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Old March 4th 11, 02:25 AM posted to rec.photo.digital.slr-systems,rec.photo.digital
Outing Trolls is FUN![_5_]
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Default The multi-element, spherical lens-based lens MUST DIE!!

On Fri, 04 Mar 2011 01:04:51 +0000, Bruce wrote:

RichA wrote:
On Mar 3, 8:38*am, Bruce wrote:
RichA wrote:
On Mar 2, 7:44 pm, Bruce wrote:
RichA wrote:
They must eliminate the multi-element lens. It is too heavy. It is
too complex inside therefore TOO expensive. 14-17 elements? Why, in
this day of hybrid aspherics and ED glass? Pentax makes a 400mm f4
telescope with 4 elements that produces a highly-corrected (colour,
astigmatism, spherical aberration) FLAT image across an entire medium
format film plane. With FOUR elements! WHERE is the Nikon or Canon
lens that can do that with so few elements? For about $3k. Takahashi
(Japanese) has a similar lens. And those lenses do it diffraction-
limited without stopping down! Multi-element lens systems based on
spherical lenses are DINOSAURS.

You need to compare the MTF of those telescopes with the MTF of top
quality fixed focus DSLR lenses of comparable focal lengths. I
suspect that will give you the answer you need.

A top quality 400mm DSLR lens is about $10,000 and still won't be
diffraction-limited wide open.

Then let's all shoot through telescopes and see how good they are.


Modern Photography in 1977 tested a TeleVue (Pearl River, New York,
maker of high-performance apochromatic telescopes) Renaissance
telescope. This was a 4" aperture, 500mm f5.0 refractor retailing for
about $2000.00. It beat every lens they had EVER tested when it came
to contrast and resolution. TeleVue has had 6 interations
(improvements) of that scope since then, each of them providing better
performance. The scope is now called the NP101 and is similar to what
is called the Petzval design, 2 main elements up front, a smaller
doublet midway down the tube. And there are even better telescopes
than the TeleVue available from places like AstroPhysics, TEC, etc.
The average camera lens is corrected to about 1-2 waves (yellow-green
light). The average telescope is 1/4 wave, a high-end scope like
those mentioned is 1/10th wave. The mirror that went into the Hubble
was 1/100th wave. The reason telescopes have to be so accurate is
that unlike a camera lens that is used at what is called, Prime focus
(no extra magnificational elements) a telescope may be used in
conjunction with projection optics. Imaging trying to put a 10-20x
teleconverter on a camera lens and trying to get an image out of it.
They do it with telescopes all the time, to shoot planets.



If all this is true, why aren't we all using telecopes instead of
telephoto lenses?

Perhaps because a telescope costing $2000 in 1977 would be way out of
reach in today's money. Plus, it would produce an inverted image.
Plus, there is a lot more to optical performance than resolution and
contrast.




I suggest you read any "Optics 101" literature that is readily available
before you ever talk about anything even remotely related to the subject
again. Then you wouldn't so swiftly remove all doubts about your blatantly
obvious fool-troll status.

(Hint: all camera lenses also produce an inverted image.)


You obviously have an obsession about telescopes. Don't worry, in a
few days it will go away, and you will start ranting about something
else that is completely unrelated, but equally implausible.

Do try to smile. ;-)