Greetings,
Here is another SPORRS blast from the past dug up from about a year ago and
dusted off for repost for the benefit of the newer members interested in F
stops.
--Reposted from previous SPORRS post--
To: SPORRS
From: Dave Cohen
Re: F stops
Several people asked me about, and have discussed this stuff with me
recently, and although most of us may already know this stuff, here it is
again:
The name "F stop" derives from the old (metal - like a dark slide) "stops"
that photographers used to slide into old view cameras to change their
apertures. ("F" was for Focal). The number of the "stop" is how much
light is allowed to pass through it (what size hole). The basic F stops
are all square roots of two because each progressive or digressive stop
lets in double or half as much light as the stop before or after it.
(Forgive me for this, but) the larger the NUMBER of the "f stop", the
smaller the opening (hole) through which the light passes through.
Example: Twice as much light goes through the aperture of the lens (and
strikes the film) at f 11 as it would at f 16. Here is why:
The doubling or halving of light can be figured (and then rounded off) by
using multiples of 1.4. We will start with f 1 (which I am sure than none
of us have on our lenses!).
1 x 1.4 = 1.4
1.4 x 1.4 = 1.96 (2.0)
1.4 x 2.0 = 2.8
1.4 x 2.8 = 3.92 (4.0)
1.4 x 4.0 = 5.6
1.4 x 5.6 = 7.84 (8.0)
1.4 x 8.0 = 11.2 (11.0)
1.4 x 11.0 = 15.4 (16.0)
1.4 x 16.0 = 22.4 (22.0)
1.4 x 22.0 = 30.8 (32.0)
1.4 x 32.0 = 44.8 (45.0)
1.4 x 45.0 = 63.0 (64.0)
Some genius decided that you should round them up (mostly). If you try
this and use the actual numbers, then you end up off by a half integer(?!,
whole number, not an f number) by f 11 and then around an integer and a
half after that! Confused? Apparently mathematics deductive reasoning
doesn't work perfectly for photography. Shutter speeds are another fiasco
entirely. Anyone care to dig into that one?
Dave Cohen
Photographer
[EMAIL PROTECTED]
--> SPORRS: Serious Photographers of Railroad Related Subjects
