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[OM] Re: f-stops (Was: Solstice web-site news)

Subject: [OM] Re: f-stops (Was: Solstice web-site news)
From: hiwayman@xxxxxxx (Walt Wayman)
Date: Fri, 14 Jan 2005 15:23:10 +0000
I promise to let this go -- that is, right after I add one more quote.  If 
there's one thing I can't resist, it's temptation, and in pulling a couple of 
things off the shelf, I realized I had overlooked one credible, authoritative* 
and unimpeachable source.  So, here goes:

"The lens aperture is simply the diameter of the lens opening, expressed as a 
fraction of its focal length.  Thus, a lens of 4-inch focal length with a 
diameter of one inch has a relative aperture of 4/1, or 4.  the aperture 
designation is expressed as f/4, indicating that the aperture is the focal 
length/4.  Another 4-inch lens that has a diameter of 1/2 inch would be an f/8 
lens.

"The aperture indicates the amount of light that the lens will transmit to the 
film.  Since the apereture is expressed as a fraction of the focal length, all 
lenses set at f/8 (or any other aperture) transmit the same intensity of light 
to the film.  This amount of light is proportional to the area of the lens 
aperture (and therefore to the square of the diameter); the f/4 lens described 
is twice the diameter of the f/8 lens, but transmsits four times as much light.

"The aperture inscribed on the front of the lens mount is the largest for that 
lens.  For practical photography we need a means of reducing the aperture to 
give us control of the intensity of the light reaching the film. [...]  The 
adjustable aperture today takes the form of an iris diaphragm, a series of 
metal blades that make different size lens openings depending on the setting of 
a control ring."

"The New Ansel Adams Photography Series/Book 1: The Camera,"  Ansel Adams, 
1980, pgs. 46-47

Walt

*This time I spelled it correctly, putting in the sylble I invertently omted 
previly

--
"Anything more than 500 yards from 
the car just isn't photogenic." -- 
Edward Weston
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