I guess my math doesn't agree with yours. I come up with 1/3 stops of
1.000
1.095
1.225
1.414
1.549
1.732
2.000
I didn't use meters or tables. After all, they have things like a
progression of shutter speeds of ...1/30, 1/60, 1/125, 1/250, which
don't follow the correct math sequence. I used simple math to convert
changes in area of the front element to diameter and then to focal ratio
(f-stop), i.e., a decrease of 1 stop is a decrease of the area of the FE
by 1/2, a decrease of 1/2 stop is a decrease of the area of the FE by
1/4, etc. I know that this doesn't mean anything in practice, but it
does in theory.
As 1.8 is closer to and slightly larger than 1.789, I conclude that it
1/4 stop faster than 2.0, not 1/3. Lama, that's the reason I used 1/4
stops in the first place, all the apertures I was illustrating fell on
1/2 or 1/4 stops increments except f1.2, which is partway between the 2
sets of increments.
Moose
John A. Lind wrote:
That's definitely on the right track, but not quite completely
correct. Nearly all interim f-stops are on 1/3 stop increments, not
1/4 stop increments. f/1.2 is one of the exceptions . . . 1/2-stop
faster than f/1.4.
Some common lens speeds at other than full stops:
f/1.2: 1/2-stop faster than f/1.4
f/1.7: 1/2-stop faster than f/2
f/1.8: 1/3-stop faster than f/2
f/3.5: 1/3-stop faster than f/4
f/4.5: 1/3-stop slower than f/4
f/6.3: 1/3-stop slower than f/5.6
These all taken from my Weston meters (1/3-stop: Master V; 1/2-stop:
Ranger 9) and confirmed by my math regarding 1/3-stop and 1/2-stop
increments. In general, f-stops and film speeds run in 1/3-stop
increments.
-- John
At 16:43 3/31/03, Moose wrote:
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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