# Light meter

2 replies to this topic

### #1 Tom Hepburn

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Posted 29 September 2008 - 08:32 PM

I hope no one minds me posting on this forum as opposed the lighting forum.

I have an old analog Sekonic light meter. I?ve already called Sekonic and they said they would call back and didn?t. Anyway, I?ve been using a Kodak exposure disk to do set my fstop. And to do that, I need the reading if foot candles. No problems so far as I can get those on the face of my meter as long as the high slide is NOT in. However when I put in my high slide on a bright day I lose the 1:1 ratio of needle=footcandles.

So in other words using the high slide renders the footcandle display useless. Can someone tell me what the footcandle display needs to be converted to if the high slide in? This could all be obvious, but if so I can't figure it out.

Here is a scan of the light meter face.

Face

Thanks,
Tom
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### #2 Hal Smith

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Posted 29 September 2008 - 10:00 PM

I hope no one minds me posting on this forum as opposed the lighting forum.

So in other words using the high slide renders the footcandle display useless. Can someone tell me what the footcandle display needs to be converted to if the high slide in? This could all be obvious, but if so I can't figure it out.
Thanks,
Tom

Here's an educated guess. There is a ratio of exactly 5 stops (20 to 640) between the "L" and the "H" arrows. 5 stops is a ratio of 2^5 = 32 so I'm guessing putting the high slide in multiplies the displayed footcandles by a factor of 32...IE: if the meter reads 10FC it's actually 320FC.

You could directly measure the difference between the two slides by putting in the high slide, creating a fairly bright source, something like some diffusion in a good sized frame in front of a 1K fresnel, seeing what the high slide reads, and then knocking the fresnel down with successive layers of ND3 gel. If I'm right about the ratio of 32, exactly 5 layers of ND3 will bring the low slide reading to match the high slide's.

But even if I'm wrong there's some stack of ND that will bring the low readiing equal to the high. ND numerical ratio equals 10^(ND#/10) where ND# is the number alongside ND, like 3 for ND3, 6 for ND6, etc. for the sum of the entire stack of ND. For instance three layers of ND3 equals ND9, the numerical ratio is 10^(9/10) = 8 (your calculator will actually give you an answer of 7.943, see below for reason why).

For the ULTIMATE GEEK...the 3 in ND3, 6 in ND6...is actually 3.010, 6.020, etc. if you want to get exact answers in the above formulas.
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### #3 Chris Keth

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Posted 29 September 2008 - 11:53 PM

The problem with trying to use that meter without a high slide is that in many light conditions, the needle will max out and the reading will be useless.
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