Fibre optic probe

[Greg Riutzel]

I just bought a used Gossen fibre optic probe for use with a Luna Pro F.(analog) I wanted to use it for ground glass measurements. The readings with it are wa y under by 3 or 4 stops the other,(incident or reflective), readings. It is the same whether studio or outdoor. It came with the box, but no intructions. Loo king for some ideas, especially those with field experience. Gossen's website i s no help, it only has referencs to sales and reps. I got it through a private sale. The readings are taken with apertures wide open, bellows usually adjusted around 10' to infinity. I would gratefully appreciate any replies.

Thanks, Greg

[Per Volquartz]

If the meter readings are consistant you are OK.

Just change the ASA setting on your Gossen + probe meter. If you use Tri-X at your reg. meter setting at f. inst. asa 160 your new asa may be 1280...As long as you understand that the film did not change, but your metering did. All you are doing is fooling the meter to do what you want it to do. If the readings are not consistant you have gotten yourself a piece of German junk...

So, start by changing the ASA settings on your new meter+probe and compare...

Or perhaps you could try the US distributor for instructions...

[Per Volquartz]

Slight change in the math:

From asa 160 to asa 20 - not from asa 160 to asa 1280...gulp...

[Per Volquartz]

Long Monday...sorry...

[Per Volquartz]

In other words:

ASA 160 If your regular meter reading is f.11 at 1/125 and your meter+plus probe reading says f.32 at 1/125 you can change your ASA setting on your meter+probe to ASA 20. The meter+probe reading should now say f.11 at 1/125.

[Struan Gray]

I have a fibre probe for my Calculight XP and it behaves the same way. The probe itself loses light compared to a spot or reflective meter, mostly because there is no lens to couple light into the start of the fibre bundle. Then the ground glass loses some more, so you can indeed end up three to four stops down from a direct reading.

The difference is constant, so as others have said, you just have to apply a constant correction, either by re-setting the ASA on the meter, or by adding the relevant number of stops in your head.

If your ground glass doesn't have a fresnel you may find the meter reading uneven as you move from the centre to the corners. I.e. the meter sees the same 'hot spot' as your eyes do, and unless the fibre probe has a way to angle the end, your meter reading will change with position on the ground glass, and by more than the usual cos^4 that the film sees. Again, you can calibrate once by pointing the lens at a wide diffuse source like a lightbox, and then apply corrections in use.

Quantum Instruments have the manual for their fibre probe online. Otherwise, www.craigcamera.com or www.oldtimercameras.com are the most comprehensive sources for manuals in the US/UK respectively.