This summer I visited a flea market near my mother's home town as we often like to do. After some time looking at various items ranging from junk to hectic, I found a pretty much brand new Praktica super TL2 from 1977, with its stock 50mm/2.8 lens, its ever-ready case and a camera bag. The seller wanted 15€ for it, I bargained it for 10€.
Fast forward home, I put a new battery in it, did a quick check on everything and realized the light meter was way off. It responded in a coherent manner (i.e. doubling ISO and halving exposure gave constant needle position) but compared to an exposition app on my smartphone as a reference, it was always off by anything between +1 and +2 stops. Not great.
I first suspected that the camera needed some old kind of mercury battery but everybody online seemed to tell that all Praktica cameras used voltage-insensitive electronics for the light meter. Then it was probably just uncalibrated, after sitting unused for decades. Bad news. But then I found this post on a forum explaining how to do that. This guy's post was extremely helpful and I managed to properly do the recalibration thanks to it. But I figured he got a few things wrong and I wanted to propose a more systematic approach other than trial and error. So I wrote a kind of tutorial - I post it here as it may be useful to someone else.
In order to perform the calibration, you first need to remove the battery holder and the battery and then unscrew to bottom lid of the camera. It is held in place by 4 small screws. You will need a very fine precision screwdriver because the slit is actually very thin. The first time I removed it, some old glue kept it in place so I gently applied a bit of pressure with a flat head screwdriver.
Take care not to lose those 4 screws and the film rewind button that will inevitably fall off. With the bottom plate removed, you can now access the PCB and perform the calibration.
You will need the circuit to be powered on and you will have to put back the battery in place. A good trick is to use a rubber band or a piece of tape:
You are now ready to calibrate the instrument. It is done by three variable resistors, called VR1, VR2 and VR3. To understand what we are doing, we need to have a look at the circuit:
The light meter operates with a Wheatstone bridge configuration: the photosensitive cell is in series with resistor VR1, in a voltage divider configuration. The needle meter compares this voltage to a reference voltage, set by a potentiometer under the ISO/shutter speed dial.
You have three resistors to tune VR1, VR2 and VR3. A bad VR1 value will affect the meter over its whole scale. VR3 will mostly impact the behavior of the meter with high lights and VR2 with low lights.
You can locate the three resistors on the PCB:
Since you have three resistors to tune, you need to tune it against at least 3 different light situations, ideally spanning over the whole range of the meter. I picked the following situations:
- my neighbor's house facade on a bright sunny day at f/4: high light
- same but at f/11: medium light
- the inside of my room at f/2.8: low light
Not that my "low light" scene is not that dark, but this is as low as the meter can actually get the needle to deflect accurately - the embedded light meter in this camera is not particularly sensitive ; by night/dark conditions, this meter is not actually useful.
I then used an Android app called "Light meter" to check what the actual exposure values should be at 50 ISO. It suggested 1/500, 1/60 and 1/8. I then tried to match these reading with the camera by adjusting the potentiometers:
1. First point at the medium light scene, set the shutter speed to the value given by the app and and with a screwdriver slowly adjust VR1 until it is accurate.
2. Then, pointing at the high light scene (adjusting aperture and shutter speed in consequence) and adjust VR3.
3. Then, pointing at the low light scene, adjust VR2. Don't worry if you hit the maximum or minimum of a resistor, try to get the best value you can.
4. Then, go back to point 1. The adjustment of VR2 and VR3 will have affected the setting for the medium light scene so tune it again. You will need to cycle through points 1, 2, 3 several times, but by the third or fourth iteration, you should only be making minor adjustments.
Don't be too zealous with a perfect calibration, this light meter is not incredibly accurate in the first place.
With the light meter properly calibrated, you can reinstall everything together. You need to pay attention to the film rewind button which can be tricky to hold in place as it has the tendency to pop. Screw back the four tiny screws and you are ready to go outside take pictures!
Fast forward home, I put a new battery in it, did a quick check on everything and realized the light meter was way off. It responded in a coherent manner (i.e. doubling ISO and halving exposure gave constant needle position) but compared to an exposition app on my smartphone as a reference, it was always off by anything between +1 and +2 stops. Not great.
I first suspected that the camera needed some old kind of mercury battery but everybody online seemed to tell that all Praktica cameras used voltage-insensitive electronics for the light meter. Then it was probably just uncalibrated, after sitting unused for decades. Bad news. But then I found this post on a forum explaining how to do that. This guy's post was extremely helpful and I managed to properly do the recalibration thanks to it. But I figured he got a few things wrong and I wanted to propose a more systematic approach other than trial and error. So I wrote a kind of tutorial - I post it here as it may be useful to someone else.
In order to perform the calibration, you first need to remove the battery holder and the battery and then unscrew to bottom lid of the camera. It is held in place by 4 small screws. You will need a very fine precision screwdriver because the slit is actually very thin. The first time I removed it, some old glue kept it in place so I gently applied a bit of pressure with a flat head screwdriver.
Take care not to lose those 4 screws and the film rewind button that will inevitably fall off. With the bottom plate removed, you can now access the PCB and perform the calibration.
You will need the circuit to be powered on and you will have to put back the battery in place. A good trick is to use a rubber band or a piece of tape:
You are now ready to calibrate the instrument. It is done by three variable resistors, called VR1, VR2 and VR3. To understand what we are doing, we need to have a look at the circuit:
The light meter operates with a Wheatstone bridge configuration: the photosensitive cell is in series with resistor VR1, in a voltage divider configuration. The needle meter compares this voltage to a reference voltage, set by a potentiometer under the ISO/shutter speed dial.
You have three resistors to tune VR1, VR2 and VR3. A bad VR1 value will affect the meter over its whole scale. VR3 will mostly impact the behavior of the meter with high lights and VR2 with low lights.
You can locate the three resistors on the PCB:
Since you have three resistors to tune, you need to tune it against at least 3 different light situations, ideally spanning over the whole range of the meter. I picked the following situations:
- my neighbor's house facade on a bright sunny day at f/4: high light
- same but at f/11: medium light
- the inside of my room at f/2.8: low light
Not that my "low light" scene is not that dark, but this is as low as the meter can actually get the needle to deflect accurately - the embedded light meter in this camera is not particularly sensitive ; by night/dark conditions, this meter is not actually useful.
I then used an Android app called "Light meter" to check what the actual exposure values should be at 50 ISO. It suggested 1/500, 1/60 and 1/8. I then tried to match these reading with the camera by adjusting the potentiometers:
1. First point at the medium light scene, set the shutter speed to the value given by the app and and with a screwdriver slowly adjust VR1 until it is accurate.
2. Then, pointing at the high light scene (adjusting aperture and shutter speed in consequence) and adjust VR3.
3. Then, pointing at the low light scene, adjust VR2. Don't worry if you hit the maximum or minimum of a resistor, try to get the best value you can.
4. Then, go back to point 1. The adjustment of VR2 and VR3 will have affected the setting for the medium light scene so tune it again. You will need to cycle through points 1, 2, 3 several times, but by the third or fourth iteration, you should only be making minor adjustments.
Don't be too zealous with a perfect calibration, this light meter is not incredibly accurate in the first place.
With the light meter properly calibrated, you can reinstall everything together. You need to pay attention to the film rewind button which can be tricky to hold in place as it has the tendency to pop. Screw back the four tiny screws and you are ready to go outside take pictures!
