"ChrisCoaster" <ckozicki@[EMAIL PROTECTED]
> wrote in message
news:1d80c7c0-736a-4b58-a9db-82fe5e46e832@[EMAIL PROTECTED]
>A few months ago I wandered into this great forum and launched a
> discussion about the not-so-manual aspects of the "manual" mode on
> some compact(non-SLR) digital cameras.
>
> My chief complaint was that when zooming, the iris(f-stop value) was
> automatically decreased when zooming(increasing focal length).
>
> It was stated here that that is in fact what happens when you increase
> focal length - you reduce light intake. Fair enough.
>
> But how difficult, and for that matter how costly, would it be for
> manufacturers to allow a setting that "freezes" f-value at the
> operator's choice when zooming.
>
> That is, keep f=2.0 or 3.0 or whatever while zooming out to, say, 4x
> or 6x optical, to really narrow the field of focus on the subject.
> That's what the really expensive manual SLRs allow one to do, right?
>
> http://www.flickr.com/photos/jsome1/1241705216/
>
> just curious the cat,
>
> -ChrisCoaster
In order to help you get your head around this optical phenomenum, try
this
experiment:
Take two or three toilet paper tubes, or paper towel tubes if you really
want to exagerate the effect, and look through one. Note how large the
hole
appears at the far end of the tube. If you have an ambient light meter,
you
could even put the little bulb covered sensor to the end of the tube and
try
to measure how much light comes down the tube.
Next, tape another tube onto the end of the first, and look down the tube
again. If you have a third tube, move your eye back and forth between the
two tubes, noting how much of a difference you note in the apparent size
of
the opening at the end of the tube. Keep in mind that the hole has not
changed it's size, it just appears considerably smaller at the greater
distance. If you have the light meter, check the reading again and note
the
difference in how much light gets transmitted down the tube once it is
lengthened.
In order to maintain the same amount of light, you would actually have to
change the shape of the tube from a cylinder to a cone, continually
widening
the end of it as you increase the length of the tube. Obviously, it would
be all but impossible to design a zoom lens to increase the diameter of
the
outer elements as it zooms out, so lens manufacturers have to find
otherways
to compensate for the virtual reduction in lens diameter.
It's not a matter of manufacturers purposely cheaping out in order to save
a
few dollars. It's a matter of how much you as a consumer are willing to
pay
the manufacturer to think up ways to beat Mother Nature at her own game.
Hope this helps,
Dudley
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