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Praktica MTL5-B: How to calibrate the light meter?
Old 02-02-2016   #1
AGX2015
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Praktica MTL5-B: How to calibrate the light meter?

Hi,

i have bought an old Praktica MTL 5B that use M42 lenses.

It's like new, but there is a problem, the light meter is not precise, how to recalibrate it?

Some one can help me?

I read that on the botton there are three potentiometer that i need to adjust, but i don't have a guide how to to and what type of value I need to use, i can use the value of my DSLR to calibrate it?

I hope that some one can help me, because actually the camera is not usable.
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Old 02-02-2016   #2
Sarcophilus Harrisii
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Quote:
Originally Posted by AGX2015 View Post
Hi,

i have bought an old Praktica MTL 5B that use M42 lenses.

It's like new, but there is a problem, the light meter is not precise, how to recalibrate it?

Some one can help me?

I read that on the botton there are three potentiometer that i need to adjust, but i don't have a guide how to to and what type of value I need to use, i can use the value of my DSLR to calibrate it?

I hope that some one can help me, because actually the camera is not usable.
The first thing you need to do, is to work out whether or not you have a MTL5 or MTL5B, because they are two different models. Your thread title refers to the MTL5. Your post above refers to the MTL5B. So which one is it?

Without looking it up I have a feeling they might have used two different types of battery. Wasn't the original 5 fitted with a 1.35 Mercury cell? I seem to recall the 5B, being later, may have moved on from that.

Pursuant to that you need to establish what is the correct battery to use for the one you have and confirm you're using it.
Get back to us when you've done the above.
Cheers,
Brett
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Old 02-02-2016   #3
AGX2015
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Thanks, i have done a typo in the title (corrected).

My camera is this:

- MTL-5B
- it uses the 1,5 Volt battery

I open the botton and i see that there are three potentiometer.

Actually the lightmeter is working, but is clear that the lecture is wrong and i suspect that the previous owner has change some value of the potentiometer.

How to calibrate again the reading of the meter?
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Old 02-02-2016   #4
Sarcophilus Harrisii
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Re your initial post. The MTL5B is a mechanical camera. The battery is only needed for the light metering function. So it is actually perfectly usable as it is, providing you meter separately.

I wouldn't be particularly inclined to reference the adjustment of the meter based on a digital camera. I prefer, if necessary, to make comparisons against a constant light source with a trustworthy film camera and ideally one using the same type of meter cell (Ie. selenium, CdS, silicon, etc). This helps prevent deviations based on cell sensitivity to different colour temperatures, something that would definitely be an issue, if, for instance, you were using a selenium meter late in the day to try to calibrate a CdS meter.

I strongly recommend doing some basic checks, prior to attempting any adjustments. Ie. battery voltage to spec. Battery contacts present, clean and in good condition. Lens aperture functioning correctly, and stop down/metering switch working properly.

What is the condition of the viewfinder like? Is it bright and clear? The MTL5B actually has a very good finder in my opinion. But if it has darkened, this may suggest that deterioration of the jointing surfaces could be affecting the meter accuracy.

You have assumed that the meter is completely functional, and simply badly adjusted. This may in fact, not be the case at all. There are many other reasons a meter will go out of range. Some of these include:
  1. Deterioration of the CdS cells altering their resistance values;
  2. Old cement becoming opaque and changing the amount of light reaching the cells where they're attached to the pentaprism;
  3. Deterioration of the prism itself (as mentioned above);
  4. Contamination by prism mounting foam, dust etc. impacting the light reaching the cells (depending on how they're installed);
  5. Wiring continuity faults in the meter circuit affecting current flow;
  6. Galvonometer faults, such as damaged or detached windings, needle off its perch or obstructed by debris of various sorts, bad electrical connections.
If the meter is likely to respond well to adjustment, it should already be reacting to various light levels in a consistent and proportionate manner but simply be out by a certain value, and out consistently. If it isn't, adjusting potentiometers is unlikely to improve matters at all (and will probably make things worse).

Assuming the meter is basically functioning correctly and is simply out by a stop or three across the range, it is possible (and I stress the word "possible") that you can dial it back to spec. But it's not a 5 minute job. At least, not without a manufacturer's test facilities and setting instructions. Meters often used a tiered, or primary/secondary, adjustment process, that facilitates accuracy across the full range of EV to be obtained. So making an adjustment to one pot won't necessarily get it ideal at all EV and probably won't. It may simply Eg. improve accuracy at higher EV, but not at lower EV (or vice-versa), and even make accuracy at the opposite end of the range worse. Hence, the presence of two or three pots, each of which may need to be balanced against the other and adjusted in a particular order (as they would have been at the factory no doubt) to get best performance. One of them may well be a calibration pot for the circuit that gets input voltage to spec. Many older cameras had one such in series with the battery check circuit (which, as I recall, the Praktica lacks). So I assume you have a multi meter, and know how to use it.

Before you touch anything, take a good, sharp, digital image, showing precisely where each pot is set. If I were doing the process, I would make comparison readings with another SLR using the same type of meter cell, using a prime lens of identical focal length and speed, set to the same focusing distance, and carefully take some precise daylight readings off a grey card as my initial reference. Having done this, I'd carefully reset one pot, checking and re-checking for improvement. If no improvement or a decrease in accuracy, reset pot to original setting and try next pot and so on.

Assuming you can achieve an increase in accuracy at a higher EV, you then need to repeat the process at lower EV levels. Using the same light source, so, assuming daylight, initially, this means moving into some shade and repeating the measuring process above. If low EV level accuracy is not good, you will then need to check the other pots in turn as mentioned above, to inspect the effect of adjusting these.

I'm quite fond of the MTL series and their kin. They're not perfect, but I handle a lot of second hand equipment which I unearth from various sources, and I've had much better luck with the functionality of 1970s Prakticas than I have with Eg. 1970s Pentaxes. Including their meters, which are often still working and accurate in my experience. But bear in mind that the camera is now several decades old, and that whilst you should endeavour to achieve perfect accuracy, this is a standard which may not realistically be possible with an as found vintage camera. Ultimately, you may have to settle for acceptable performance across the medium to high EV range (being the more commonly experienced imaging light range for most people) and accept that at lower EV, the meter is no longer optimum.

Also bear in mind that MTL5Bs, whilst worthy cameras, are not exactly rare or expensively priced, and, if having a 100% functional light meter is a must for you, you may be better served investing in another, fully functional example.
Cheers,
Brett
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Old 02-02-2016   #5
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AS far AS I know you can use 1,5 Volt cells with all MTL cameras but I don't know how to adjust the Meter. A battery check is strongly recommended
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Old 02-02-2016   #6
Joao
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Quote:
Originally Posted by AGX2015 View Post
Hi,

i have bought an old Praktica MTL 5B that use M42 lenses.

It's like new, but there is a problem, the light meter is not precise, how to recalibrate it?

Some one can help me?

I read that on the botton there are three potentiometer that i need to adjust, but i don't have a guide how to to and what type of value I need to use, i can use the value of my DSLR to calibrate it?

I hope that some one can help me, because actually the camera is not usable.
Have you compared the lightmeter readings with other cameras or have you tried to shoot some frames ? Try the camera with some film - if you have not done it yet. Don't take any action based only on different readings from different cameras
Good luck
Joao
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Old 02-03-2016   #7
k__43
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is it a constant error - like always two stops off? then I'd just say live with it and adjust the ISO accordingly. If it's a gain or linearity error i.e. the error changes from one to another EV then you have to adjust it.
I'd say if you are the adventurous type use paint to mark the current position and start trying. Measurement circuits are mostly wheatstone bridges and my guess one pot is for the offset (start point), one for the gain and one for the max. voltage (endpoint).

Also - don't use a DSLR as a reference light meter - who knows what company xyz is doing there today, digital is not as standardized as film AFAIK. Use a handheld lightmeter or another camera you trust. Check with slide within it's linear range (no long exposures) afterwards and see if you are getting your exposure right (probably best to measure of greycards to not involve scenery & lightning position as another error).

If you needle moves rather erratic then your CdS is on it's way to the graveyard or the battery contacts are corroded (try this first).

I know an excellent repairman for Praktica in Berlin but the repair (while not expensive) might be more than a replacement camera.
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Old 02-03-2016   #8
k__43
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Quote:
Originally Posted by Sarcophilus Harrisii View Post
Re your initial post. The MTL5B is a mechanical camera. The battery is only needed for the light metering function. So it is actually perfectly usable as it is, providing you meter separately.

I wouldn't be particularly inclined to reference the adjustment of the meter based on a digital camera. I prefer, if necessary, to make comparisons against a constant light source with a trustworthy film camera and ideally one using the same type of meter cell (Ie. selenium, CdS, silicon, etc). This helps prevent deviations based on cell sensitivity to different colour temperatures, something that would definitely be an issue, if, for instance, you were using a selenium meter late in the day to try to calibrate a CdS meter.

I strongly recommend doing some basic checks, prior to attempting any adjustments. Ie. battery voltage to spec. Battery contacts present, clean and in good condition. Lens aperture functioning correctly, and stop down/metering switch working properly.

What is the condition of the viewfinder like? Is it bright and clear? The MTL5B actually has a very good finder in my opinion. But if it has darkened, this may suggest that deterioration of the jointing surfaces could be affecting the meter accuracy.

You have assumed that the meter is completely functional, and simply badly adjusted. This may in fact, not be the case at all. There are many other reasons a meter will go out of range. Some of these include:
  1. Deterioration of the CdS cells altering their resistance values;
  2. Old cement becoming opaque and changing the amount of light reaching the cells where they're attached to the pentaprism;
  3. Deterioration of the prism itself (as mentioned above);
  4. Contamination by prism mounting foam, dust etc. impacting the light reaching the cells (depending on how they're installed);
  5. Wiring continuity faults in the meter circuit affecting current flow;
  6. Galvonometer faults, such as damaged or detached windings, needle off its perch or obstructed by debris of various sorts, bad electrical connections.
If the meter is likely to respond well to adjustment, it should already be reacting to various light levels in a consistent and proportionate manner but simply be out by a certain value, and out consistently. If it isn't, adjusting potentiometers is unlikely to improve matters at all (and will probably make things worse).

Assuming the meter is basically functioning correctly and is simply out by a stop or three across the range, it is possible (and I stress the word "possible") that you can dial it back to spec. But it's not a 5 minute job. At least, not without a manufacturer's test facilities and setting instructions. Meters often used a tiered, or primary/secondary, adjustment process, that facilitates accuracy across the full range of EV to be obtained. So making an adjustment to one pot won't necessarily get it ideal at all EV and probably won't. It may simply Eg. improve accuracy at higher EV, but not at lower EV (or vice-versa), and even make accuracy at the opposite end of the range worse. Hence, the presence of two or three pots, each of which may need to be balanced against the other and adjusted in a particular order (as they would have been at the factory no doubt) to get best performance. One of them may well be a calibration pot for the circuit that gets input voltage to spec. Many older cameras had one such in series with the battery check circuit (which, as I recall, the Praktica lacks). So I assume you have a multi meter, and know how to use it.

Before you touch anything, take a good, sharp, digital image, showing precisely where each pot is set. If I were doing the process, I would make comparison readings with another SLR using the same type of meter cell, using a prime lens of identical focal length and speed, set to the same focusing distance, and carefully take some precise daylight readings off a grey card as my initial reference. Having done this, I'd carefully reset one pot, checking and re-checking for improvement. If no improvement or a decrease in accuracy, reset pot to original setting and try next pot and so on.

Assuming you can achieve an increase in accuracy at a higher EV, you then need to repeat the process at lower EV levels. Using the same light source, so, assuming daylight, initially, this means moving into some shade and repeating the measuring process above. If low EV level accuracy is not good, you will then need to check the other pots in turn as mentioned above, to inspect the effect of adjusting these.

I'm quite fond of the MTL series and their kin. They're not perfect, but I handle a lot of second hand equipment which I unearth from various sources, and I've had much better luck with the functionality of 1970s Prakticas than I have with Eg. 1970s Pentaxes. Including their meters, which are often still working and accurate in my experience. But bear in mind that the camera is now several decades old, and that whilst you should endeavour to achieve perfect accuracy, this is a standard which may not realistically be possible with an as found vintage camera. Ultimately, you may have to settle for acceptable performance across the medium to high EV range (being the more commonly experienced imaging light range for most people) and accept that at lower EV, the meter is no longer optimum.

Also bear in mind that MTL5Bs, whilst worthy cameras, are not exactly rare or expensively priced, and, if having a 100% functional light meter is a must for you, you may be better served investing in another, fully functional example.
Cheers,
Brett
oh, missed this post! Great writing there.
Do what he says



btw. a MTL5B was my dad's camera - it developed a transportation problem which was very pronounced with the thicker bases of Kodak films (when we were able to buy those) - my dad always used Fuji then. The said repairman told me that a repair wouldn't be economic tho.
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Old 02-04-2016   #9
AGX2015
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Some one can help me find a photos of the potentiometer of another camera like mine?
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Old 02-05-2016   #10
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Quote:
Originally Posted by AGX2015 View Post
Some one can help me find a photos of the potentiometer of another camera like mine?
I'll try to look into mine this weekend. No promises there is a lot on my desk in the moment
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Old 02-05-2016   #11
baachitraka
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I may forget about the meter in MTL 5B, which is rather cumbersome to use and instead use an external meter.
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Old 02-09-2016   #12
AGX2015
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Originally Posted by k__43 View Post
I'll try to look into mine this weekend. No promises there is a lot on my desk in the moment
I hope to hear you
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