but functioning meter, and used this procedure to convert the meter to a silver oxide 1.55v standard and bring it back to reasonable accuracy. Now, the meter agrees with my Gossen Super Pilot on uniform test fields (walls/paper of a certain brighness, etc). In use, it agrees within about 1 stop with the Super Pilot...which might be caused by the fact that the Pilot is an averaging meter with a wider field of measurement than the CL's semi-spot field.
I wanted to copy the text of this procedure here for the RFF record:
The original text I found at http://photo.net/bboard/q-and-a-fetch-msg?msg_id=00C1kg
By the way, do this at your camera's risk. I think if you're careful at worst you'll make a wonky meter more wonky.
-------------------------------------
Some basics on CdS-lightmetering
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.
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.
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.
LDR's have some unwanted features:
* They are not only light dependent but also temperature dependent. This effect is compensated in the electronic circuit described above.
* 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.
* And last: LDRs change their characteristic over time. And that's the reason why older lightmeters must be calibrated from time to time.
Finding the circuit inside the CL
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.
Beneath this foil you will see 4 little holes. With a little (electronic) screwdiver one can turn trimming resistors through this holes.
The holes and the corresponding trimmers are numbered by Leica as follows:
* 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.
* 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.
Trimming procedure
1. 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).
2. Set the film sensitivity dial to 17 DIN (approx. 50 ASA).
3. Set the shutter dial to 1/1000s.
4. 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.
5. Set the shutter dial to 1/30s. Measure again with your second lightmeter and transfer the aperture. Turn R4 until the needle is OK.
6. 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.
7. 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.
8. Leica announces that this calibration will give you a tolerance of 2 DIN (film sensitivity).
9. 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.
10. 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.
I got a bargain basement Leitz Minolta CL with a wonky, but functioning meter, and used this procedure to convert the meter to a silver oxide 1.55v standard and bring it back to reasonable accuracy. Now, the meter agrees with my Gossen Super Pilot on uniform test fields (walls/paper of a certain brighness, etc). In use, it agrees within about 1 stop with the Super Pilot...which might be caused by the fact that the Pilot is an averaging meter with a wider field of measurement than the CL's semi-spot field.
I wanted to copy the text of this procedure here for the RFF record:
The original text I found at http://photo.net/bboard/q-and-a-fetch-msg?msg_id=00C1kg
By the way, do this at your camera's risk. I think if you're careful at worst you'll make a wonky meter more wonky.
-------------------------------------
Some basics on CdS-lightmetering
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.
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.
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.
LDR's have some unwanted features:
* They are not only light dependent but also temperature dependent. This effect is compensated in the electronic circuit described above.
* 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.
* And last: LDRs change their characteristic over time. And that's the reason why older lightmeters must be calibrated from time to time.
Finding the circuit inside the CL
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.
Beneath this foil you will see 4 little holes. With a little (electronic) screwdiver one can turn trimming resistors through this holes.
The holes and the corresponding trimmers are numbered by Leica as follows:
* 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.
* 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.
Trimming procedure
1. 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).
2. Set the film sensitivity dial to 17 DIN (approx. 50 ASA).
3. Set the shutter dial to 1/1000s.
4. 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.
5. Set the shutter dial to 1/30s. Measure again with your second lightmeter and transfer the aperture. Turn R4 until the needle is OK.
6. 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.
7. 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.
8. Leica announces that this calibration will give you a tolerance of 2 DIN (film sensitivity).
9. 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.
10. 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.
