Does anyone have any actual, hands-on experience with the Gossen Luna Pro Fiber Optics Probe accessory attachment? This is the device used to measure light directly at the film plane, by metering from the ground glass surface?

Questions: How accurate and sensitive are the readings taken from the ground glass, compared to spotmetering the scene? Would one measure light with the lens wide open, or stopped down to the taking aperture? If measuring light with the lens wide open, would one have to compensate/calibrate for individual differences in maximum aperture for each lens used (for example: f/5.6 vs. f/8)?

Additionally: Would the exposure compensations, necessary either with filters or close-up bellows extensions, be more precise, as opposed to applying filter or bellows extension factors? Does the device have to be used under a darkcloth, to avoid metering errors from extraneous light sources, when measuring light at the ground glass? Would the use of a fresnel lens, or a Beattie screen, actually increase the measured light at the ground glass surface, and require metering compensation?

Any thoughts would be appreciated. Sergio.

-- Sergio Ortega (s.ortega@worldnet.att.net), April 27, 1998

Answers

Metering in this method would take into consideration all bellows and filter corrections (its now TTL). AS far as the calibration of the unit, I dont know. I would imagine the mfr. could give you an idea on how its used.

-- Ron Shaw (shaw9@llnl.gov), April 27, 1998.