Just happened to be playing with this too. I took my Weston Master III outside with the Invercone installed and took an incident measurement. Here in UT sunny 16 is really sunny 16.66 and the Weston measured 350. So, with an ISO of 400, the square root is 20 (which translates into f16.66 conveniently enough!) and the shutter speed would be 1/350. Normalized, that would be f16 at about 1/600 shutter speed, which is absolutely right on the money! So, the magic number in UT is 350 on the Weston III. I also have a foot candle illuminometer which tells me that a bright sunny day is around 12-13K foot candles, depending on atmospheric conditions.
I don't know what the units are on the Weston (I have heard it is foot candles and my Sekonic 398A has a scale in foot candles) but both the Weston and the 398A say a bright sunny day in UT will be very close to 350 'units' (actually the Sekonic measured about 400). I am sure AA was using a Weston to derive his formula and it works perfectly on my Sekonic and Weston.
The Sekonic is dead on (at 400) and I think that the Weston (mfd. in the 50s) may be slightly tired. But, of course, we don't know if the units on the Weston are indeed the same units on the Sekonic, similar, yes.
PS. I have gone to the Sekonic website and downloaded their EV/Lux/FC conversion chart and was able to correlate EV numbers to their FC numbers on my Sekonic 398A and the Sekonic Illuminometer.
Last edited by Fred Aspen; 06-08-2011 at 01:02 PM. Click to view previous post history.