Hmmm. Interesting reading, but there's more to the F-ing stops. f-stops,
I learned from old timers, is a ratio of aperture to lens size, which is why
it's written 1:1.8 or 1:4.5 on your lens front piece. For example, f-1 means
that the aperture is the same as the focal length. f-8 means that the
aperture is1/8th of the focal length (try it on any of your lenses). As the
aperture gets smaller, light only passes through the center of the lens, thus
slightly changing the properties of the light rays hitting the focal point
(hence the star patterns at night -- and notice that the number of point on
the star is identical to the number of leafs in the iris).
Lenses around the edges are less than perfect parabolas, causing the
light rays to not be exactly on the focal point. That's why wide open lenses
look funny compared to the same lens at f-8; the images are in focus, but
they have a wax paper look. The bigger diameter a lens has, the more
difficult it is to make, which is why they cost so much. When you pay the
big bucks for Lieca (biggest bucks of all) or Hallselblad or Nikon or Canon,
you're paying for perfect parabolic shape around the edges. And that's why
those 300 f-2.8s are so friggin huge at the front element. Canon does the
most to manufacture high quality lenses with big rear elements to also let as
much light as possible hit the film plane. Cheap and early-era lenses were
only good in the center, and with a small enough hole (aperture), a lens
isn't required at all to make an image. For those who aren't aware of this,
try poking a pin hold in a piece of paper and holding that paper off the
ground on a sunny day. By moving the paper, you'll eventually focus the
image of the sky onto the ground -- that's pinhole photography, and modelers
still use it today for its maximum depth of field.
There's more to f-stops than I've written, but the above is the basic theory.
....Mike Del Vecchio
---------------------
Forwarded message:
From: [EMAIL PROTECTED] (Dave Cohen)
Sender: [EMAIL PROTECTED]
To: [EMAIL PROTECTED]
Date: 97-09-26 16:54:13 EDT
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
--> SPORRS: Serious Photographers of Railroad Related Subjects
