Calibrating a Pentax Digital Spot-meter

henpe · Aug 12, 2019

Assuming you have access to a controlled light source or other reference meter, is it possible to calibrate a Pentax Digital Spot-meter? How? I have been searching for some kind of service manual, but without any luck. Off course I can just disassemble my meter, look for some trimpots and do some backwards engineering, but I prefer to have a firm idea on the schematics before I start. Any advice?

Thanks in advance!

PS: not interested to send my meter overseas (US) for calibration in a repair shop; to expensive and complicated to deal with shipping...

Sirius Glass · Aug 12, 2019

Send it off to Quality Light Metric. I use him to calibrate my Pentax Digital Spot Meter. George charged me $88US plus shipping in October [actually I drove over and dropped it off and picked it up the next day.] and when I had some questions last month he had me send it in and he calibrated it for free. He is highly recommended and the movie industry in Hollywood uses him very heavily. https://www.facebook.com/pages/Quality-Light-Metric/554305868044042

MattKing · Aug 12, 2019

I don't think Sirius caught the fact that the OP was unwilling to deal with the complexities and expense involved when sending equipment thousands of kilometres for calibration.
Otherwise, I would agree with him.

ic-racer · Aug 12, 2019

To continue with posts that don't answer the original question, I like my Sekonic meter because the calibration factor is easily manipulated in a submenu of its digital operating system; no screwdriver needed!

RalphLambrecht · Aug 12, 2019

henpe said:
Assuming you have access to a controlled light source or other reference meter, is it possible to calibrate a Pentax Digital Spot-meter? How? I have been searching for some kind of service manual, but without any luck. Off course I can just disassemble my meter, look for some trimpots and do some backwards engineering, but I prefer to have a firm idea on the schematics before I start. Any advice?

Thanks in advance!

PS: not interested to send my meter overseas (US) for calibration in a repair shop; to expensive and complicated to deal with shipping...

I'd be worried to mess something up.

Sirius Glass · Aug 12, 2019

RalphLambrecht said:
I'd be worried to mess something up.

Like doing recalibration without a standard. Calibrating to nothing is not calibrating.

henpe · Aug 13, 2019

Just to be clear:
I am confident that I understand the complexity of the task and that a some specialists equipment is needed. I do have access to calibrated light sources, lab grade photo-spectrometers, light intensity meters, integrating spheres etc. I am also confident that I have the technical skills required to do the calibration.
To avoid guesswork and to avoid spending a lot of time doing reversed engineering I am now looking for a schematics, a service manual, or similar, to help me understand the schematics of the Pentax. Basically, I want to know how to adjust bias and/or gain of the lightmeter, or possibly if the calibration is done according to some other regimen.

Anyone with access to the schematics or service manual? Would it be possible to share, please?

I hope no one find this post as blunt, not intended!

Best regards
Henrik

neilt3 · Aug 15, 2019

I don't have any Pentax meters , my spot meter and other meters are all Minolta .
In mine the adjustment point is in the battery compartment hidden with a square plastic flush fitting cover .
No markings are on it , presumably so people don't have a fiddle with it.
I had to adjust one of my meters as it was well out compared to my other meters and cameras .
I used an evenly lit wall with artificial light in the house to test things .
I took a light reading with my spot meter , then a couple of cameras , then my 3 other meters , a Minolta flash meter IV , Autometer III and Autometer IIIF .
The one that was out was adjusted by turning the adjuster by two of the markings .
All my meters now give consistent readings both under test and in the field .

Took about ten minutes to do my testing and thirty seconds to do the adjustments and retest .

Is there not an unmarked cover on yours ?

Chan Tran · Aug 15, 2019

Calibration must be done with at least 2 adjustment and not 1. My Minolta meters have the 1 screw to bias the meter but that's not calibration. I know that with today's digital DSLR and Mirrorless the meter calibration is done via a USB cable.

Chan Tran · Aug 15, 2019

henpe said:
Assuming you have access to a controlled light source or other reference meter, is it possible to calibrate a Pentax Digital Spot-meter? How? I have been searching for some kind of service manual, but without any luck. Off course I can just disassemble my meter, look for some trimpots and do some backwards engineering, but I prefer to have a firm idea on the schematics before I start. Any advice?

Thanks in advance!

PS: not interested to send my meter overseas (US) for calibration in a repair shop; to expensive and complicated to deal with shipping...

However why don't you check the meter and determine how much it's off before trying to do calibration. With all those instruments you should be able to do that easily.

henpe · Aug 15, 2019

I have done that: It's off by approx 1 stop

Arvee · Aug 15, 2019

henpe said:
I have done that: It's off by approx 1 stop

If you know the amount of error, simply adjust film speed to compensate. I have a number of meters and attach a dot label to the back indicating correction factor. Much simpler/cheaper than getting each meter calibrated. I use Sunny 16.5 since I live at 5K+ elevation as a constant source for calibration.

Luckless · Aug 15, 2019

Arvee said:
If you know the amount of error, simply adjust film speed to compensate. I have a number of meters and attach a dot label to the back indicating correction factor. Much simpler/cheaper than getting each meter calibrated. I use Sunny 16.5 since I live at 5K+ elevation as a constant source for calibration.

Given that the tool is capable of being calibrated, I'm not sure why someone would really want to settle for mucking about with a workaround instead of fixing it so it is works correctly...

Seems like suggesting one use their large front window to get in and out of their house instead of fixing the lock on their front door.

Sirius Glass · Aug 15, 2019

neilt3 said:
I don't have any Pentax meters , my spot meter and other meters are all Minolta .
In mine the adjustment point is in the battery compartment hidden with a square plastic flush fitting cover .
No markings are on it , presumably so people don't have a fiddle with it.
I had to adjust one of my meters as it was well out compared to my other meters and cameras .
I used an evenly lit wall with artificial light in the house to test things .
I took a light reading with my spot meter , then a couple of cameras , then my 3 other meters , a Minolta flash meter IV , Autometer III and Autometer IIIF .
The one that was out was adjusted by turning the adjuster by two of the markings .
All my meters now give consistent readings both under test and in the field .

Took about ten minutes to do my testing and thirty seconds to do the adjustments and retest .

Is there not an unmarked cover on yours ?

https://www.cnn.com/2019/08/14/tech/windows-10-microsoft-security-update-trnd/index.htmlf

The question is "Are your other meters correct or off by the same amount?" Mine were off from 3/4 f/stop to 1 1/4 f/stop thus appearing to agree but still wrong. Therefore to do the job correctly one needs a calibrated light source or to send the meters to be calibrated. Of course if your photographs are not important the why bother to calibrate at all?

neilt3 · Aug 15, 2019

Sirius Glass said:
https://www.cnn.com/2019/08/14/tech/windows-10-microsoft-security-update-trnd/index.htmlf

The question is "Are your other meters correct or off by the same amount?" Mine were off from 3/4 f/stop to 1 1/4 f/stop thus appearing to agree but still wrong. Therefore to do the job correctly one needs a calibrated light source or to send the meters to be calibrated. Of course if your photographs are not important the why bother to calibrate at all?

My cameras metered just fine with all film including E6 .
My meters gave the same readings .
Spot meter and cameras set to spot metering with a grey card filling the spot metering area , autometers measuring the same light that hits the card .
All gave consistent results in a variety of lighting conditions that resulted in correctly exposed film , which is kinda the point of using one when I'm using cameras that don't have a built in meter .
Unless I'm using flash I tend to use my spot meter , and with large format film especially I'd rather not waste film .

All except one that is , who was giving a much lower figure consistently .
The numbers it was giving didn't agree with what my brain and eyes could see , and what I thought the exposure should be .
Guess which one I adjusted ?
It now gives figures I can trust .

Regards you link provided , were you intending to link to something on topic , or just for information to anyone on W10 ?

ic-racer · Aug 15, 2019

Bill Burk · Aug 15, 2019

henpe said:
Just to be clear:
I am confident that I understand the complexity of the task and that a some specialists equipment is needed. I do have access to calibrated light sources, lab grade photo-spectrometers, light intensity meters, integrating spheres etc. I am also confident that I have the technical skills required to do the calibration.
To avoid guesswork and to avoid spending a lot of time doing reversed engineering I am now looking for a schematics, a service manual, or similar, to help me understand the schematics of the Pentax. Basically, I want to know how to adjust bias and/or gain of the lightmeter, or possibly if the calibration is done according to some other regimen.

Anyone with access to the schematics or service manual? Would it be possible to share, please?

I hope no one find this post as blunt, not intended!

Best regards
Henrik

Haaa

I have a calibrated light source 100 foot lamberts. But its color temperature is matched to Weston Master era meters. Great for them. But not for modern meters.

Bill Burk · Aug 15, 2019

ic-racer, is that the digital spotmeter? I didn't know it used 9v source, had a high-low switch and a light bulb.

p.s. Though it's off a stop at one particular light level, it may have a linearity issue as well - (might be accurate in low light for instance but off in daylight). So if you can calibrate it that would be great.

Sirius Glass · Aug 16, 2019

neilt3 said:
My cameras metered just fine with all film including E6 .
My meters gave the same readings .
Spot meter and cameras set to spot metering with a grey card filling the spot metering area , autometers measuring the same light that hits the card .
All gave consistent results in a variety of lighting conditions that resulted in correctly exposed film , which is kinda the point of using one when I'm using cameras that don't have a built in meter .
Unless I'm using flash I tend to use my spot meter , and with large format film especially I'd rather not waste film .

All except one that is , who was giving a much lower figure consistently .
The numbers it was giving didn't agree with what my brain and eyes could see , and what I thought the exposure should be .
Guess which one I adjusted ?
It now gives figures I can trust .

Regards you link provided , were you intending to link to something on topic , or just for information to anyone on W10 ?

Contact information if someone wants to use him.

neilt3 · Aug 16, 2019

Sirius Glass said:
Contact information if someone wants to use him.

The link provided is for a CNN news alert to a security risk on windows 10 p.c's , not a service centre / repair man for light meters .

Alan Edward Klein · Aug 16, 2019

henpe said:
Just to be clear:
I am confident that I understand the complexity of the task and that a some specialists equipment is needed. I do have access to calibrated light sources, lab grade photo-spectrometers, light intensity meters, integrating spheres etc. I am also confident that I have the technical skills required to do the calibration.
To avoid guesswork and to avoid spending a lot of time doing reversed engineering I am now looking for a schematics, a service manual, or similar, to help me understand the schematics of the Pentax. Basically, I want to know how to adjust bias and/or gain of the lightmeter, or possibly if the calibration is done according to some other regimen.

Anyone with access to the schematics or service manual? Would it be possible to share, please?

I hope no one find this post as blunt, not intended!

Best regards
Henrik

I only have one IIIF. How would I calibrate it?

Sirius Glass · Aug 17, 2019

How about

neilt3 · Aug 17, 2019

Sirius Glass said:
How about

View attachment 229061

No doubt about it , that's definitely the one now !
Wrong side of the world for me , but will be very useful for folk on your side of the pond .
As it happens , even over here , I seem to remember being recommend to them before as a very good service centre .
Cheers .

ic-racer · Aug 17, 2019

Bill Burk said:
ic-racer, is that the digital spotmeter? I didn't know it used 9v source, had a high-low switch and a light bulb.

p.s. Though it's off a stop at one particular light level, it may have a linearity issue as well - (might be accurate in low light for instance but off in daylight). So if you can calibrate it that would be great.

That schematic is for the one in the picture below. It is the only Pentax spot repair manual I know of. Though there may be more. I wound up not needing the manual, because I got a Sekonic meter instead. I don't have the service manual for my Sekonic meter, but probably will never need it. The Sekonic is nice because it has a digital calibration via a menu and if it breaks I can just get a new one.

Chan Tran · Aug 17, 2019

Sirius Glass said:
https://www.cnn.com/2019/08/14/tech/windows-10-microsoft-security-update-trnd/index.htmlf

The question is "Are your other meters correct or off by the same amount?" Mine were off from 3/4 f/stop to 1 1/4 f/stop thus appearing to agree but still wrong. Therefore to do the job correctly one needs a calibrated light source or to send the meters to be calibrated. Of course if your photographs are not important the why bother to calibrate at all?

I want a good meter regardless whether I would take any photograph or not.

Calibrating a Pentax Digital Spot-meter

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