# Adam's Exposure Formula contracts with the Addative Photographic Exposure System (APEX)

Discussion in 'Darkroom Developing and Printing' started by Steven Woody, Jan 14, 2007.

1. ### Steven WoodyGuest

in Adams's book 'Negative', page 66, he showed a Exposure Formula:

" to use the exposure formula, take the film speed number ( on the ASA
scale ) and determine its approximate square root. This number is
remembered as the key stop for that firm. for example, a film rated as
ASA 125 has a key stop of f/11. at the key stop, the correct shutter
speed in seconds to expose a given luminace on Zone V is the reciprocal
of the luminace expressed in c/ft^2. thus for a sureface that measure
60c/ft^2, we would use a shutter speed of 1/60 second at the key stop."

but if using the APEX ( any one heard that? ), since Av + Tv = Bv +
Sv, where Sv ( 125 ASA ) = 5.4, Bv ( 60c/ft^2 ) = 3.3, Av ( f/11 ) = 7,
so Tv = 5.4 + 3.3 - 7 = 1.7 which is about 1/4 second.

what's the wrong? thanks.

-
woody

Steven Woody, Jan 14, 2007

2. ### JJGuest

It is more clear if you graph it out but it is still unnecessary to go
to lengths to understand it. It is an approach and not the ultimate
method. Adams still dodged, burned and bleached his way to happiness.

Learn to expose for significant shadows, develop for highlights and be
happy.

JJ, Jan 14, 2007

3. ### Gregory BlankGuest

It is more clear if you graph it out but it is still unnecessary to go
to lengths to understand it. It is an approach and not the ultimate
method. Adams still dodged, burned and bleached his way to happiness.

Learn to expose for significant shadows, develop for highlights and be
happy.[/QUOTE]

& use a light meter.

Gregory Blank, Jan 14, 2007
4. ### Steven WoodyGuest

[/QUOTE]

i just wonder, in this exposure, which one is wrong, Adams or APEX ?

-
woody

Steven Woody, Jan 15, 2007
5. ### Peter IrwinGuest

60 candles per square foot is a brightness level of around 7.5

That brings your total to 5.9 or very nearly 1/60th of a second.

I think you looked up an incorrect table for brighness.
In the Focal enclclopedia a bv of 7 is 45 canles/ ft^2
and a Bv of 8 is 90 candles/ft^2.

Peter.

Peter Irwin, Jan 15, 2007
6. ### JJGuest

& use a light meter.[/QUOTE]

Uh, yeah, and a lightmeter.

and World Peace!

JJ, Jan 15, 2007
7. ### Steven WoodyGuest

thank you. after read you post and did a research on the net, i can now
draw a conclusion that,

1, what adam mentioned 60c/ft^2 is a brightness of the scene. 60c/ft^2
brightness equals to a Bv value of 7.56 ( you are right ).

2, what i previously methioned Bv=3.3 for 60/ft^2 is from a table that
actually list incident light values, which should be Iv instead of Bv.

thank you very much!.

-
woody

Steven Woody, Jan 15, 2007