Tom Pfeiffer wrote:
[snip]
> In reading the subsequent posts, I think my error was not using the focal
> length but the actual lens diameter in my calculations.
>
> I assumed the lenses to have the same diameter as the filters they used,
> which is not pure science by any means :^)
>
> So the radius of the 1.8 lens wide open is 52/1.8/2, or 14.44mm.
> The radius of the 1.4 lens wide open is 58/1.8/2, or 20.71mm.
Hello Tom ...
Again, my sixth grader sets me straight: The filter size for any given
lens is NOT directly related to the aperture of the lens. You can find the
diameter of the lens opening (aperture) for any given f-stop by dividing
the focal length by the f-stop. For example, the 50mm f/1.0L that we
all have in our bags has a maximum aperture of 50mm. When it's stopped
down to f/1.4 the aperture is about 35.7mm (and the area and light
gathering ability is halved). Stopped down to f/2.0 the area is again
halved and the diameter is 25mm, so the light gathering ability is
one fourth of what it was at f/1.0. Each full stop step reduces the
area to one half of the previous value, the equivalent of halving the
film's ASA number or halving the exposure time.
-Nick
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