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Discussion in 'Camera Building, Repairs & Modification' started by ic-racer, Sep 16, 2013.
I picked up a nice Vivitar flash meter for almost nothing. Looks brand new, but readings are off.
Inspection of insides reveals no signs of tampering. Trimmers don't have enough range to get it correct...must be something else...
Inspection of the front looks like a little piece of ND plastic is missing from the photoresistor opening. No problem, plenty of graded ND available in old control strips.
With a little trial-and-error, I found a piece of film with a density to exactly match the response a newer flash, known to produce acceptable exposures. Linearity is good without having to adjust the internal trimmers; 3ft, 6ft, & 12ft flash distances demonstrate the appropriate 2 stop drop.
and the verdict is………...
Did you miss post #3?
No, I saw that and am trying to roll it around in this ole noggin.
In novicespeak, does it work well now throughout the range? I guess it's the 2 stop comp you mention that is confusing me?
edit: okay I get it, the distance falloff is what you're saying…correct?
Double the distance between a flash and the meter and it should read 2 stops less for each doubling of the distance. That is the test for linearity of the response; no need for any calibrated light source, you just need an accurate way to measure distance and a repeatable flash.
So, yes it works perfectly.
Again, that was just the linearity test. The cheap-and-dirty calibration involved flashing a known to be good SB-28 Nikon flash, set to non-TTL auto, at various f-stops and reading the reflected value with Vivitar meter set to 'reflected light.' White wall, black wall, gray wall, it does not matter. Distance does not matter either. When the meter read the same f-stop as the flash was set for, it was considered 'calibrated.'
Realize that these inexpensive digital units just are accurate to a single f-stop. This Vivitar branded unit was also sold as Polaris or Shepherd FM800 and Novatron used to market one also:
I think it's really excellent you made this one come back to the living with a piece of spent film no doubt!
Bravo my man!!!
That is interesting -- I bought a Shepherd FM 900 many years ago and it still works with the 9 volt battery !
Here you go, Bruce. It's really pretty simple. Law of inverse squares - http://hyperphysics.phy-astr.gsu.edu/hbase/vision/isql.html