I recently bought a new Gossen Luna Pro S to back up my Sekonic 608. Big mistake.

I have been testing the two meters in incident mode, thinking both are "state of the art" meters, and both would be somewhat close in readings.

Rule #1 in photography - Assume NOTHING!

The readings were consistently off, with the Gossen reading .5 - 1.5 stops higher, at color temp ranging from 2500K to 9500K. I would have been happy with the readings having consistent differences, but this is surely NOT the case.

At first I thought one of the meters was defective. I assumed the Gossen since I had good exposures using the Sekonic for a few years. (but of course not as perfect as I would like) But, after speaking to Bogen (USA importer of Gossen meters) I made a new, but of course dissapointing discovery.....

There are no International standards on meter calibration, therefore variances are the norm. Gossen and most European photographic manufactures, calibrate their meters at 5600 degrees Kelvin, i.e. "Daylight" setting. Most Japanese manufacturers calibrate at 3400 degrees Kelvin, a "Tungsten" setting. This 2000 degree Kelvin difference in color temperature causes a difference in readings.

From my test results, the readings are very sensitive to color temp. It's my guess they are both accurate at the color temperture they were calibrated to. But outside that specific color temperture, it seems the readings are not as accurate. Does anyone have any have further insight into this?

Armed with this new information, it's not quite so easy to calibrate film to a given meter, as a single calibration test will only calibrate the meter / film when shooting under the same color temperture light which the calibration test was performed at. So, if you shoot under different lighting color temp lighting (such as in Landscape Photography), then ideally, you should calibrate meter / film under a range of color temp lighting.

To be fully prepared for field photoraphy, it seems you must know the corrections for the light meter under all color temps that can be expected. When in the field, you start with a color temp meter rerading, then take light meter reading. Adjust light meter reading according to a "cheat sheet" you carry. Does anyone have a better suggestion?

For MF and 4x5, I just bracket, but I plan to shoot a lot of 8x10, whereas brackets can get quite costly. Of course using Negative film vs. color chrome film will add some additional leeway for the calibration issue, but I much prefer the chrome film for scanning. TYIA