> > > He also said that if we use pi to more than two decimal places,
and
> > > use the square root of 2 instead of 1.4 we'll get more accurate
results.
He should have used the word "precise" because "accuracy" improves
little if you don't know the actual aperture to better than 1.4 vs
1.8. Now if I had a 1.8 in my hands I would measure the light
hitting the film plane with a Minolta Booster and compare it to my 1.4
and answer the question ... but then to no better than 0.1 stop
because that is the readability of my meter.
> > > He also told me to remember that "f-stop" is NOT aperture, but
the
> > > ratio of aperture to focal length.
Actually its not: it is an arbitrary sequence of numbers that is
approximately close to that ratio. Ask your son "why f22 not f23?"
which is MUCH closer ;o)
> > Exactly, which is why you can't use the ratio (1.4 or 1.8) in the
formula.
> > You have to use actual sizes. Light transmission is dependent on
the area of
> > the opening the light passes through.
Once you get diffraction playing a part (very small apertures) the
light intensity hitting the film plane starts to lose its exact
relationship to the physical area of the hole. (the diffraction
occurs around the edges) but that is getting nerdy ...
> Had to go to my seventh grade son for this one Chuck. He said that
since
> the focal length is a constant, it can be ignored and it is proper
and
> okay to use the focal ratio.
Ha ha ha ... he forgot to mention that the focal lengths of the 50
f1.8 and 50 f4 may not even be the same ;o)
> To rub it in a little, the sixth grader pointed out that the 1.4
f-stop
> on your lens is NOT the exact figure; it should be 1.414213562 at
the
> very least ... or more accurately, it should be the square root of
2.
Ah, grasshopper ... it depends on how the lens testers calculated the
f-stop. If they measured the light and factored in "1.40000000"
(because close enough is good enough) then using root 2 might
introduce error. And are they really quoting f- or t-stops anyway?
Oh ... FWIW, 2/3 stop is about right for the difference in these two
lenses ;o)
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