Re: Fixed Aperture Pentax Zoom

On Mon, 9 Feb 2009 19:46:21 -0500, "Peter" <peternew@xxxxxxxxxxxxxxxxxxx>
wrote:
: "Robert Coe" <bob@xxxxxxxx> wrote in message
: news:sg7no4hu4kk7851ei649bptostovmkhm5o@xxxxxxxxxx
: > On Wed, 4 Feb 2009 07:50:22 -0500, "Peter" <peternew@xxxxxxxxxxxxxxxxxxx>
: > wrote:
: > : "Robert Coe" <bob@xxxxxxxx> wrote in message
: > : news:4rnbo45c73la5mrodg7eqnnm5r54n8t9dv@xxxxxxxxxx
: > : > On Sun, 01 Feb 2009 11:03:41 -0500, Alan Browne
: > : > <alan.browne@xxxxxxxxxxxxxxxxxxxxx> wrote:
: > : > : The Canon 300 f/4 is $1,200 but the
: > : > : Canon 300 f/2.8 is over $4,000.
: > : > :
: > : > : The f/2.8 is almost 4x more expensive to be 1 stop faster.
: > : > :
: > : > : The real point is that the longer the focal length, the more
: > expensive
: > : > : it is to make a lens faster as the lens elements get larger.
: > : > :
: > : > : This is why many photographers refer to the lens as "glass". They
: > are
: > : > : talking about the whole lens (lens barrel and lens elements).
: > : >
: > : > Well, actually they're talking about the lens elements. The barrel, I
: > : > think
: > : > it's safe to say, is almost never made of glass. ;^)
: > : >
: > : > It does, of course, make some sense to ignore the barrel in such
: > cases.
: > : > Unless
: > : > it can't manage to hold the elements in place, the characteristics of
: > the
: > : > barrel are nearly irrelevant. (Not totally irrelevant, however.
: > Presumably
: > : > Canon makes their larger lenses white because they're concerned that
: > : > thermal
: > : > expansion of the barrel might have optical consequences.)
: > :
: > :
: > : If the barrel has the same coefficient of expansion as the glass,
: >
: > It doesn't ...
:
: Not certain about that. Do you have any place I can see a comparison? My
: Chemical Rubber Handbook, 35th Edition, doesn't cover modern glass and
: composits. ( I used modifications 0f some of its foruulas to mix my own
: developing agents.

I was assuming (probably erroneously) that the barrel would be steel or
aluminum (i.e, some metal). Most metals are more subject to thermal expansion
and contraction than glass is. My gut tells me that plastic is less subject to
expansion than glass, but I don't know for sure.

: > : why would it need to be white? Or, is that marketing hype
: >
: > ... But even if it did, a lens that has undergone thermal expansion, even
: > without distortion, is unlike to faithfully preserve all its optical
: > properties.
:
: That would apply to thermal contraction as well. I am not an optical
: physicist, but think that a quality lens design would be ideal at some
: median temperature. I think that any temperature varient would naturally
: cause some change in optical quality.

Yes, I think so too.

: > : OTOH Why wouldn't the opposite be true when dong cold weather shooting?
: >
: > Maybe it *would* be better to use a black lens on a cold day, especially
: > in
: > the sun. But absorption of sunlight aside, a black lens loses heat faster
: > through radiation than a white lens does.
:
: I don't understand the physcal reason for your statement. I always thought
: that black tended to absorb radiant heat and white reflected radiant heat.

What absorbs better also radiates better, for the same reason. Black objects
absorb heat faster, but give it back faster.

: >And in really cold weather, cameras already don't work all that well.
: >
: that makes sense, but I am not really sure what you mean by not working
: that well.

Most (all?) electronic devices work best at STP. If you look carefully at the
specs for your digital camera, you'll see that there's a maximum and minimum
temperature at which it's supposed to work correctly. The latter may be higher
than you might have thought.

Bob
.

Relevant Pages

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