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08-30-2018, 12:40 PM   #1
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Pentax Analog Spotmeter V calibration

Hey guys,

Not entirely sure this is the best place to put this, but I was wondering if thereís any way to validate the calibration of an analog Spotmeter V, and if itís found to be off for recording exposure, a procedure for calibrating the Spotmeter for accurate reads.

The reason why I ask is my Spotmeter had a cold solder at some point, and while i was fixing it after it failed, Iím not entirely sure that itís correct in its calibration anymore...

08-30-2018, 01:04 PM   #2
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QuoteOriginally posted by shadowjester Quote
Hey guys,

Not entirely sure this is the best place to put this, but I was wondering if thereís any way to validate the calibration of an analog Spotmeter V, and if itís found to be off for recording exposure, a procedure for calibrating the Spotmeter for accurate reads.

The reason why I ask is my Spotmeter had a cold solder at some point, and while i was fixing it after it failed, Iím not entirely sure that itís correct in its calibration anymore...
Eric probably can do this, send him an email.

Phil.
08-30-2018, 03:55 PM   #3
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While this won't help fix it, if you have access to another 1 degree spot, you could do a lot of A/B test to see if you get the same reading on both. That not "real" calibration because the other meter could be off, but it might tell you if its close the where it should be.
08-30-2018, 04:07 PM   #4
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The go-to person for light meter repair/CLA is George Milton in Los Angeles (Hollywood). All the local film, TV/Video, pro photographers and camera repair shops send their meters to him.

Light Meter Resurrection - Quality Light Metric - PentaxForums.com

The quick and dirty test is just to use a grey card as the target.

BTW: George Milton CLAed my Gossen Luna Pro F a number of years ago and did a splendid job. I highly recommend him.


Last edited by Not a Number; 08-30-2018 at 04:23 PM.
08-30-2018, 05:59 PM   #5
dms
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On a clear day with the sun fairly high, a reading of the blue sky should be a reliable f/16 at 1/iso sec. With your meter you may have to place it on zone V (a midtone), as opposed to a simpler meter.

---------- Post added 08-30-18 at 06:22 PM ----------

I checked one source (Dunn and Wakefield, Exposure Manual, 3rd edition, page 175-6) where he [Dunn] discusses the calibration of the SEI spot meter which he was the (or one of two) designers for it. He says a reading of the haze-free blue sky with a high sun (above about 45 degrees) is a method for calibrating it. The value will be 10,000 lumens per sq. ft. which he says corresponds to 1/125 sec at f/8 with ASA (ISO) 25 film. This would be the same as 1/125 sec at f/16 at iso 100--which is very close to the 1/100 sec, using my prescription above. I have seen others recommend metering the blue sky and 1/iso at f/16 (e.g., 1/100 sec at iso 100) for this calibration, but don't recall now where.

Last edited by dms; 08-30-2018 at 06:24 PM. Reason: Added about zone V.
08-30-2018, 06:48 PM   #6
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Thanks guys for the advice; I'll give the blue sky evaluation a try and see what it comes out to; any idea on how to actually adjust it if it is off?
08-30-2018, 06:52 PM   #7
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Sorry, but having given the above information, I should make a minor correction for completeness.

Dunn's description that I discussed above (on his page 177) does not directly state the reading is from the blue sky--rather it is of a midtone surface reflecting the sun/skylight. Elsewhere he does say the reading of the blue sky under the conditions given above is a check on the meter ("exposure meter middle-tone check level"), except that is on page 165, Table 6.1, where he explicitly says so.

---------- Post added 08-30-18 at 07:02 PM ----------

QuoteOriginally posted by shadowjester Quote
Thanks guys for the advice; I'll give the blue sky evaluation a try and see what it comes out to; any idea on how to actually adjust it if it is off?
I don't have/have not used that meter, but it should have an adjustable screw/knob. A quick and effective way (to first do it w/o touching said screw/knob) is to adjust (bias) the iso value. E.g., if the meter yields 1 stop too much exposure, then double the input iso, as compared to the actual iso value. Of course this presupposes the calibration is off by the same amount at all light levels (which is likely the case). Whether this is truly the case is another matter, and comparing to another meter should address this possibility.
08-31-2018, 08:07 AM   #8
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In the older Spotmeters there are five potentiometers/trimmers inside the meter (case must be opened) that adjust the calibration. A calibrated light source is required for accurate calibration (Note that Dunn above mentions a specific lumens per sq. ft).

Note: the external zero adjustment screw zeroes the needle mechanically and not the actual calibration in response to light levels. The zero adjustment is done with the power off.

Again, a quick and dirty test is to use a grey card at midday on a sunny cloudless day and see how close it is to the "Sunny 16" rule. If it's in +/- 0.5 EV I'd say it's good enough.

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