Bob Talbot wrote:
>
> > > > 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)
Thanks Bob! Just exactly what I needed to get my 12 & 13 year old
smart ass sons to do some thinking!
-Nick
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