Maybe a daft idea.

[Ed Bray]

Has/Does anyone use/d their spotmeter to take a reading from the ground glass to compensate for a change in lens, aperture or change for bellows factor.

Is this a daft idea? is so why?

[IanG]

I've used a Spotmeter to make comparisons of focus screens and fresnels combinations but you always get some light relfecting off the screen itself.

Ideally you'd need something like a Sinar metering back with it's probe.

Ian

[Ed Bray]

I tried it out with the meter pressed against the ground glass to prevent extraneous light affecting the result. lens focussed on infinity.

When using my Symmar-S 240mm I got a reading of 10.1EV f5.6, ISO100 from a bright area on the ground glass, when I closed down the aperture by full stops I got readings as follows:

f8 - EV9.4
f11 - EV8.5
f16 - EV7.6
f22 - EV6.6
f32 - EV5.6
f45 - EV4.7

All readings from the same spot, so apart from the initial reading which is only 0.7 of a stop (makes me believe that the stated aperture of f5.6 is more likely f6.3) all other readings are within +/-0.1EV.

Then with my Symmar-S 150mm f5.6 I got the following results:

f5.6 - EV9.3
f8 - EV8.6
f11 - EV7.5
f16 - EV6.6
f22 - EV5.5
f32 - EV4.7
f45 - EV3.6
fully closed (f64?) - EV2.5

So again pretty close to 1EV steps +/- 0.1EV apart from 0.2EV between f22 & f32 and the 0.7EV between wide open and f8, again leads me to suspect fully open aperture more f6.3 than f5.6.

Next I tried measuring the bellows factor using the 150mm at f8 focused to both infinity and focused to 4ft, then 2ft, 18", 12" and 9" (the shortest I can focus with the 150mm on my Horseman LX).

Distance...........Bellows Length............EV
inf......................... 6"......................9.0
48".........................7"......................8.7
24".........................8"......................8.4
18"........................10".....................8.0
12"........................14".....................7.2
9"..........................22".....................6.0

So from my bellows measurements 48" to 24" gives a 7"- 8" bellows movement which equates to 1/3 stop measurement from meter gives EV 8.7 - 8.4 = 0.3 stop
24" to 18" gives an 8" - 10" bellows movement which equates to 0.6 stops measured by meter EV 8.4 - 8.0 gives 0.4 stops
18" to 12" gives a 10" - 14" bellows movement which equates to 1.0 stops measured by meter EV 8.0 - 7.2 gives 0.8 stops
12" to 9" gives a 14" - 22" bellows movement which equates to 1.3 stops, measured by meter EV 7.2 - 6.0 gives 1.2 stops

18" to 9" gives a 10" to 22" bellows movement which equates to 2.3 stops, measured by meter EV 8.0 to 6.0 gives 2 stops
24" to 9" gives a 8" - 22" bellows movement which equates to 3.0 full stops, measured by meter EV 8.4 - 6.0 gives only 2.4 stops

I must have too much time on my hands!

[Gary Beasley]

My minolta flash meter has a probe option for use on ground glass. I never messed with it because it has to be calibrated to the camera before you can get any useful information from it.

[Old-N-Feeble]

You can get a probe for a Luna Pro SBC too. I had one many years ago but never used it much. You not only must compensate for light loss at the center of your GG but, with a Fresnel and WA lenses, that can change from center to edge. Close to center it works fine with a simple adjustment of the meter provided you stay away from markings such as grids and lines. Also, some GG have a center spot that may read differently. The Sinar metering back doesn't have any of these issues but it may have others... I don't know. Lastly, if you want to read through a CF you'll have to close down to your shooting aperture and that'll be darned hard to see even just for metering plus stray light will become a bigger factor.

[Ed Bray]

I tried it at all apertures all the way down to a marked f45 and beyond, I didn't need to see the image on the screen, just keep the meter in the same place. No extraneous light was measured.

On thinking about it, it was actually quicker just to measure the bellows extension and use the difference in the bellows length to give me my exposure.