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 dir ectly at the film plane, by metering from the ground glass surface?
Questions: How accurate and sensitive are the readings taken from the ground gl ass, 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 th e lens wide open, would one have to compensate/calibrate for individual differe nces in maximum aperture for each lens used (for example: f/5.6 vs. f/8)?
Additionally: Would the exposure compensations, necessary either with filters o r 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, actua lly increase the measured light at the ground glass surface, and require meterin g compensation?
Any thoughts would be appreciated. Sergio.