Well, that's not quite correct, but close, 1.0 -> 1.2 -> 1.4 -> 1.6 ->
2.0 is more like it. Considering all the variables, f1.8 and f2 are
essentially the same speed in use. I suspect some clever fellow with a
marketing bent realized many years ago that an upcoming f2.0 design
could become f1.8 at almost no cost and one up the competition. Then, of
course, everybody had to have an f1.8 standard lens :-)
Do remember that f-stops are simply mechanical ratios of the clear front
element diameter and the focal length. Actual f-stop will vary with
normal sample variations in focal length, etc. Actual light transmission
varies with individual lens and coating design.
The simple math is that 1/4 stop increments (to more decimal places than
have any possible practical meaning) from f1 to f2 are
1.000
1.069
1.155
1.265
1.414 (sqrt of 2)
1.512
1.633
1.789
2.000
As you can see, 1.8 is 1/4 stop faster than 2.0 and 1.4 is 1 stop faster
than 2.0. 1.2 doesn't fall directly on either 1/4 stop or 1/3 stop, but
is roughly 1/2 stop faster than 1.4.
Moose
Andrew Dacey wrote:
I could be wrong here, but I believe that 1.0 -> 1.2 -> 1.4 -> 1.8 -> 2.0 are
all half-stop increments. So the 1.4 is 1/2 stop faster than the 1.8 and the 1.2 is a
further 1/2 stop faster still.
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