Originally posted by Just1MoreDave It's misleading or wrong to subtract 1.2 from 1.4 and turn the decimal into a fraction of a stop. The scale doesn't work that way. The difference between f1.2 and f1.4 (which I'm writing incorrectly but won't change

) is a half-stop, 0.5 Ev.

Allow me to bloviate on this a little.

The f/stop scale for full-stops goes like this:

f/1.0

f/1.4

f/2.0

f/2.8

f/4.0

f/5.6

f/8.0

f/11

f/16

f/22 etc

The half-stops (at the fast end) go like this:

f/1.0

f/1.2

f/1.4

f/1.7

f/2.0

f/2.4

f/2.8

f/3.4

f/4.0

f/4.8

f/5.6 etc

The difference between f/1.2 and f/1.4 is the same as between f/4.0 and f/4.8, which is 0.5 EV aka 1/2-stop. Those jumps look weird because they're based on the square root of 2, about 1.4. Each full stop lets in half or twice as much light through a roughly circular iris. Doubling the area of the iris circle doesn't double its diameter; it increases by... the square root of 2!

Note: It's common for lensmakers to fudge their ratios a little. We see lenses marked f/1.7 or f/1.8, or f/1.9 or f/2.0, and we see many at f/3.5, which is 2/3 stops from f/2.8. I read that the famous FA50/1.4 is actually more like 52/1.5. Somehow, marking a lens at f/1.9 instead of f/2, or f/3.9 instead of f/4, makes it seem faster. Just like 99 cents is way less than a buck, eh?

Some makers used different numbers; I have an old USA-made Argus Cintar 50/3.5 that's marked f/3.5--4.5--6.3--9--12.7--18 (and I'll be selling it soon, if anyone want a collector's item). But with our dSLRs we can step through the square-root-of-2 scale in 1/2-stop or 1/3-stop intervals, and thus pick almost any aperture we wish. Such power! Hold me back!