A tale of two meters

Perhaps someone with expert experience in meter bench-testing can explain a problem I had last year.

On the printed recommendation of a LF leading photographer, who had had two spot meters perfectly matched by a very reputable meter calibration service, I sent my Zone VI-modified Pentax analog spot and my much-less-recently calibrated Gossen Luna Pro-F to the same place, since the readings of the two varied widely and inconsistently. The meters returned, about 1 1/2 stops different.

I had another photographer confirm the reading discrepancy, just in case I had lost my mind.

I set up a carefully-made test target, metered it with both meters (both incident and reflected for the Gossen), recorded the readings in a table, and photographed the three set-ups (direct sun, open shade, interior indirect lighting). After notifying the service of the issue, I returned the meters with results, set-up photos, and further explanation.

When they returned again, the same problem persisted, suggesting that something in the meters' design differences made bench testing ineffective -- and that no off-bench test has been made. I resorted to using the Gossen's compensation feature to get them very close.

My question is why this evident failure in bench testing occurred.

I'd appreciate responses that offer some insight on this, if any is to be had, rather than questions on whether or not I had paid attention to possible sources of flare or infrared-emitting materials in my set-ups, or had used saturated colors that might be read differently by the meters. (None of the above. I'm not perfect, but I have run a lot of tests over the past five decades.)

Thanks.

Same problem in 2009 before I started LF

I gave up and use one meter only

Interestingly my strobes exactly move in stops, matching one meter


Fingers in ears...

Spot meter sees a very narrow angle. LunaSix Pro-F sees a wide angle. Unless you compare their reflected readings from a very large very uniformly illustrated wall they'll disagree. Why are you surprised that they disagree? And why are you surprised that a wide angle incident reading disagrees with a reflected spot reading, or, for that matter, with a reflected reading?

First, where did you send your meters? Quality Light Metric in Hollywood is my only choice.

Second, many meters are calibrated to different specifications, i.e., different "Zones" if you wish. Some meters are set to place in Zone V, some in Zone VI and some in between. Then there's the elusive K-factor (there was a discussion on all this here or over at Photrio some time ago; pretty technical IIRC).

Third, incident meters measure light falling on the meter; reflected meters measure light reflected from a surface. Unless your test surface is calibrated to match the incident meter, your readings will differ.

Fourth, just like watches, no two meters agree. I've got three Pentax digital spotmeters; they vary from one another by 1/3-stop or so. I just have separate E.I.s for the different meters.

If your meters are calibrated to factory specifications (maybe test your LunaSix against other LunaSixes and your Zone VI modified meter against other Zone VI modified meters to check), then they may simply be designed to give you different readings, i.e., to be used differently.

FWIW, I had a Zone VI modified Soligor that read 1 1/3 stops different than my Zone VI modified Pentax digital spotmeter. It took two trips to Richard Ritter to get them even close...

Meters aren't the precision instruments we like to think they are. They're still better than nothing, however.

Best,

Doremus

I think Doremus explained it all very well. I would simply add that IMO a modified Zone VI meter cannot be compared against any other non-modified meter because:

1. As already stated above, some meters are calibrated to Zone V while others are set at Zone VI. A Zone VI calibrated meter is set for Zone V.

2. Fred specified other meter modifications such as baffling, flare elimination, etc, in a quest to force the light meter to mimic film's response to daylight; specifically, Tri-X. Note: IIRC one of his newsletters detailed all of this throughly.

3. Light meters don't respond to different colored light (temperature) in a linear fashion. The same meter may well provide two different readings in daylight (5000K) vs tungsten (3200K).

4. All of this really doesn't matter because no two meters (even same type and manufacture) will read identically; generally speaking, anyway.

For fact, I own a Soligor 1 degree spot meter and a Pentax 1 degree digital spot meter, both modified by Zone VI, that don't match exactly at several different EV values. Best to pick one meter, test for proper exposure and development, then use only that meter. If you change meters for whatever reason, re-test, and continue your photography.

Just a clarification, if it makes any difference. My Gossen is, as stated, a Luna Pro-F (F for reading strobe as well). I take readings from large, evenly illuminated walls as well, but since the spot meter reads only a degree, I also compared spot readings from about 15 ft. with Gossen reflected readings from about 3 ft., careful to ensure that my presence was not affecting the incident illumination in some way.

A simple off-bench test would have revealed the difference; it could not have been performed. Anyway, as I said, with the Gossen compensation dialed in, they're now certainly close enough for my B&W work. I was just wondering.

Observations:

*What was the "carefully set up test target"?

*What was the light source used?


Spot meters are calibrated and designed to read light reflected from a certified calibrated 18% gray card using a 5000 Kelvin light source as the point of reference for calculating exposure time_f-stop_film speed.

Incident meters are calibrated to produce the same based on light "falling" on the diffuser.

If the light source is not 5000 degrees Kelvin, moderately diffused, and absolutely consistent light intensity these test are likely not valid. Different colors will produce a different spot meter reading than 18% gray.

Having done test like this in the past using a Minolta Spot F and Minolta flash meter 3 and 4, they agreed within 0.1 f-stop under highly controlled studio conditions using high quality Elinchrome strobes, Plume wafer light box, Kodak certified gray card.

Meter reading is intended to produce a specific film density with a source of 18% gray.

Anything less will produce non-accurate results.

** Remembered and found this previous post that is related to light meter testing. Go to post# 20. That is how color transparency film was tested, matched to the specific batch of film, lab processing's resulting density, and overall lens_lighting_light meter_lens f-stop calibration and all related to achieve a target film density measured using a calibration certified color densitometer which allows calibration of a given light meter.
https://www.largeformatphotography.i...ght=elinchrome


Bernice

Quote:

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Originally Posted by Ulophot

I set up a carefully-made test target, metered it with both meters (both incident and reflected for the Gossen), recorded the readings in a table, and photographed the three set-ups (direct sun, open shade, interior indirect lighting). After notifying the service of the issue, I returned the meters with results, set-up photos, and further explanation.

When they returned again, the same problem persisted, suggesting that something in the meters' design differences made bench testing ineffective -- and that no off-bench test has been made. I resorted to using the Gossen's compensation feature to get them very close.

My question is why this evident failure in bench testing occurred.

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