isn't 1.2 only half a stop faster than 1.4?
Actually, it's one third of a stop faster.
, it's both, or if you prefer, neither. On the half-stop scale, f/1.2 is 1/2-stop wider than f/1.4, but on the 1/3 stop scale, f/1.2 is 1/3-stop wider than f/1.4. See the wikipedia page on f/stops
Mathematically, f/1.2 really is closest to a 1/2-stop, since 21/2
Ã—0.5 = 1.1892), whereas 22/3
Ã—0.5 = 1.2599, which personally I'd round to f/1.3.
Of course, while the difference is meaningful in terms of depth of field, and in terms of the total amount of light reaching the sensor, in terms of the actual amount of light being recorded
by the sensor, it's essentially a moot point. The only difference between f/1.2 and f/1.4 is really how much of a stealth ISO boost
the camera applies to compensate for the fact that at apertures that wide, the additional light is at an angle too oblique for the sensor to detect (i.e. at f/1.2, the camera is adding 1/3 to over 1/2 of a stop to the ISO speed, but not reporting that gain in the EXIF).