I suppose in Adams times the candle/square foot reading from the light meter was converted into and exposure value through the use of same table, disk, etc. Probably the formula, more than a way to avoid use of the light meter, was a way to avoid the use of a disc calculator, making the calculation mentally.
The stated intent of the formula was that it "facilitates rapid exposure calculation mentally" but still, one must take a reading and that reading gets dropped into a formula.
The problem Adams was trying to avoid was "setting dials on meters".
The meters he was working with (the tools available when the book was written) didn't spit out "proper" camera settings like modern electronic meters.
If you search 'light meter' on ebay you will see a few meters which are not for photographic use but give a light level reading for industrial/business use such as installing office lighting. This formula could be used with one of those meters if it measures in the right units.
The Exposure Formula is associated with the quote "Chance favors the prepared mind," so I take it to mean that knowing the formula and using it can help in a pinch when a meter fails or is unavailable.
Originally Posted by markbarendt
No because these meters return incident candlepower measurement units and the formula is tied to reflected candlepower measurement units.
Originally Posted by Steve Smith
I don't assert that the four stop difference is a constant. But can anybody say? Maybe it is just that.
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.
Ansel's formula requires a reading. No reading, no answer.
Originally Posted by Bill Burk
This isn't a rule of thumb thing like sunny 16, or an exposure table.
If you set the Weston to ISO 100 (or Weston 80) and set it for a reading of 400 it will give the standard sunny 16 combination of f16 and 1/125 so perhaps the Weston is scaled in foot candles.
Originally Posted by Fred Aspen
[QUOTE=markbarendt;1192835]Ansel's formula requires a reading. No reading, no answer.[QUOTE]
You are right. I gave Ansel's formula another reading. He only gave one example where the moon was 250 candela per square foot. Now that is a reasonable constant brightness that could be worth memorizing.
And he reminds that estimating exposures from memory like this and Sunny 16 are emergency measures unlikely to result in a fine print.
So taking meter readings is important.