Winsor,
I'm sure you know the basic scale, f1, f1.4, f2, f2.8, f4, f5.6, f8, f11,
f16, f22, f32, f45, f64, f90. To determine the difference between odd f
numbers simply subtract the smaller number from the larger number between
two adjacent f stop values. The difference determines the number of steps
between the two. Now simply calculate the percentage the odd number is of
the total steps and you have it. For instance, f1.7 . Subtract 1.4 from 2
and you get six steps. F1.7 is three steps away from both 1.4 and 2. That
means f1.7 gives one half stop less light than f2 and one half stop more
light than f1.4 . f1.8 would calculate out to two thirds stop more than f1.4
than and one third stop less than f2. The difference between f4 and f5.6 is
sixteen steps, so f5 is ten steps greater than f4 or approximately two
thirds of a stop greater, and one third stop less than f5.6 . Works for all
the f stop values. Just a quick math problem in your head. By the way,
Olympus is not the only manufacturer to assign odd f stop values. Almost all
of them do. Lets them advertise a faster lens.
Regards,
John Austin
Oakland, CA
Winsor Crosby wrote:
>I have a question for you knowledgeable people on this list. Olympus
>probably takes the prize for assigning odd FStop numbers to the widest
>aperture on their lenses. I am never quite sure, since FStops are not
>linear or arithmetic, how much more light I am letting in wide open than
>the nearest true FStop. What is the relationship between F1.2 and F1.4,
>F1.8 and F2, F3/5 or F3.6 and F4, F4.5 and F5.6, F5 and F5.6, F6.3 and F8,
>F6.5 and F8? Did I leave any out? Are these odd F-numbers 1/3, 1/2, 2/3 of
>a stop wider or are they essentially the same?
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