The difficulties can be related to the physical size of the glass. The
larger the glass, the more difficult it is to accurately grind and surface
the glass, and no glass can be ground and surfaced perfectly. I run into an
analogous problem with parabolic antenna design. Each part of the surface
must be true over the entire surface to within a very small fraction of a
wavelength. Further, modern glasses are very homogeneous, but they are not
perfect. The larger the glass, the harder it is to control dispersion caused
by this. These problems are related to the actual size of the lens, not the
f/stop. This is the reason that the longer the lens, the slower it is
(generally) designed and the more likely it is to use an expensive, low
dispersion glass. In other words, the same problems that make a 50mm f/1.2
lens more difficult to make, more costly, yet often "softer" than a 50mm
f/1.7 are the same ones that make long lenses more difficult to make, more
costly, yet usually "softer" than shorter lenses. Getting these lenses to be
useful with physically big glass (fast & long), along with lower sales
volume, is what makes these lenses really expensive. This is why Pentax's
longest refractor lens is A* 1200 mm ED [IF] is an f/8 lens yet costs as
much as a car.
Regards,
Bob...
Democracy is two wolves and a lamb voting on what to have for lunch. Liberty
is a well-armed lamb contesting the vote!
From: "Rob Studdert" <[EMAIL PROTECTED]>
> On 6 Jan 2001, at 12:07, Wieland Willker wrote:
>
> > Yoshihiko wrote:
> > > BTW, I also noticed the relatively low resolving powers of long
> > > lenses compared with those of short or normal lenses, though I
> > > don't have the correct answer for that question yet.
> >
> > In my naivity I think this must have at least something to do with the
distance.
> > The farther away you are from the object the more problems you have,
e.g.
> > tripod-vibrations, air-move, temperature gradients etc. The nearer you
are to
> > the image the more light from it hits the lens unchanged.
> >
> > Maybe one should keep the distance constant and change the image size
for those
> > measurements?
>
> The usual procedure when lens testing using standard test charts is to use
a
> fixed magnification (such as 55 or under in the case of the USAF test
chart)
> so that calculation of the actual lppmm are easily scaled. Therefore even
a
> 600mm lens would generally be no longer than 33m from the test target.
>
> I don't know why it is generally the case that longer lenses have poorer
> absolute resolution than their shorter focal length counterparts. However
what
> does seem to be apparent is that within a lens family normal lens lengths
(ie
> those that preserve normal perspective) and those that are of generally
> symmetrical design generally provide higher resolutions under test
conditions.
>
> For more information on lens testing resources see:
>
> http://www.ozemail.com.au/~distudio/lenstesting.html
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