THE MECHANICS
of photography are based on the number 2, its reciprocal
1/2, or square root 1.4. These numbers make photographic
technique easier to understand, easier to use, easier to
calculate.
Standard shutter speeds, 1, 2, 4, 8, 15, 30, 60, 125, 500,
1000, etc., follow this pattern of 2. Moving to the next
higher number reduces the time the light impinges the film
by one half. Moving to the next lower number doubles the
time.
Lens openings f/stops), 1.2, 1.4, 2, 2.8, 4, 5.6, 8, 11,
16, and 22 follow the same pattern. Each number is
multiplied by 1.4 to arrive at the next higher number and
that reduces the volume of light by one half.
[Some calculations are rounded off to the next number, 11
* 1.4 = 15.4.]
f/1.2 is a half f/stop. 1/2 stop calculations are based on
1.2. Think of that the next time a camera sales person
tries to convince you to spend a couple of hundred extra
bucks for an f/1.2 instead of an f/1.4.
ISO films speeds follow the same pattern of 2.
ORVIL STOKES is publisher of PHOTOWORK NEWSLETTER, now in
its tenth year. (317 E Winter Ave, Danville IL 61832;
Voice/Fax: 1 217 442-3075.) $10 per year. Canada $11. He
also conducts the annual Orvil Stokes Workshop.
