David Rosen (golem@Capital.Net) wrote:
[...]

: : > That f1.8 is basicly in the middle [between 1.4 and 2.0].


: Very nice. I was the one who stated it
: originally to be a mere 1/3 stop, just
: because it is common knowlwdge, even tho
: I can square my roots with any crowd.

: F:1.7 is the std, recognized 1/2 stop.
[...]

Greetings:

In case anyone cares, here's a table of exact (to three digits)
fractional f/stop increments between 1.4[14] and 2.000:

1/2 stops increments: 1.414 .. 1.682 .. 2.000
1/3 stops increments: 1.414 .. 1.587 .. 1.782 .. 2.000
1/4 stops increments: 1.414 .. 1.542 .. 1.682 .. 1.834 .. 2.000

(Similarly, it can be determined that an f/3.5 lens is 2/3
stops slower than an f/2.8 lens, while an f/2.5 lens is 1/3
stop faster...)

Thus, a lens marked "xx mm f/1.8" could be either 1/4 stop or
1/3 stop faster than a true 1/2.0 lens -- since both 1.782
and 1.834 would be rounded to 1.8 -- but not a full 1/2 stop
faster...

By the way, how do we know that the 85mm f/1.8 AF-Nikkor is 1/3
stop faster rather than "merely" 1/4 stop faster (than a 2.0
lens)? Given the above numbers, either possibility is about
equally plausible.

Sincerely,

Michael Loretan
guest@milton.econ.virginia.edu