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The Exposure Formula

Ansel Adams developed this method about 80 years ago. It’s utter simplicity to use once you understand it. Later in life, Adams explained it well. It’s simpler and faster than any exposure measurement system. It literally takes seconds to calculate an exposure.


We assume here that you’ve tested for film speed (low values) and Zone VIII or IX (high values). So - at the minimum time to print black through the film base plus fog: (1) Zone I is a tone barely lighter than maximum black – and Zone III has some discernable detail; and (2) Zone VIII prints as a light gray barely darker than the white paper base. Good?


(Read this through – the loose ends are picked up later, for clarity.)


Film speed and key f-stop.


Your key stop is the f-stop corresponding to the square root of the film speed.


Huh? Square root? It simply means “what-times-what equals the approximate film speed you use.” That’s it. My speed for Foma 400 sheet film is 200, so my key stop is 16. (16 x 16 = approximately 200). If your film speed is 100, your key stop is 11. For 50, it’s 5.6 – that’s close enough. If it’s in between, use the lower stop. These are very small differences.[1]


Candles per square feet and shutter speeds


Don’t be put off by candles per square foot – c/ft2. It’s a known standard for illumination. For example, OSHA uses footcandles to determine safe illumination levels for industry. The scale is geometric, like shutter speeds (f-stops are logarithmic – you don’t need to know that).


Anyway, once you calibrate a meter, you can instantly calculate exposure using this system. The old Weston meters read illumination in c/ft2, so you didn’t need to interpolate anything. The c/ft2 reading gave you a shutter speed which yielded a Zone V print value, when used with the key stop.


Example – meadow grass in shade measures 25 c/ft2. A shutter speed of 1/25 (1/30) at the key stop gives a Zone V print value. Dark tree bark in shade reads 5 c/ft2. You want it on Zone III. For me, 1/5 (1/4) at f16 gives me Zone V; and Zone III is 2 stops less exposure, so 1/25[2] (1/30) at f 16 gives me Zone III. Easy. Another – white painted clapboards in sun read 2000 c/ft2. A shutter speed of 1/2000 at f16 gives me Zone V, so 1/50 (1/250) at f32 gives me Zone VIII.[3] Easy. No rotating dial numbers or zone meter scales.


Other than speed, the most important element of this formula is that it teaches you to understand light. After a while, you’ll look at a glaring snow field and know it’s 2000 or 4000 c/ft2. You’ll also know that the same snow in shade is always 2 stops less. When I’m setting up my tripod, I’ll glance over at a tree or a forest floor and think, ‘I’ll bet that’s 2 or 5 c/ft2’ (less in winter – Rochester’s a pretty dark place light-wise).


For the famous Moonrise picture, the light was fading fast, and Adams couldn’t find his meter. But he knew the c/ft2 value of the moon at dusk. So he based his exposure on that. He’d have missed the picture otherwise.


Interpolation with ‘modern’ light meters


The Exposure Formula gets jumbled in your head because meters don’t read in candles per square foot. They read in EV numbers, which are arbitrary. So you need to take another step – convert the EV number to c/ft2. It’s not all that tough.


On the Gossen SBC and Minolta spot meters I use, at ISO 200, EV 10 will give me 10 c/ft2. So 11 is 25(1/30), 9 is 5(1/4) and so forth. Draw a comparison scale on an index card until you have them memorized.


Do enough work, and apply this formula, and after a while you won’t need a meter in familiar situations.



[1] Do this – take a calculator, tap in your film speed and hit the square root key – that’s your key stop. [2] Older view camera shutters progress in a slightly different manner than modern ones: ½, 1/5, 1/10, 1/25, 1/50, etc. Modern shutters have 1/15, older ones don’t. Just eliminate 1/15 – the differences are so tiny, you’ll never see it. [3] Or its equivalent – 1/25 (1/30) at f 32, or 1/10 (1/8) sec at 45, etc.

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