Ansel Adams Exposure Formula

[dave_f2]

<p>So in p.66 of Book II, Ansel Adams outlines his exposure formula:</p>

<p>Square root of your ASA give you the key stop<br>

Take your meter reading in c/ft^2 (foot-candles) and the inverse is the shutter speed at the key stop.</p>

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

<p>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.<br>

Is there a difference between foot candles and foot candles squared? What am i doing wrong?</p>

[ellis_vener_photography]

<p>Foot candles squared = foot candles X foot-candles</p>

<p>so yeah I think there is a difference. </p>

[JDMvW]

<p>Um, have you considered metering the range of zones in the picture with a light meter and using the resultant exposure?</p>

<p>I've heard that many meters nowadays simply read off exposure and even have zone markings. ;)</p><div></div>

[dave_f2]

http://en.m.wikipedia.org/wiki/Foot-candle

 

wikipedia defines foot candle as lumninance over area so it's always square feet

[kevin_kolosky]

<p>If you are talking about a key stop for a sunny day, I have been using this formula for the past 30 years and it has worked well for me.<br>

Just use F16 and the speed of the film<br>

So, for example, You are shooting in a sunlit area with ISO 100 film Set f16 and 1/100 of a second and shoot. A bit hazy - stop up to F11, a bit cloudy stop up to F8. Lots of clouds F5.6. <br>

Works very well. I shot hundreds of weddings without a meter using this method.</p>

[paul_hoyt]

<p>Using your exposure meter, Adams' is telling you that at ISO 100 the seconds that coincides with f/10 is the reflectance value in c/ft^2; 1/125 sec is 125 c/f^2. If the manufacturer of the meter has a K-factor then the ISO must be adjusted; the value is not correct. He had calibrated meters at his workshops and would test his against the students to adjust out the K-factor for his students. If you have a calibrated foot-candle meter available, you could calibrate your meter accordingly. The exposure formula drives the calibration of the meter, not visa versa. How do you know the reflectance value is 100 c/ft^2? <br>

Also, we are talking about reflectance values not incident readings.</p>

<p>Paul</p>

[profhlynnjones]

<p>As JDM said, consider the range of tones, meter the brightest area that you wish to have barely perceptibel detail, then measure the darkest tone that you wish to have barely perceptible detail, the difference should be between 3 and 4 stops, if not, exposure/development must be altered. This corrected exposue will probably be somewhat different from a grey card/incident exposure, this is the very nature of landscape photography. <br />If zone system actually worked, there would not be 2,000 to 3,000 different books and pamphlets written trying to explain (justify) it. It is basically unsound technology.<br>

F stops should be based on depth of field and point of focus, not square roots of anything. <br>

Lynn</p>

[paul_hoyt]

<p>Sorry Lynn, The Photographer <strong>works</strong>, the Zone System is just a tool. It is a practical way to evaluate the scene and make sure you have enough exposure to render detail in the shadows and decide how much development for the highlights. Adams was not the first to propose a system for exposure and development, he was just prolific at advancing a system through his technical manuals and workshops.</p>

<p>Paul</p>

[dave_f2]

<p>The K factor would not explain being 5-6 stops off. Please read the actual exposure formula in Ansel Adam's book. It has nothing to do with Zone system as it is just a formula to get to middle grey exposure. What tones you want to put into middle grey is completely up to the photographer.</p>

[dave_f2]

<p>Paul, regardless of talking about incident or reflectance, the formula should stand, no? Aside from the K factor, what is the difference between taking an incident reading and a reflectance reading off 18% grey? Furthermore, the Sekonic 785C doesn't read FC in reflectance.</p>

[paul_hoyt]

<p>The formula E = (I)(T) is true regardless of the type of meter. But an incident meter only reads the light falling ON the subject; it does not lend itself to measuring the light reflected from the various objects in the scene and adjusting the exposure accordingly. My question still stands: you claim in your post you are measuring 100 FC of reflected light, how do you know it is 100 FC? According to your meter, the reflected light is 8 FC; ISO 100, F/10, 1/8 sec. </p>

[bebu_lamar]

<p>The Sekonic readout in FC is really lumen/ft^2 and not Cd/ft^2 which is a luminance (reflected light) reading. If you use the Sekonic and readout in Cd/m^2 and divide the reading by 10.7 then you have the value of Cd/ft^2. It's still about 1/3 to 1/2 stop off. </p>

[bebu_lamar]

<p>Ansel criticized about the use of K factor in fact his formula using a K factor of 10.76.</p>