Comments at bottom, after two quotes.
At 6:04 AM +0000 11/26/02, olympus-digest wrote:
>Date: Mon, 25 Nov 2002 15:10:59 -0800 (PST)
>From: AG Schnozz <agschnozz@xxxxxxxxx>
>Subject: RE: [OM] Mind Bender (intermediate focal length?)
>
> >"Effectively" is the weasel word that essentially makes this
> >question meaningless.
>
>Not quite. Affectively would be meaningless, effectively is
>referring to the actual field/angle of view.
>
>This question came up one day when I was photographing with Joel
>W. and we were comparing 135mm lenses. I stated that I felt
>that the 135mm was the mid-focal length between 100mm and 200mm
>just based on "how the image looks in the viewfinder". He
>disagreed on principle and that was the end of it. We haven't
>spoken since. (just kidding).
>
>Anyway, I was cruising the books the other day and came across a
>lens comparison chart which inluded angle-of-coverages. Ahar!=20
>I knew it!
>
>I think this is a "modern math" thing. If multiplications are
>supposed to be rough multiples of focal length then why is 135mm
>the oddball? Hmm????
At 6:04 AM +0000 11/26/02, olympus-digest wrote:
>------------------------------
>
>Date: Tue, 26 Nov 2002 10:39:57 +1100
>From: Wayne Harridge <wayneharridge@xxxxxxxxxxxxxxx>
>Subject: Re: RE: [OM] Mind Bender (intermediate focal length?)
>
> > AG Schnozz <agschnozz@xxxxxxxxx> wrote:
> >
> > Anyway, I was cruising the books the other day and came across a
> > lens comparison chart which inluded angle-of-coverages. Ahar!
> > I knew it!
> >
>
>Angle of coverage, what sense does that make ? You want area of coverage,
>that makes the fl 141mm !
The problem is that field of view varies from lens design to lens design, even
if the focal length is the same, so using FOV values to determine something
like "intermediate" is only approximate.
Perspective effects (and magnification) do follow focal length alone, ant
that's what we are usually looking to vary in some methodical manner.
Perspective effects vary inverse proportionally with focal length, so half the
focal length yields twice the perspective distortion.
The standard mathematical way to find the value equidistant in ratio from two
surrounding values is the geometric mean: Take the square root of the product
of the two surrounding values.
In the present example, we get Sqrt[ 100 * 200 ]= 141.421mm, call it 140mm.
140/100= 1.400
200/140= 1.429
So, 140mm is almost exactly eqidistant in ratio from 100mm and 200mm.
Joe Gwinn
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