On Tue, Oct 17, 2000 at 11:31:20PM +0100, David Jenkinson wrote:
> Dear all
> But, and here's the rub.....as one of the answers pointed out,*all* zoom
> lenses, by definition *should* be variable aperture, shouldn't they? After
> all, the focal length changes without the physical size of the opening by
> which light leaves the lens altering one way or the other. At least, it
> doesn't seem to on my 65-200/4. Bafflingly, the 35-70/3.5-4.5 I used to
> have did change size, as the aperture was stopped down a tad at the 70mm
> end. (If you've got one, try it.) To me this means that any lens with a
> fixed aperture must be a compromised design in some way, and also why isn't
> my 65-200/4 a 65-200/2.8-4 or something like that. The only thing I could
> think of was that the lens is actually a 65-200/3.7674757 - 4.3323343, and
> for simplicity's sake it gets corrupted to 4.
A F/4 apperture for 200mm needs 50mm in diameter, for 65mm only 16.25mm.
A 50mm diameter apperture for 65 mm would be F/1.3 !
Imagine: 65-200mm F/1.3-4
This is a value which is not copeapble in terms of optical quality for a zoom.
So the diameter of the apperture must change in (practically) any zoom lens:
35mm / F3.5 => 10mm
70mm / F4.5 => 15.6mm
So the value of a variable aperture is a better lens-size/ aperture ratio.
It is difficult to evaluate the diameter of a lenses aperture with looking
trough
the front or back of a lens. Because you`re viewing the aperture trough a sort
of loupe,
which can make the image of the aperture locking larger or smaller! This
magnification-rato
is a tecnical specification for a lens, dependant on the optical construction.
On a telephoto-lens, or a wide angle (retrofocus lens) this ostensivley
apperture
size is signifficant different when viewed from the front or back!
And it is hard to guess what optical effects take place in a certain zoom lens.
hope this helps.
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