Depends on what you mean by "halfway point". I take the halfway point
to be the one that results in an *area* 1/2 the size of the next step up
and double the size of the next step down. Using that as the measure
the halfway point between 100 and 200mm is determined by applying the
square root of 2, either 100 * 1.41 or 200 / 1.41 and gives midpoint
focal length as 141mm. The midpoint between 200 and 400 is twice that
or 282mm.
For a baseline "normal" lens of 50mm these half/double steps would yield
a lens array of 13, 18, 25, 35, 50, 70, 100, 140, 200, 280, 400, 560
For a baseline "normal" lens of 43mm we would have
15, 22, 30, 43, 60, 86, 120, 172, 240, 344, 480
Most of the standard focal length primes are from the 50mm as normal
sequence with a couple thrown in from the 43mm as normal. But now we
return to the mystery. We don't typically have either 60 or 70 so the
prime lenses are misssing a step.
Chuck Norcutt
AG Schnozz wrote:
> And to add further confusion to this, the 135mm focal length is
> actually the halfway point between 100mm and 200mm. But is 300
> halfway between 200 and 400 or is it 270? Is 75 halfway between
> 50 and 100 or is it 65?
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