In article , Winsor Crosby <wincros@xxxxxxx> 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?
>
>Winsor
Take the ratio of f/#'s and square them to work out how much more/less
light intensity you are getting.
So an f/1.2 is 1.36 times faster than an f/1.4, or 0.69x an f/1,
f/3.5 is 1.31x an f/4, or 0.64x an f/2.8,
f/4.5 is 1.55x an f/5.6, or 0.79x an f/4,
f/5 is 1.25x an f/5.6, or 0.64x an f/4,
f/6.3 is 1.61x an f/8, or 0.79x an f/5.6 and
f/6.5 is 1.51x an f/8 or 0.74x an f/5.6
These figures correspond to the change in light intensity over the
standard f/#'s, so for example if you were using f/4 at 1/250th of a
second then the shutter speed at f/3.5 would be 1/(1.31 x 250)th, or
1/327.5th of a second to give the same exposure.
Remember a stop is a factor of two change in the light intensity, so to
convert the light intensity changes to fractions of a stop, take the
logs and divide by log(2). eg. f/3.5 is 1.31x faster than f/4, and
1.31x is +0.389 stops.
Within the accuracy of the lens manufacture and the standard aperture
progression, which isn't exactly a doubling at each step, these 'odd'
apertures do indeed correspond to an extra half or third of a stop over
standard aperture sizes.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
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