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06-22-2020, 05:31 PM   #1
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Spotmatic F open aperture metering issue

My Spotmatic F metering is off, over-exposing by about 1 1/2 stops. What's interesting is if I flip the metering switch the meter reading is only off by 1/2 stop. This is with SMC Takumar lenses. When I test with a super Takumar, its the same 1/2 stop off (which is good enough for me). So, it appears the variable resistor moved with aperture changing is the issue. I'm wondering is anyone has experienced this. Maybe some corrosion or buildup in the variable resistor is effecting this...changing the resistance. Getting to that resistor pad is difficult...requires removing the front standard to get to it to clean... Something I don't want to risk doing on an otherwise perfectly working camera.



06-22-2020, 08:39 PM - 1 Like   #2
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QuoteOriginally posted by tonyzoc Quote
My Spotmatic F metering is off, over-exposing by about 1 1/2 stops.
You had a similar problem with your ESII, right? Did you get a diagnosis for it? Are you comparing the meter against another camera or handheld metering or guessing from the negatives?

QuoteOriginally posted by tonyzoc Quote
What's interesting is if I flip the metering switch the meter reading is only off by 1/2 stop.
You can't do stop-down metering with an SMC/S-M-C series lens. This is in the camera manual. The results will always be improper.

QuoteOriginally posted by tonyzoc Quote
When I test with a super Takumar, its the same 1/2 stop off
The stop-down metering and open-aperture metering are trimmed separately and by different means. There may be an issue with the variable resistor, though that should not affect the stop-down metering, assuming that the coupler is properly positioned. FWIW, my SP F's meter was off a bit when I sent it for CLA and the fix involved replacement of the meter cells.

Good luck!


Steve
06-23-2020, 02:53 AM   #3
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QuoteOriginally posted by stevebrot Quote
You had a similar problem with your ESII, right? Did you get a diagnosis for it? Are you comparing the meter against another camera or handheld metering or guessing from the negatives?



You can't do stop-down metering with an SMC/S-M-C series lens. This is in the camera manual. The results will always be improper.



The stop-down metering and open-aperture metering are trimmed separately and by different means. There may be an issue with the variable resistor, though that should not affect the stop-down metering, assuming that the coupler is properly positioned. FWIW, my SP F's meter was off a bit when I sent it for CLA and the fix involved replacement of the meter cells.

Good luck!


Steve
I don't know if it's the same as the ESII, as I didn't try it with stopped down metering. Buts off by about the same amount. I'm comparing exposure vs a few cameras.. my MX and K-70...also a Vivitar 230LX.
I've read you can't do stopped down metering with SMC lenses, but when I do, it matches exactly to the readings I get with a 50/1.4 Super Takumar stopped down.

06-23-2020, 09:56 PM   #4
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I would suspect that the meter cells have aged and you need to adjust the meter calibration VR. My recollection is that there is a separate VR for stop down. After 45 years CDS cells can age out of calibration and a stop and a half is not unheard of.

06-24-2020, 12:26 PM - 1 Like   #5
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Hello. I have just adjusted the meter of my Spotmatic-f which suffered similar symptoms than yours. If you are comfortable removing the top of the camera is not difficult.


There are two different circuits for stop-down metering and for open-iris metering with SMC takumars. The first one can be adjusted by a small pot which can be accessed by removing the film reminder discs in the rewind film axis. You need a known good meter (or camera) and 2 or 3 different light conditions. In my experience is difficult to adjust the high level light AND the low level light so you just make a compromise.


For adjusting the open-diaphragm you have to remove the top cover. There are 2 little pots around the viewfinder: the left one is for the high level and the right one for the low level. You can achive a better adjustment with this procedure than with the previous one.


I have found invaluable these manuals: Pentax Manuals
For removing the top cover "National Camera Manual for Spotmatic"

and for the meter adjustment: "Pentax Spotmatic F Meter"

If you need details I would be glad to help. Regards
06-24-2020, 01:20 PM   #6
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QuoteOriginally posted by wupdigoj Quote
Hello. I have just adjusted the meter of my Spotmatic-f which suffered similar symptoms than yours. If you are comfortable removing the top of the camera is not difficult.


There are two different circuits for stop-down metering and for open-iris metering with SMC takumars. The first one can be adjusted by a small pot which can be accessed by removing the film reminder discs in the rewind film axis. You need a known good meter (or camera) and 2 or 3 different light conditions. In my experience is difficult to adjust the high level light AND the low level light so you just make a compromise.


For adjusting the open-diaphragm you have to remove the top cover. There are 2 little pots around the viewfinder: the left one is for the high level and the right one for the low level. You can achive a better adjustment with this procedure than with the previous one.


I have found invaluable these manuals: Pentax Manuals
For removing the top cover "National Camera Manual for Spotmatic"

and for the meter adjustment: "Pentax Spotmatic F Meter"

If you need details I would be glad to help. Regards
Yay! Thanks for sharing this. As you mentioned, securing a proper light source for the high and low calibrations is tough. A shop would have a calibrated light source. We have to make due with something else.


Steve
06-25-2020, 02:45 PM   #7
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QuoteOriginally posted by wupdigoj Quote
Hello. I have just adjusted the meter of my Spotmatic-f which suffered similar symptoms than yours. If you are comfortable removing the top of the camera is not difficult.


There are two different circuits for stop-down metering and for open-iris metering with SMC takumars. The first one can be adjusted by a small pot which can be accessed by removing the film reminder discs in the rewind film axis. You need a known good meter (or camera) and 2 or 3 different light conditions. In my experience is difficult to adjust the high level light AND the low level light so you just make a compromise.


For adjusting the open-diaphragm you have to remove the top cover. There are 2 little pots around the viewfinder: the left one is for the high level and the right one for the low level. You can achive a better adjustment with this procedure than with the previous one.


I have found invaluable these manuals: Pentax Manuals
For removing the top cover "National Camera Manual for Spotmatic"

and for the meter adjustment: "Pentax Spotmatic F Meter"

If you need details I would be glad to help. Regards
Thanks, I'm familiar with removing the top. One thing I noticed in the meter manual...it seems the high and low adjusts effect the ends of the exposure graph pictures...basically increasing or decreasing the slope of the line. This Leeds me to think at some EV I would get a correct exposure. If I'm seeing low light (EV 6 say) still low by 2 stops, and a bright source (EV 16) also low by 2 stops, will adjusting these pots work? Maybe the graph isn't really representative of what happens on adjust. Looking at that graph...it seem something in the middle would be still reading correctly...like EV 10-11...

06-26-2020, 09:28 AM   #8
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I do not remember the graph, but the process is as follows:
- Point the camera to a strongly lighted surface(e. g. EV 15 or 16) and adjust the left pot until the meter is centered using correct f number, speed and iso.
- Repeat the process using approx. EV 8, adjusting the right pot.

Most probably the high light calibration would be now slightly out: adjust again and iterate until both are right. Check intermediate values.

Probably you will have to shield the cds cells from light not coming directly from the pentaprism, use black tape or something like that. Take care when adjusting the pots: they are easily scratched.
Regards

06-30-2020, 12:38 PM   #9
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QuoteOriginally posted by wupdigoj Quote
I do not remember the graph, but the process is as follows:

- Point the camera to a strongly lighted surface(e. g. EV 15 or 16) and adjust the left pot until the meter is centered using correct f number, speed and iso.

- Repeat the process using approx. EV 8, adjusting the right pot.



Most probably the high light calibration would be now slightly out: adjust again and iterate until both are right. Check intermediate values.



Probably you will have to shield the cds cells from light not coming directly from the pentaprism, use black tape or something like that. Take care when adjusting the pots: they are easily scratched.

Regards
Thanks. I may give this a try.

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