[pure-silver] Re: Metering 135 vs 120
----- Original Message ----- From: "Mark Blackwell" <mblackwell1958@xxxxxxxxx> 
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Tuesday, August 26, 2008 7:48 PM
Subject: [pure-silver] Re: Metering 135 vs 120
I use a Minolta IVF I believe and it has an adjustment buried in the back so a tech could make the adjustments IIRC.
Mine can be either a reflective or incident light meter, but to be a reflective meter it requires an additional attachment. Unless you are using the extra attachment that makes it a spot meter, then its a incident meter and that introduces a number of issues.
Let me suggest a couple of tests. Richard please chime in here if you see some things that would help.
I think you pretty much have it. Of course, the sunny 16 rule is not precise because the strength of sunlight varies with the seasons and with latitude, but in most places it will be close. Few modern meters use selenium cells but some do and there are lots of old ones around. Selenium cells rarely go completely dead but they often become non-linear. The usual symptom is reading low for high light intensities. Since most selenium meters have two overlapping ranges one can test for a reading which is in the overlap area. It should be essentially the same for both ranges. If the cell is bad it will usually read low on the low range where the pointer is near full scale in comparison to the same ligth read on the high range where the meter is toward the low end of the scale. Selenium meters do not have calibrating adjustments and there is no way to correct the progressive error of a bad cell other than replacing it. The only warning I have about using gray cards is to be careful of how its lighted. None I've seen has a truely diffusing surface. If not illuminated carefully there can be some specular reflection toward the meter which will make it read high. Of course you can also use a white card and correct the reading by a factor of five. Kodak used to recommend this method for measuring exposure for graphic arts work. Gray cards are often too small for some meters, usualy one can find white surfaces with essentially no limit as to size. There is an ISO standard for exposure meters, actually I think there are two, one for incident and another for reflected light meters. These describe the calculation of exposure from the measured light intensity to ISO film speeds. Unfortunately ISO standards are not available on-line and can be quite expensive to buy. I think its a bit of a racket but so are a lot of things we put up with every day. 

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Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 
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