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• 06-07-2011, 07:19 AM
davidave
So in p.66 of Book II, Ansel Adams outlines his exposure formula:

Square root of your ASA give you the key stop
Take your meter reading in c/ft^2 (foot-candles) and the inverse is the shutter speed at the key stop.

So if shooting ASA100, the key stop would be f/10. And if luminance value is 100FC, then the exposure is 1/100.

I'm trying to get this to work with my Sekonic 758C. I'm not coming in close. At f/10 and 100FC with ASA 100, my exposure reading is 1/8 second. Even taking into account a K factor, this is orders of magnitudes off.
What am i doing wrong?
• 06-07-2011, 07:24 AM
Mustafa Umut Sarac
I have no idea but may be he writes in metric terms ? Did you check for it ?
• 06-07-2011, 07:30 AM
davidave
Yes. Definitely foot candles, not meter candles.
• 06-07-2011, 07:39 AM
MaximusM3
Just out of curiosity, as I have no idea of how this formula work, what would be the advantage of this versus taking a simple incident meter reading? Of course, it sounds like it could be valuable if one doesn't happen to have a meter and, like Ansel, be able to guesstimate luminance values.
• 06-07-2011, 08:12 AM
davidave
I find it useful to be able to work in a luminance value as it is easier to figure out contrasts then go think about exposure after that. And you would still need a meter for a luminance reading.
• 06-07-2011, 09:33 AM
Christopher Walrath
Slight increase of aperture, slight decrease in duration. 1/125. E=i(t)
• 06-07-2011, 09:34 AM
Usagi
Quote:

Originally Posted by davidave
I find it useful to be able to work in a luminance value as it is easier to figure out contrasts then go think about exposure after that. And you would still need a meter for a luminance reading.

Do you mean EV values as luminance value?
EV is easy - but c/ft^2, sounds complicated. Does any modern exposure meter even give option for unit like c/ft^2?

I wonder what is it's standard (metric) equivalent. The thing that always bugged me when reading Adams' books - the use of units that are not used in science.
• 06-07-2011, 09:35 AM
Diapositivo
I presume at the time of Adams there were light meters around which gave you only the candles per square foot values, and you had to convert them in photographic values. It's the same measurement, if I get it right, expressed with a different unit of measure.

My Minolta spot meter F comes with a conversion table between EV at ISO 100, candles per square meter, and foot-lamberts. E.g. EV 14 is equivalent to 2300 cd/m^2 or 670 fL.

If you have to use a light meter, and you use it for photography purposes, then it's better to use one which gives you straight EV values, or maybe exposure "couples". I see no advantage in having a measure in another unit and then converting it to your purposes.
• 06-07-2011, 09:36 AM
davidave
@Chris,

I'm not getting anything close to that. Try metering something that is 100FC at f/11 at 100ASA, not anywhere near 1/125.
• 06-07-2011, 09:36 AM
Christopher Walrath
Addendum. That will be a slight increase in exposure, of course. But that can be accounted for in printing.
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