--- Ken Durling <[EMAIL PROTECTED]> wrote:
> The thread about viewfinder image brightness brought
> up something I
> realized I'm unclear on: that is prime lens vs
> constant aperture zoom
> vs variable aperture zoom. Since aperture is an
> expression of
> fraction of focal length, "constant" aperture zooms
> are really
> "constant ratio" zooms, aren't they?
Yes, that's right. Except that aperture really MEANS
ratio, so constant aperture and constante ratio are
saying the same thing.
> The actual
> aperture size must
> change with changing focal length, mustn't it?
Yes. More precisely, the diameter of the aperture
changes as the focal length changes.
> Could that in any way
> explain the difference in viewfinder brightness
> asked about between a
> prime and a zoom?
Not really, unless the compensation isn't working
correctly
>
> In a variable aperture zoom, if you dial in max
> aperture, and then
> zoom out, the aperture increases in steps, i.e 4,
> 4.5 5.6., and each
> step remains constant for a certain percentage of
> the zoom range.
That's a limitation in the display, not an accurate
reflection of what's going on. In a variable aperture
zoom, the diameter of the aperture DOESN'T change as
the lens zooms. So if you vary the focal length by
10mm, the effective aperture changes because the ratio
changes--but the change may be too small to be
reflected in the display.
> This obviously means that you aren't getting precise
> f-stops
> throughout the range, which is probably part of the
> lesser cost of
> such lenses. In a "constant aperture" zoom how is
> this avoided? Or
> is it? Is there actually a continuous range of
> aperture available?
Your EOS should still be adjusting the shutter speed
in reaction to the actual f-stop, not the one
displayed on the screen, regardless of the cost of the
lens.
In constant aperture zooms there is mechanical (or in
the EOS case maybe electronical <g>) compensation. As
the lens zooms shorter, the diamter of the aperture
decreases.
Now this does raise an interesting question. Dos the
aperture actually change size as you zoom, or only
when the lens is stopped down for picture taking? I
think it must vary in real time, because I don't
notice changes in brightness as I zoom my 70-200.
=====
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
***********************************************************
