Meter calibaration

[Eric Rose]

The following is what I snagged from the Bogen Site on Gossen meters. It's in their FAQ section:
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My Gossen meter gives me a slightly different reading than my in-camera meter. Which one is correct?

Because there are no International Standards on this topic, variances can occur. Gossen and most European photographic manufactures, calibrate their meters at 5600o Kelvin, a "Daylight" setting. Most Japanese manufacturers calibrate at 3400o Kelvin, a "Tungsten" setting. This 2000o Kelvin difference in color temperature causes a difference in readings of approximately 1 F-stop.
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This is interesting as I use a Pentax spotmeter and a Gossen LunaPro for incident readings. Now if I am reading the shadows with my spotmeter and the color temperature seems to be a variable in the equation it would stand to reason that if I'm reading the shadow detail of a red object rather than a blue object the reading is going to be different for each even though the same amount of shadow light is there. This is in addition to the error caused by the meter trying to make everything 18% grey.

There is an interesting thread on PN that can be read here:

http://www.photo.net/bboard/q-and-a-fetch-...g?msg_id=000ud4


Would this only be an issue for chromes and not B&W? Gee and I just thought I had it all figured out!!

Eric

 

[clogz]

Well, well....and I have always thought that light meters read the intensity of the light and that for colour temperature you used a different meter.
And Gossen - I may be slightly biased for having had a Polysix meter that was always 1-1.5 stop off compared to any in built meters. I sent it back to the importers twice, to no avail. As luck would have it I dropped this source of frustration and got myself a Minolta V, which is a joy to use. No more differences.

 

[Tom Duffy]

Eric,
Color temp of the ambient light would certainly be an issue for metering for black and white. After first reading about this effect in a magazine article, I performed a test myself. I spot metered the black cover of a stereo speaker (black with some detail) in daylight and tungsten, using a zone VI pentax meter. I bracketed shots on the same roll of film and printed all negs for same enlarger time to produce same black tone on paper. the neg metered in tungsten light required just over a stop of additional exposure to produce the same detail in the black as the daylight metered shot.

Since then, I've tried to add a stop of extra exposure whenever metering in tungsten.

I never tried this with different meters, but now I suppose I ought compare the Zone VI meter with the ttl meter in my 35mm camera...

What's really disturbing is that my Zone VI meter is Japanese, which according to Gossen is already calibrated for 3200 k. Does this mean that the gossen, calibrated for 5600 k, is off by 2 stops in tungsten light?

 

[Jorge]

Hmmm...what I understood is that meters were calibrated to a reflection standard, 18% or 12% whatever the manufacturer wishes. Certainly the color will have an effect on the response of the meter but I think people are confusing meter response to film response. B&W film, with exception of TMX or ACROS have a lower response to tungsten light than to daylight. I beleive in Toms case he was seeing the film response and the meter was accurate measuring the amount of light. This is why is recommended to test film response to tungsten light.

 

[Ed Sukach]

There are a number of considerations in the measurement of light.

Light is a form of radiant energy. In the case of photographic light meters, the spectrum of sensitivity of the meter is important. Ideally, it would correspond with the Spectral Sensitivity Curves of *ALL* film. The response curves would necessarily have to be limited to spsecific spectral areas, and would match the sensitivity levels at those areas.

When we speak of a film - or meter - "balanced" for 5500K, we are describing a spectral curve that approximates (best effort basis) the spectrum and light levels of "Noonday Sunlight at 40 degrees North Latitude - etc.", not an extremely narrow band at only one single frequency - laser-like.

There are lots of compromises: It is extremely difficult to get the meter to have the same sensitivity levels as sunlight in the first place. Films differ widely in their sensitivity curves (example: look at the spectral sensitivity of Infra-red film), and in particular, the light measured, in itself, is more than likely NOT have the same spectral composition as Daylight at 40 degrees.. etc.

Then - added to all the "compromises" we must include - metering technique - Incident or reflected? When we use "incident" to we deliberately point the sphere at the source of light, or the camera itself? How does one "point" a sphere anyway?

Fred Picker of Zone IV (did I get that right?) tried to eliminate some of the compromises by adding filters to Honeywell 1 Degree Spotmeters to respond more closely to the spectral sensitivty of average black and white films. Great, but that probably diminished their accuracy with color films.

In the last analysis, we probably do not deserve to get exposure as close as we do - even with the fancy-shmancy 10,320 point matrix blue-pont sorcerer's dust compensated systems..

I suppose a truly accurate meter could be built. Sensors would analyze the light intensity and specrtrum of average and specific points on the subject: that information would feed and control various (and many) filters and match that information to programmed Film Spectral Response curves which would be further processed to respond to any correction filters on the camera - and any biases caused by lens coating.... and, whoops, nearly forgot... some super-intelligent future Processing Prediction software.

There! Sounds like everrything. Now to make the thing the same size as the average meter and keep the price under a few hundred dollars. And then, SELL it to a bunch of "Digidumbos" whose greatest acheivement so far is figuring out which end of the camera to point at the subject....

 

[Eric Rose]

I was always second guessing my meters. I had a hand held from Zeiss (Euro), a Gossen (Euro), a Pentax Spotmeter (Jap.) and several Nikons also Japanese. The Nikons were very close to the Pentax but the Zeiss and Gossen were not. I always thought the Pentax and Nikons were out and reduced the IE accordingly. The Zeiss and Gossen always gave me dead on chromes, once the usual high velocity fudge factor is adjusted for.

Darn I hate testing, but now I think I might have to. Actually I will be out with some very anal LF types (and I say that in a nice way) this weekend so will compare my meter to theirs and see what exposures we would come up with given an identical subject and effect wanted.

Will it ever end!!