Originally posted by WPRESTO To add just a bit. Two film speed (=light sensitivity) rating systems were in widespread use from the end of WWII to about 1980.
1) The older German DIN system which is a logarithmic scale. Without explaining the technicalities of the math, it was set up so that 3 numerical steps doubled or halved the sensitivity, eg; DIN 21 film requires one f-stop or one shutter speed more exposure for the same density as DIN 24 film. To put it in more modern terms, DIN 21 is one EV step less sensitive than DIN 24.
2) The American ASA system, somewhat later in development, was an arithmetic system. Doubling the numeric value meant one f-stop or one shutter speed or one EV step change in sensitivity. If ASA 200 film required 1/125 second @ f8, then ASA 400 film required 1/250 @ f8 (or 1/125 @ f11) to record the same density.
The current ISO system is closer to the American ASA system than the German DIN system. A digital sensor set to ISO 400 requires EV settings (f-stop + shutter speed) similar to old ASA 400 film for "correct" exposure. This is somewhat appealing psychologically. Both our f-stop values and our shutter speed values are essentially arithmetic (not logarithmic) systems based on doubling and redoubling values, although for f-stops because it's based on a diameter but light transmission changes with area (= the square of 1/2 the diameter), there is an alternating doubling (f1.4 doubles to f2.8, but that is 2 shutter speeds difference, so you need the intermediate f2 for one shutter speed difference, so F1.4 doubles and redoubles to 2.8, 5.6, 11, 22 while f2 doubles and redoubles to 4, 8, 16, etc).
For those of us who used ASA ratings from our first 35mm camera, the ISO values mean something. I know what ISO 800 is like in terms of probable f-stops and shutter speeds*, I know when I would want or not want that ISO sensitivity. DIN 16 or EV 11 are not so instantly meaningful.
*In bright sun, about 1/3000 @ f8. What EV would this be? A high value, but I could not guess.
The Suny day rule say: 1/X, isoX @ f/16 so 1/3000 would be iso 3000 @f/16. With f/8 we gain 2EV so that would be iso750, f/8, 1/3000.
"The basic rule is,
"On a sunny day set aperture to f/16 and shutter speed to the [reciprocal of the] ISO film speed [or ISO setting] for a subject in direct sunlight."" from wikipedia:
https://en.wikipedia.org/wiki/Sunny_16_rule
And there a few other things one could remember for the apperture to use in different conditions.
f/22: Snow/sand
f/16: Sunny
f/11: Slight overcast
f/8: Overcast
[...]
But of course, a lot depend what you mean by bright sun, here this is the "sunny day" setting.
Other than that apperture, iso, shutter speed all evolve the same by a factor, just not the same, that quite regular for a mathematian to have the square root of 2 instead of 2 as a factor for apperture (Approx 1.4). 100% logical and expected