Ken Durling wrote:
> On Mon, 30 Jul 2001 15:11:14 -0400, you wrote:
>
> >whereas the 28-105/3.5~4.5 is
> >a "constant-aperture" lens (the maximum diameter of the diaphragm doesn't change
> >(in principle), therefore the f-stop does change with focal length).
>
> If this were *strictly* true, shouldn't we see a proportional change
> in f-stop to the range of focal lengths? 28-105 is a range of about
> 3.5x and 3.5-4.5 is what, 2/3 stop?. My 75-300 has a zoom range of
> 4x, but is a f/4-5.6 lens, one full stop, or 2x. My 28-135 a bit
> more than 4x, and is 3.5-5.6. My math is failing me now, but you get
> the picture. So to speak.
Well, as I noted, in some variable f-stop lenses the physical aperture isn't in fact
constant, including the two I have (20-35 and 28-105). So perhaps that's what's going
on.
Since the physical size of the opening of an f/3.5 lens is the focal length divided
by 3.5, the diameter of the opening at 28 mm should be 8 mm, and the diameter at 105
mm should be 30 mm. In this instance, the diameter at 105 mm is 260% larger than it
is at 28 mm. At a focal length of 105 mm, f/4.5 should yield an opening of 23.3 mm;
the physical diameter at f/4.5 is about 20% smaller than at f/3.5 (at 105 mm).
. . . (Just thinking "aloud" here). . . .
If we change the focal length from 105 mm to 360 mm (about 3.5x) and calculate the
physical diameter, we get 103 mm for f/3.5 and 80 mm for f/4.5. Taking f/3.5 as the
example we see that the physical opening at 360 mm is about 290% larger than at 105
mm--not that different from the change we saw when changing length from 28 mm to 105
mm. The difference in the size of the opening between f/3.5 and f/4.5 at 360 mm is
about 20%, or about what we saw in the 28-105 mm example.
What does this tell us?
It's been a LONG time since I did any math of any complexity, so I give up.
But, for the example of the 75-300, the physical size of the opening at 75mm and f/4
should be 18.75 mm, and at 300 mm and f/5.6 it should be 53.6 mm. My guess is that if
you investigate your lens you'll find that the physical diameter of the diaphragm
when wide open is not constant when you zoom, otherwise the maximum f-stop at 300 mm
would be . . . f/16 (300/18.75). Similarly, for the 28-105/3.5~4.5, if the physical
size of the aperture did not increase some during zooming the maximum aperture at 105
mm would be f/13.
Of course it seems so simple now. The problem was the assumption that variable f-stop
lenses employ diaphragms that have the same maximum physical diameter across the zoom
range, when in fact they may have a limited amount of increase in that diameter in
order to allow them to be useable at the long end--just not enough increase to allow
them to have a constant maximum aperture.
Of course, the basic idea remains unaffected: if the physical size of the maximum
aperture does not change in a zoom lens, the maximum f-stop does; if the size changes
proportionally to the change of focal length, then the maximum f-stop remains
constant. In some designs, the physical opening does increase but not in direct
proportion to the focal length, so you get a smaller maximum f-stop at the long end,
but not as small as if there were no increase at all.
BTW Ken, I meant to include your comment about using the DOF preview in my previous
message, in which I quoted someone else under your name. Sorry!
Craig
>
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