--- Ken Durling <[EMAIL PROTECTED]> 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.
In actuality, many variable aperture lenses probably
do provide some mechanical compensation of aperture,
but not enough to maintain a constant aperture. Such
is probably done for reasons of size and cost, and
optical performance (to maintain 3.5 at the 105mm end
of the zoom would require better glass and either more
elements, or more complex aespherical elements, or
larger diamter glass (or all three)).
I simplified for the sake of argument and to make the
example more clear.
=====
Bob Meyer
I wish I knew what I know now, when I was younger...
http://www.meyerweb.net/epson
__________________________________________________
Do You Yahoo!?
Make international calls for as low as $.04/minute with Yahoo! Messenger
http://phonecard.yahoo.com/
*
****
*******
***********************************************************
* For list instructions, including unsubscribe, see:
* http://www.a1.nl/phomepag/markerink/eos_list.htm
***********************************************************
