Q: Testing Exposure Meter Linearity Using ND Filters

[Peter Collins]

First, I have searched the Forum as best as I know how, and I cannot find/extract information to help me with this problem.

I propose to test my exposure meters using ND filters--not variable or grad--just uniform ND2, ND4, and ND8. The plan is to meter a bright, uniformly-lit surface, note the reading, then successively apply the filters by placing them in front of the meter. (I recognize that there may be some reflected light off the meter 'window' and the filters.) Three questions:
1. Can this plan be improved?
2. Can ND filters be combined/superimposed, e.g., placing an ND4 in front of an ND2?
3. If an ND4 (1/4 of the reflected light is passed) is placed in front of an ND2 (1/2 the reflected light is passed), is the result that 1/8 of the amount of reflected light is passed?
Thanks for helping me out!

[Bob Salomon]

Why?

Are you getting inconsistent readings?
Are you constanly getting incorrect exposures?

[Kirk Gittings]

First you would need to test the ND filters for accuracy no? I would suggest top of the line ones from like B&W. I have found cheaper ones to be all over the place.

[ic-racer]

Yes, that is how I do it to re-calibrate meters. Calibrated light sources are rare and expensive, but 'calibrated relative illumination' is easy to achieve with ND filters. Combine them as needed.

[Peter Collins]

Bob and Kirk,
Thanks for the questions and suggestions. I have a Gossen Luna Pro, a Sekonic 378, a banged-up Pentax Digital Spot, and a Weston Ranger 9 recently serviced by Quality Light Metric. What to trust? The Ranger 9--maybe. The Pentax Digital Spot thus seems away off at higher reflectances, e.g., EV 17+ I just want to be able to use it, or junk it. Also, the Luna Pro is a hand-me-down from a third party, and it seems inconsistent.

I'm looking at a set of Tiffen NDs--ND2, ND4, ND8 as a set for 67mm (what I use for filters). Cost is $59. B+W is about 4 times as much....

[Bill_1856]

A Ranger 9 recently serviced and calibrated by Quality is as good as it's gonna get. Just accept it and don't waste your time trying to "improve" their job.

[Struan Gray]

Compared to the metal-on-glass filters used in technical applications, photographic ND filters are not calibrated and, more importantly, they are not spectrally neutral. You may get a reasonably straight line plot in your tests, but you won't know if small deviations are due to the filters or the meter - or to some complicated interaction between both of them (their IR response, for example).

If you have two polarisers, you can plot intensity vs. angle as you rotate one of them in front of the other (if you have circular polarisers, you'll need to reverse the front one). This plot will have the form of a cos^2 wave (usually with a small vertical offset), and deviations from that functional form will be a better giveaway of meter nonlinearity than wobbles in an uncalibrated straight line. You'll need a way to set angles. Fitting the cos^2 can be done by hand, but a fitting program on a computer is faster and better.

[DrTang]

why not send all the meters to the guy who calibrated the wesson? get them all calibrated since he seems to have a light source to do it... then they will all be 'on'..more or less and relative to each other

[ic-racer]

A handheld light meter is not a piece of laboratory research equipment. The commonly available ND filters are more than adequate for testing and calibrating.
http://www.hoyaoptics.com/pdf/ND03.pdf

[Maris Rusis]

I test light meter linearity in my backyard at night. With a test card illuminated by a light bulb on a stand I take a meter reading. Then I move the light to twice the distance. The reading should be two stops lower; inverse square law and all that. Then I double the distance again and check the reading. The procedure is repeated until the light meter runs out of sensitivity. I write any non-linear correction factors on a small piece of card that I stick to the side of the meter for future reference.