I recently acquired a fiber optic probe for my Gossen Luna-Pro F light meter. I see little black specks in each end of the probe that's not dirt. Is this cause for concern ? The readings off the ground glass are not the same as incident reading by the lens. Obviously I also need to locate some instructions. Any help is immensly appreciated.

Best regards,

Greg

-- Greg Riutzel (riutzelg@peak.org), March 16, 2002

Answers

You need to calibrate the meter to work with the probe so that it gives you the proper reading.

I have the Booster II for the Minolta meters, and the method of calibration (which should be similar for your probe) involves taking a reflective reading of a uniformly colored surface like a grey card, then taking a ground glass reading with the lens focused at infinity and wide open, then adjusting an adjustment dial on the Booster so that the groundglass reading matches the normal reflective reading. You should do this for each lens, and you need to do it again if you change your groundglass.

-- David A. Goldfarb (dgoldfarb@barnard.edu), March 16, 2002.

Greg,

Although I can't help with the instructions or the difference in readings, the black specks are probably individual fibers that are broken. No light is being transmitted from one end to the other via the broken fiber, hence, the black specks. A small number of broken fibers won't affect the reading. Even with a large number of broken fibers, you could apply a correction factor if they're randomly scattered through the fiber bundle. Good luck!

-- Ted Brownlee (omfbh@aol.com), March 17, 2002.

Hello!

I have one of these fibre optics for my Gossen Profisix. While it may be so that the black specks are indeed broken fibres, I don't think that one or two should hinder you from getting a good reading. As for the incident metering vs your metering off the groundglass, they are really two very different types of metering and should by all means give you different readings.

The fibre optic is firstly doing the metering through the lens (taking into account any corrections when doing close-ups for example), and secondly it does its metering on a very tiny spot, whereas the incident meters on a very broad spectrum. In any case, I think that you probably will get a more accurate metering, broken fibres or not, with the fibre optic.

The fibre optic is also usable as a density meter for negatives, and of course it works well as a spotmeter for tiny objects or areas.

Try www.craigcamera.com for a manual, and if you really want know the definitive answer about the broken fibres, then try http://www.gossen- photo.de/uk/index.htm

Good luck with the metering!

-- Jimi Axelsson (jimi@earthling.net), March 17, 2002.