Exposure Value (EV) System
(aka APEX: Additive System of Photographic Exposure)
In the 1950s there was a move to simplify exposure calculations, particularly for the masses of people who did not want to memorize the ƒ/stop - shutter speed progressions and combinations, and whose eyes crossed when looking at a meter dial with its jumble of numbers and hash marks. Really, it wasn't a bad idea.
It never caught on, but there is evidence of its implemention on meters and cameras, and I do still see variations of it mentioned here and there on the net.
There are three parts to this.
The shutter speed and aperture dials would be calibrated with a series of numbers in addition (or instead of, but not likely) to their normal markings. The numbers would be:
Any combination of the two numbers that add up the same are equivalent exposures. So let's say you meter the scene and it reads EV 10. Any of these combinations give you EV 10:
|10 + 0 (1 sec @ f32)|
|1 + 9 (1/2 sec @ f22)|
|2 + 8 (1/4 sec @ f16)|
|3 + 7 (1/8 sec @ f11)|
|4 + 6 (1/15 sec @ f8)|
|5 + 5 (1/30 sec @ f5.6)|
|6 + 4 (1/60 sec @ f4)|
|7 + 3 (1/125 sec @ f2.8)|
|8 + 2 (1/250 sec @ f2)|
|9 + 1 (1/500 sec @ f1.4)|
|10 + 0 (1/1000 sec @ f1)|
The nice thing about this is that it's easy to find equivalent speed/aperture pairs without having to memorize the ƒ/stop-shutter speed progression. It's intuitive, though it's not helpful for understanding depth-of-field or motion-stopping.
In order to get the EV, the photographer either needs a meter that can read out (or calculate) EV, or a chart that describes various light levels and what the EV probably is. (Example: "Light sand or snow in full sun with distinct shadows: EV 16. Typical scene in full or hazy-bright sun with distinct shadows: EV 15. Typical scene in hazy sun with indistinct shadows: EV 14". etc.) Wikipedea has a typical EV description chart.
This is where things get a little hairy. From the meter side of things, you need the amount of light and the film speed to figure out the EV. We'll tackle the film speed part of things next, but right now we'll talk about the meter, which gives us the Light Value.
The LV value on the meter can be converted into (or from) footcandles.
Some meters read directly on the LV scale, particularly German ones from the 1950s, where this whole thing was being hatched. Since most meters had a calculator dial however, the dial could factor in the film speed and give you the EV directly my Sixtry and my Luna Pro both do this).
If you have an older meter that reads out in footcandles? You can convert it to LV and then set the camera.
III. Film Speed
This is the last part of the system, and it's the least implemented. The idea was to have another additive number for the film speed, which film manufacturers like Kodak and Agfa and Fuji would publish along with the ASA/DIN/ISO speeds. Maybe they did it at one time, but they don't do it now. You can, however, look up the number on the chart and mark it yourself.
Putting It All Together: The SLAT Rule
So here is where all this ties up into a nice bow. SLAT is a mnemoic for the equation:
S + L = A + T
where S = film speed, L = light value, A = aperture and T = time (shutter speed)
So for example, if your film speed is 6 and the meter says Light Value 7, that equals EV 13. You set the camera controls to anything that equals 13 and you're golden.
What if you're shooting into backlight? Bump the EV up one or two numbers depending on how severe you think it is. What if your subject is particularly dark? Drop the EV number down one or two.