Mirror lenses

Does anyone here use a mirror lens. I read about circular deformity in out
of focus backgrounds. How bad is this really? Can anyone point to a picture
location, good and/or bad?

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S. Cargo wrote:
Morris Coleman wrote:


Does anyone here use a mirror lens. I read about circular deformity in out
of focus backgrounds. How bad is this really? Can anyone point to a picture
location, good and/or bad?

--


Here's a pretty good writeup with sample images..

http://www.photozone.de/3Technology/lenstec5.htm

I've tried one.. The article pretty well sums things up :-)

While in general, I agree with the above article, it should be
noted that the statements made apply to cheap consumer photo
mirror lenses and are not general properties of mirror "lenses."
In astronomy circles, high quality mirrors can and do
out perform lenses, and in fact for "telephoto", i.e. the longer
focal lengths that need to be fast, mirrors far outperform
lenses. For astronomical planetary images, refractors (lenses) must be
on the order of f/15 to get diffraction limited results in all colors.
Mirror systems, especially with a corrector plate (weak lens), of either
Maksutov, Schmidt-Cassegrain, Schmidt-Newtonian, and Wright-Newtonian
design can have excellent performance (ie diffraction limited). A well-
designed system has high contrast too, in opposition to the above web
page. BUT, like lenses, a good mirror system is not cheap.
So, the bottom line is: which is better a $500 mirror telephoto,
or a %500 lens telephoto? Answer: they are both probably poor.
I am not aware of a high quality camera mirror telephoto
on the market, unlike many high quality astronomical telescopes.
If you want a reasonably fast, manual focus mirror telephoto,
an astronomical telescope can do very well, when the mirror diameter
is about 6-inches or larger (this means the secondary mirror
can be large enough to field the light cone for 35mm cameras).
Smaller than this size, and the performance suffers trying
to field the light to the full film plane (assuming you are
attaching a 35mm or DSLR camera body to the system).

I have tested a 6-inch diameter, f/5 Newtonian telescope as a
telephoto lens (760 mm f/5):

http://clarkvision.com/newt-tele1

The results show that it is much sharper than a 100-400 mm Canon L IS lens.
The sharpness is limited by my ability to focus. I built this system
for a couple of hundred dollars. Note the link at the bottom of the page
describing much better optical designs.

If you want to use your telephoto for wildlife, especially moving wildlife,
you need autofocus, for example, see:

http://www.clarkvision.com/galleries/gallery.bird

As you can see, after my experiments with the homemade mirror lens
(which came out great), I chose to go the real telephoto route.
The ease and autofocus ability of a high quality telephoto
lens (e.g. a Canon 500 mm f/4 L IS for about
$5,700) was well worth the price.

Roger

"Roger N. Clark (change username to rnclark)" wrote in
message
I have tested a 6-inch diameter, f/5 Newtonian telescope as a
telephoto lens (760 mm f/5):

http://clarkvision.com/newt-tele1

The results show that it is much sharper than a 100-400 mm Canon L IS
lens.
The sharpness is limited by my ability to focus. I built this system
for a couple of hundred dollars. Note the link at the bottom of the page
describing much better optical designs.

Amazing results; however, would have been slightly more interesting had you
compared your mirror lens to something like Canon's 400 2.8L - 'course, you
still had the 100-400 covered in cost, if not in portability

If you want to use your telephoto for wildlife, especially moving
wildlife,
you need autofocus, for example, see:

http://www.clarkvision.com/galleries/gallery.bird

As you can see, after my experiments with the homemade mirror lens
(which came out great), I chose to go the real telephoto route.
The ease and autofocus ability of a high quality telephoto
lens (e.g. a Canon 500 mm f/4 L IS for about
$5,700) was well worth the price.

No argument there - I rounded my 400 2.8 (non IS) up for $4500 new - price
became more manageable after the IS model came out..........they are amazing
not only optically but by size and weight also.

Nice birds, BTW! Liked the panorama mock up, good work.

Shoot'em up, w/wo a mirror, Agfa, Fuji, Kodak and all the rest will love you
for it!!

Jim

Roger N. Clark (change username to rnclark) artikulierte sich am 14 Jan
2004 wie folgt:

S. Cargo wrote:
I have tested a 6-inch diameter, f/5 Newtonian telescope as a
telephoto lens (760 mm f/5):

http://clarkvision.com/newt-tele1


Roger,

highly interesting results. However, that wouldn't be a real solution
for an outdoor photographer.

Greets from Germany

Thomas

You're in luck; I took a picture a few years ago to illustrate
exactly that phenomenon:

http://www.photo.net/photodb/photo?photo_id=2050704

This is a photo of my neighbor's Christmas lights on
a tree in his front yard. I deliberately set the lens as far
out-of-focus as possible, so all we see is the donuts
of the OOF lights.

Notice that as the donuts get closer to the edge of the frame,
the side of the donut toward the middle of the frame, is cut off.
This gets worse the farther from the middle of the frame, and
is caused by the lens barrel actually vignetting part of the
mirror.


"Morris Coleman" apparently said:

Does anyone here use a mirror lens. I read about circular deformity in out
of focus backgrounds. How bad is this really? Can anyone point to a picture
location, good and/or bad?

Roger N. Clark wrote....

For astronomical planetary images, refractors (lenses) must be
on the order of f/15 to get diffraction limited results in all colors.
Mirror systems, especially with a corrector plate (weak lens), of either
Maksutov, Schmidt-Cassegrain, Schmidt-Newtonian, and Wright-Newtonian
design can have excellent performance (ie diffraction limited).

Boy Roger. I'd sure like to see the responses you'd get if you posted this on
sci.astro.amateur. :-) Haven't you ever heard of apo lenses? The finest
refractor lenses made by AP, TeleVue, Borg, Takahashi and others will beat a
mirror system of similar size any day of the week. It wouldn't even be close!
A less expensive achromat lens will not focus all colors to the same point, so
a longer focal length lens is necessary, but a modern apochromatic lens can do
it, and the lack of a central obstruction will increase contrast that is easily
noticeable. Diffraction limited isn't all it's cracked up to be either. This
is a minimum standard that most inexpensive telescope makers try to attain.
Better quality manufacturers can easily go beyond the 1/4 wave figure of a
diffraction limited scope.

Thanks!
Richard Navarrete

Astrophotography Web Page - http://members.aol.com/richardn22

RichardN22 wrote:
Roger N. Clark wrote....

For astronomical planetary images, refractors (lenses) must be
on the order of f/15 to get diffraction limited results in all colors.
Mirror systems, especially with a corrector plate (weak lens), of either
Maksutov, Schmidt-Cassegrain, Schmidt-Newtonian, and Wright-Newtonian
design can have excellent performance (ie diffraction limited).

Boy Roger. I'd sure like to see the responses you'd get if you posted this on
sci.astro.amateur. :-) Haven't you ever heard of apo lenses? The finest
refractor lenses made by AP, TeleVue, Borg, Takahashi and others will beat a
mirror system of similar size any day of the week. It wouldn't even be close!
A less expensive achromat lens will not focus all colors to the same point, so
a longer focal length lens is necessary, but a modern apochromatic lens can do
it, and the lack of a central obstruction will increase contrast that is easily
noticeable. Diffraction limited isn't all it's cracked up to be either. This
is a minimum standard that most inexpensive telescope makers try to attain.
Better quality manufacturers can easily go beyond the 1/4 wave figure of a
diffraction limited scope.

Thanks!
Richard Navarrete

Astrophotography Web Page - http://members.aol.com/richardn22

Richard,
While modern apochromats are very good, they still can't match a reflector
in fast f/ratios, like f/4. Check the specs of a Wright-Newtonian:
http://silverstar.pccenter.ru/fov.htm
(note the 8-inch f/4 system at the end of the page).
Compare to refractor specs and spot diagrams. e.g.:
http://www.apm-telescopes.de/franzoe...m/techspec.htm

There is no comparison.

An apochromat (3 lenses) has zero color at 3 wavelengths, but does have
residual color away from those wavelengths.

Roger

All the close bird pictures on
http://lewbar.tripod.com/2002/darling/darling.htm were with a hand-held
mirror lens. The first osprey photo has the doughnut out of focus spots.
Most do not. A 600mm Sigma was used.
Lew

"Morris Coleman" wrote in message
...
Does anyone here use a mirror lens. I read about circular deformity in out
of focus backgrounds. How bad is this really? Can anyone point to a
picture
location, good and/or bad?

--

Roger Clark wrote..
"Richard,
While modern apochromats are very good, they still can't match a reflector
in fast f/ratios, like f/4. Check the specs of a Wright-Newtonian:"

Have you seen the coma that will be introduced in an f4 reflector? The edges of
the frame will have severe distortion. Do you realize how big a secondary
you'll need to fully illuminate a 35mm frame? The loss in contrast over a
similar sized refractor will be significant. You can see some astrophotos I've
taken with my TeleVue 4" apo refractor on my website. The stars are sharp to
the edges.



Richard Navarrete

Astrophotography Web Page - http://members.aol.com/richardn22

RichardN22 wrote:

Roger Clark wrote..
"Richard,
While modern apochromats are very good, they still can't match a reflector
in fast f/ratios, like f/4. Check the specs of a Wright-Newtonian:"

Have you seen the coma that will be introduced in an f4 reflector? The edges of
the frame will have severe distortion. Do you realize how big a secondary
you'll need to fully illuminate a 35mm frame? The loss in contrast over a
similar sized refractor will be significant. You can see some astrophotos I've
taken with my TeleVue 4" apo refractor on my website. The stars are sharp to
the edges.



Richard Navarrete

Astrophotography Web Page - http://members.aol.com/richardn22

F/4 Newtonian reflector = horrible coma.
Catadioptric reflectors pretty minimal coma, especially the
Wright-Newtonian. Did you look at the specifications on the web page
I referenced?

At f/4, the diffraction disk size (to first minima, green light)
is 4.5 microns. Your Televue 4" apo (f/5 ?) is probably not diffraction
limited, but could still give great pinpoint star images over the
film plane, but only because detail is limited by the film.
My canon 500 mm (focal length), 5-inch aperture, f/4 lens also gives
beautiful star images over the entire field of a Canon 10D sensor,
but it is not diffraction limited either. I would pay a lot
for a Wright-Newtonian with great optics. It would outclass these
lenses in performance, even with the central obstruction.

Regarding the central obstruction and contrast, this contrast loss
is mainly into the first diffraction ring, and for general imaging as well
as stars (deep-sky astro work) this is not a big factor. It would only be
a factor in higher magnification imaging as in the planets.

You have a beautiful web site with great images. I know how hard you
have worked to get them. But look at some of the brighter stars in some
of your images. Some have halos. This is due to either residual
spherical aberration (common with lenses) or a reflection (also common in
multiple element lenses). These effects sap contrast much faster than
a central obstruction moving light into the diffraction ring.
I'm not trying to degrade your beautiful images, they really are
spectacular! I'm just trying to point out there are many factors
important in an optical system, and there is no perfect one.

Roger