At 8:14 AM -0400 8/7/00, you wrote:
In a message dated 8/3/00 12:49:12 PM US Eastern Standard Time,
jlind@xxxxxxxxxxx writes:
<< Considering lens speeds, there is often a misconception about f/3.5 versus
f/4. They are only 1/3 stop apart, not the half stop many think it is.
That's right, f/3.5 is only 1/3 stop faster. For me, f/3.5 versus f/4
wouldn't be a consideration.
>>
I was always under the impression that the difference between aperture
settings on the lens was one stop difference in light transmission. That is,
the difference is measured by light transmission, not by numerical value. If
so, how could there be a 1/2 or 1/3 difference in light transmission? At the
same shutter speed, there should be twice as much light transmitted by the
f3.5 aperture setting than by the f4. The consistency of the one stop rule is
what makes possible the exposure correlation between aperture and shutter
speeds. For example, f3.5 at 1/125 second would allow the same light
transmission as f4 at 1/60; f8 at 1/250 second would allow the same light
transmission as f11 at 1/125. In other words, to maintain exposure
consistency, if light transmission is doubled then shutter speed is halved;
if light transmission is halved then shutter speed is doubled.
Have I just been confused for the past 50 years?
Yep, you've been confused :-)
Most manufacturers put the numbers on their apertures (and the
detents associated with them) so that each stop is a factor of two.
But it's the numbers that govern. The amount of light transmitted
depends on the square of the f-stop number (which is the ratio
between aparture and focal length). You square the aperture number
because the *area* through which light can pass, which is the
determining factor, depends on the square of the
radius/diameter/diagonal/etc of the opening.
So 1.4^2=2, 2^2=4, 2.8^2=8, 4^2=16 and so forth, giving the standard
factors of two.
But most manufacterers abandon these increments when it gets to the
widest opening of a lens, because that's determined by optical
design, cost, weight and barrel-size limits and so forth. So you'll
have maximum apertures of 1.8 or 2.5 or 3.5 or 6.3 or whatever turns
the designer on... The odd aperture typically transmits roughly 2/3
the light than the next lower standard number stop would, and roughly
4/3 the amount of light of the next higher standard number, but don't
bet on it (and of course if you do the math you'll see how accurate
that "roughly" can be at best).
paul
--
Paul Wallich pw@xxxxxxxxx
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