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Mirror Lenses Revisited

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Anonymous
August 13, 2005 11:58:39 AM

Archived from groups: rec.photo.digital (More info?)

I'm just wondering, why are mirror lenses so bad? Is it because they are
poorly made or is it an inherent problem. I would have thought that apart
from the donut shaped highlights they should be better than traditional
lenses. In telescopes, Newtonian reflectors are far superior to refractor
telescopes. A mirror lens is very similar to a Newtonian reflector, while a
traditional lens is more similar to a refractor telescope. So why is it that
cameras reverse the conventional wisdom learnt from astronomy?
Anonymous
August 13, 2005 11:58:40 AM

Archived from groups: rec.photo.digital (More info?)

"Justin Thyme" <pleasedontspamme@nowhere.com> wrote in message
news:lY8Le.514$yt1.8207@nnrp1.ozemail.com.au...
> I'm just wondering, why are mirror lenses so bad? Is it because they are
> poorly made or is it an inherent problem. I would have thought that apart
> from the donut shaped highlights they should be better than traditional
> lenses. In telescopes, Newtonian reflectors are far superior to refractor
> telescopes. A mirror lens is very similar to a Newtonian reflector, while
> a traditional lens is more similar to a refractor telescope. So why is it
> that cameras reverse the conventional wisdom learnt from astronomy?

Ha! tres amusant user name.

Not.

Camera mirror lenses are hybrids, part refractor part reflector, the front
element *is* a lens, unlike a true Newtonian..
Anonymous
August 13, 2005 11:58:40 AM

Archived from groups: rec.photo.digital (More info?)

In article <lY8Le.514$yt1.8207@nnrp1.ozemail.com.au>, Justin Thyme
<pleasedontspamme@nowhere.com> wrote:

> I'm just wondering, why are mirror lenses so bad?

The biggest issue, aside from the donuts, is that the aperture is fixed
- and it's a small one. With a mirror lens, you're focusing a dim, high
magnification image (600mm f/8 in the last one I used) and can't stop
down to cover minor focusing errors.
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Anonymous
August 13, 2005 11:58:40 AM

Archived from groups: rec.photo.digital (More info?)

"Justin Thyme" <pleasedontspamme@nowhere.com> wrote in message
news:lY8Le.514$yt1.8207@nnrp1.ozemail.com.au...
> I'm just wondering, why are mirror lenses so bad?

They aren't "so bad" unless you're talking about some inferior mirror lens.
I've had Minolta 500mm f/8 and 250mm f/5.6 mirror lenses and they were
superb. (But I think most of the mirror lenses sold were much cheaper than
those and, I expect, not of nearly the same quality.)

They do, however, have some inherent problems in use. You can't change the
aperture and that is generally quite small, and actual light transmission is
even less than the f-number would lead you to believe. As I recall, my 500mm
f/8 passed only as much light as you'd expect from about f/11 in a
conventional lens. That's probably because of the secondary mirror and other
obstructions in the light path. With the film speeds generally available at
the time I that lens, especially since I mostly used slide films, that
pretty much limited it to tripod use.

N.
Anonymous
August 13, 2005 11:58:40 AM

Archived from groups: rec.photo.digital (More info?)

"Justin Thyme" <pleasedontspamme@nowhere.com> wrote in message
news:lY8Le.514$yt1.8207@nnrp1.ozemail.com.au...
> I'm just wondering, why are mirror lenses so bad? Is it because they are
> poorly made or is it an inherent problem. I would have thought that apart
> from the donut shaped highlights they should be better than traditional
> lenses. In telescopes, Newtonian reflectors are far superior to refractor
> telescopes. A mirror lens is very similar to a Newtonian reflector, while
a
> traditional lens is more similar to a refractor telescope. So why is it
that
> cameras reverse the conventional wisdom learnt from astronomy?

Could someone please post some large images from one of these things so we
can actually see what the heck we are dealing with?
Anonymous
August 13, 2005 11:58:40 AM

Archived from groups: rec.photo.digital (More info?)

Justin Thyme wrote:
> I'm just wondering, why are mirror lenses so bad? Is it because they are
> poorly made or is it an inherent problem. I would have thought that apart
> from the donut shaped highlights they should be better than traditional
> lenses. In telescopes, Newtonian reflectors are far superior to refractor
> telescopes. A mirror lens is very similar to a Newtonian reflector, while a
> traditional lens is more similar to a refractor telescope. So why is it that
> cameras reverse the conventional wisdom learnt from astronomy?
>
>

Mirror lenses can have wonderful performance.
Here is a 760 mm f/5 home made telescope/telephoto
that cost me less than $200 to build:
http://www.clarkvision.com/newt-tele1
Performance is sensor limited, not optics limited!

The problem with small telephotos that you typically see for
sale as camera telephoto lenses: they are cheaply made.

The concept that they are fixed f/ratio is bogus. You can add
a diaphragm that is off-center so as you close down, you
crop out the central obstruction. There are also optical designs
that have no central obstruction, but they tend to
have odd shapes.

In the super telephoto lens category (600 mm f/4 and up), it is
my opinion (as someone who has written ray tracing programs, and
published scientific papers based on ray tracing mirror optical
systems), that a mirror telephoto would be superior optically
to a lens system, lighter, and cheaper to make. It is only
reasonable to do this in the larger apertures because it becomes
easier for the secondary mirror to field the light cone and deliver
it to a 35mm size focal plane. If someone would just market
a focus mechanism that takes a DSLR and attaches it to a telescope
with autofocus, one could use a number of telescopes for high
power wildlife photography. I would take a 1016 mm f/4 telephoto
on my next bird photo expedition if I had autofocus (cost: about
$1000.).

With a lens, you have to have multiple elements (10 elements are common),
each with 2 surfaces. Optical glass is very expensive as it can have
no bubbles or shear (stress). You must then grind about 20 surfaces,
but they are typically spherical (with perhaps a couple of spherics,
and tolerance on the surfaces is about 1/2 wavelength. Then you must
anti reflection coat all surfaces, and accurately mount all lenses.
This add to a big expense.

A mirror system can have as little as 2 surfaces, the glass can have bubbles,
and you have only 2 mounts and 2 coatings. Optical tolerance is 1/2
that of a lens, about 1/4 wavelength (or better), but that is easy
these days. But one is flat (Newtonian), or hyperbolic secondaries,
and typically parabolic primaries. Still simpler. For a given
aperture, a mirror system is many times less costly to produce.

Why no manufacturer has done this is a good question.

After doing the above (web page) experiments to show I had the optical
quality, I bought a Canon 500 mm f/4 L IS telephoto because of the
autofocus capability. It was worth every penny--it is a wonderful
and versatile lens, and portable too.

Roger
Photos at: http://www.clarkvision.com
Anonymous
August 13, 2005 11:58:40 AM

Archived from groups: rec.photo.digital (More info?)

Justin Thyme wrote:
> I'm just wondering, why are mirror lenses so bad? Is it because they are
> poorly made or is it an inherent problem. I would have thought that apart
> from the donut shaped highlights they should be better than traditional
> lenses. In telescopes, Newtonian reflectors are far superior to refractor
> telescopes. A mirror lens is very similar to a Newtonian reflector, while a
> traditional lens is more similar to a refractor telescope. So why is it that
> cameras reverse the conventional wisdom learnt from astronomy?

Telescopes differ from cameras in that the field of view required is
very tiny, just a few degrees at most and usually less than 1 degree.

Compare that to a camera, where the field of view is much larger. With
a multi-lens system, those many curved surfaces are needed to
compensate for aberations and the fact that much of the image is not
being viewed straight-on.

A mirrored system is limited to just 2 curved surfaces. That's fine
when most of the image is being viewed from straight ahead, as in
astronomical work, but for large fields of view needed for cameras,
photos would be blurred away from the center of the picture.

Mark
August 13, 2005 11:58:41 AM

Archived from groups: rec.photo.digital (More info?)

On Sat, 13 Aug 2005 01:36:09 GMT, "Dirty Harry" <nopsam@nojust.com>
wrote:

>
>"Justin Thyme" <pleasedontspamme@nowhere.com> wrote in message
>news:lY8Le.514$yt1.8207@nnrp1.ozemail.com.au...
>> I'm just wondering, why are mirror lenses so bad? Is it because they are
>> poorly made or is it an inherent problem. I would have thought that apart
>> from the donut shaped highlights they should be better than traditional
>> lenses. In telescopes, Newtonian reflectors are far superior to refractor
>> telescopes. A mirror lens is very similar to a Newtonian reflector, while
>a
>> traditional lens is more similar to a refractor telescope. So why is it
>that
>> cameras reverse the conventional wisdom learnt from astronomy?
>
>Could someone please post some large images from one of these things so we
>can actually see what the heck we are dealing with?
>

Pick a test group, i'll put one up, full rez from a canon d rebel and
the vivitar 600. Hope I got it focused close to right.
August 13, 2005 11:58:41 AM

Archived from groups: rec.photo.digital (More info?)

On Sat, 13 Aug 2005 01:36:09 GMT, "Dirty Harry" <nopsam@nojust.com>
wrote:

>
>"Justin Thyme" <pleasedontspamme@nowhere.com> wrote in message
>news:lY8Le.514$yt1.8207@nnrp1.ozemail.com.au...
>> I'm just wondering, why are mirror lenses so bad? Is it because they are
>> poorly made or is it an inherent problem. I would have thought that apart
>> from the donut shaped highlights they should be better than traditional
>> lenses. In telescopes, Newtonian reflectors are far superior to refractor
>> telescopes. A mirror lens is very similar to a Newtonian reflector, while
>a
>> traditional lens is more similar to a refractor telescope. So why is it
>that
>> cameras reverse the conventional wisdom learnt from astronomy?
>
>Could someone please post some large images from one of these things so we
>can actually see what the heck we are dealing with?
>


Okay, I have one under exposed, one over exposed. Where do you want
them?
August 13, 2005 11:58:41 AM

Archived from groups: rec.photo.digital (More info?)

Deep Reset wrote:

>
> "Justin Thyme" <pleasedontspamme@nowhere.com> wrote in message
> news:lY8Le.514$yt1.8207@nnrp1.ozemail.com.au...
>> I'm just wondering, why are mirror lenses so bad? Is it because they are
>> poorly made or is it an inherent problem. I would have thought that apart
>> from the donut shaped highlights they should be better than traditional
>> lenses. In telescopes, Newtonian reflectors are far superior to refractor
>> telescopes. A mirror lens is very similar to a Newtonian reflector, while
>> a traditional lens is more similar to a refractor telescope. So why is it
>> that cameras reverse the conventional wisdom learnt from astronomy?


>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

The donut highlights you are probably referring to is the out of focus
background light that passes through the optical path revealing a shadow of
the secondary mirror in the optical system train. Just as any light with a
regular lens will show up out of focus as a big light blob. Nothing wrong
with these in telephoto shots. Mirror lenses are not as bad as you think,
they may be bulkier in size and weight and the f/ratio is typically pretty
high. They may also suffer much less achromatic aberration then a typical
lens that may require many more compound lens groups. They are also cheaper
to manufacture because of fewer optical surfaces to grind, polish,and
antiglare coat.

Richard
August 13, 2005 11:58:42 AM

Archived from groups: rec.photo.digital (More info?)

On Sat, 13 Aug 2005 01:46:38 GMT, Charles <ckraft@SAMTRAP.west.net>
wrote:

never mind, it's awful. Let me try to get a good shot.
Anonymous
August 13, 2005 12:25:28 PM

Archived from groups: rec.photo.digital (More info?)

In article <42FD5E37.4000001@qwest.net>, change username to rnclark
<username@qwest.net> wrote:

> The concept that they are fixed f/ratio is bogus. You can add
> a diaphragm that is off-center so as you close down, you
> crop out the central obstruction.


Is this theoretical, or are you thinking of a production lens I'm not
aware of? I've sold and/or owned a lot of mirror lenses over the years,
and they were all fixed aperture.
Anonymous
August 13, 2005 1:30:20 PM

Archived from groups: rec.photo.digital (More info?)

"Charles" <ckraft@SAMTRAP.west.net> wrote in message
news:63lqf1h0ku3jne61nv42aeq4e3oprcceho@4ax.com...
> On Sat, 13 Aug 2005 01:36:09 GMT, "Dirty Harry" <nopsam@nojust.com>
> wrote:
>
> >
> >"Justin Thyme" <pleasedontspamme@nowhere.com> wrote in message
> >news:lY8Le.514$yt1.8207@nnrp1.ozemail.com.au...
> >> I'm just wondering, why are mirror lenses so bad? Is it because they
are
> >> poorly made or is it an inherent problem. I would have thought that
apart
> >> from the donut shaped highlights they should be better than traditional
> >> lenses. In telescopes, Newtonian reflectors are far superior to
refractor
> >> telescopes. A mirror lens is very similar to a Newtonian reflector,
while
> >a
> >> traditional lens is more similar to a refractor telescope. So why is it
> >that
> >> cameras reverse the conventional wisdom learnt from astronomy?
> >
> >Could someone please post some large images from one of these things so
we
> >can actually see what the heck we are dealing with?
> >
>
>
> Okay, I have one under exposed, one over exposed. Where do you want
> them?
How about a link right here? If you want you could email and I can host
them.
--
www.harryphotos.com
Anonymous
August 13, 2005 1:33:37 PM

Archived from groups: rec.photo.digital (More info?)

"Dirty Harry" <nopsam@nojust.com> wrote in message
news:M4jLe.204222$s54.129933@pd7tw2no...
>
> "Charles" <ckraft@SAMTRAP.west.net> wrote in message
> news:63lqf1h0ku3jne61nv42aeq4e3oprcceho@4ax.com...
> > On Sat, 13 Aug 2005 01:36:09 GMT, "Dirty Harry" <nopsam@nojust.com>
> > wrote:
> >
> > >
> > >"Justin Thyme" <pleasedontspamme@nowhere.com> wrote in message
> > >news:lY8Le.514$yt1.8207@nnrp1.ozemail.com.au...
> > >> I'm just wondering, why are mirror lenses so bad? Is it because they
> are
> > >> poorly made or is it an inherent problem. I would have thought that
> apart
> > >> from the donut shaped highlights they should be better than
traditional
> > >> lenses. In telescopes, Newtonian reflectors are far superior to
> refractor
> > >> telescopes. A mirror lens is very similar to a Newtonian reflector,
> while
> > >a
> > >> traditional lens is more similar to a refractor telescope. So why is
it
> > >that
> > >> cameras reverse the conventional wisdom learnt from astronomy?
> > >
> > >Could someone please post some large images from one of these things so
> we
> > >can actually see what the heck we are dealing with?
> > >
> >
> >
> > Okay, I have one under exposed, one over exposed. Where do you want
> > them?
> How about a link right here? If you want you could email and I can host
> them.
> --
> www.harryphotos.com

And how much was that lens btw? Did you pick it up for shots of the stars
or something else?
Anonymous
August 13, 2005 3:28:37 PM

Archived from groups: rec.photo.digital (More info?)

In article <d8cLe.1843$vj.1781@pd7tw1no>,
Dirty Harry <nopsam@nojust.com> wrote:
>
>Could someone please post some large images from one of these things so we
>can actually see what the heck we are dealing with?

I didn't keep many of the shots I took with mine, but here's a test shot.
Full res JPEG, taken on a Canon EOS D30:

http://homepage.ntlworld.com/narcissus/mirrorlens.jpg

Notice how it's already struggling to pprovide the sensor with a decently
sharp image, even though the D30 was only a 3 megapixel camera. Notice also
the lack of decent contrast. Notice also the utterly vile way it's rendered
the slightly out-of-focus eyebrows.

This was a Sigma 600mm f/8 mirror lens.
Anonymous
August 13, 2005 3:29:53 PM

Archived from groups: rec.photo.digital (More info?)

Scott Schuckert wrote:

> In article <42FD5E37.4000001@qwest.net>, change username to rnclark
> <username@qwest.net> wrote:
>
>
>>The concept that they are fixed f/ratio is bogus. You can add
>>a diaphragm that is off-center so as you close down, you
>>crop out the central obstruction.
>
>
>
> Is this theoretical, or are you thinking of a production lens I'm not
> aware of? I've sold and/or owned a lot of mirror lenses over the years,
> and they were all fixed aperture.

I've not seen a production lens with it, but it is not hard.
I've done it on my telescopes.

Roger
Anonymous
August 13, 2005 3:38:58 PM

Archived from groups: rec.photo.digital (More info?)

redbelly wrote:

> Telescopes differ from cameras in that the field of view required is
> very tiny, just a few degrees at most and usually less than 1 degree.

1) We are talking telephotos here, so this is not relevant.
2) It is wrong. There are telescope designs that give wide field that
are diffraction limited. Perhaps not as wide as a wide angle
lens, but equivalent to a 100mm lens on a 35mm camera.

> Compare that to a camera, where the field of view is much larger. With
> a multi-lens system, those many curved surfaces are needed to
> compensate for aberations and the fact that much of the image is not
> being viewed straight-on.
>
> A mirrored system is limited to just 2 curved surfaces. That's fine
> when most of the image is being viewed from straight ahead, as in
> astronomical work, but for large fields of view needed for cameras,
> photos would be blurred away from the center of the picture.

No, this too is incorrect. There are single mirror as well as
multiple mirror designs.

Mirror systems in moderate to long focal lengths generally outperform
lenses (e.g. that is why big astronomical telescopes are mirror systems).

Some designs are not straight through designs, so people might find them
awkward for photography (e.g. a Newtonian). But it could also
be an advantage: for wildlife, you would not be facing the animal,
so it might have greater comfort in you being close, is it would
perceive you were looking at something else.

Roger
August 13, 2005 7:50:36 PM

Archived from groups: rec.photo.digital (More info?)

On Sat, 13 Aug 2005 09:33:37 GMT, "Dirty Harry" <nopsam@nojust.com>
wrote:


>> > >
>> >
>> >
>> > Okay, I have one under exposed, one over exposed. Where do you want
>> > them?
>> How about a link right here? If you want you could email and I can host
>> them.
>> --
>> www.harryphotos.com
>
>And how much was that lens btw? Did you pick it up for shots of the stars
>or something else?
>

Okay, I found my good tripod. Let me go out and get some more
representative pictures today, I'll try to get some donuts in ther as
well.

the lens was somewhere between 4 to 6 hunded, this was back in the
1970's so I don't remember exactly. It filled a need that I have one,
I didn't have any real use for it beyond just wanting it to make my
life complete. It didn't do that either. It is the old Vivitar
Series one cat lens.
August 13, 2005 7:51:45 PM

Archived from groups: rec.photo.digital (More info?)

On Sat, 13 Aug 2005 11:28:37 GMT, Chris Brown
<cpbrown@ntlworld.no_uce_please.com> wrote:

>In article <d8cLe.1843$vj.1781@pd7tw1no>,
>Dirty Harry <nopsam@nojust.com> wrote:
>>
>>Could someone please post some large images from one of these things so we
>>can actually see what the heck we are dealing with?
>
>I didn't keep many of the shots I took with mine, but here's a test shot.
>Full res JPEG, taken on a Canon EOS D30:
>
>http://homepage.ntlworld.com/narcissus/mirrorlens.jpg
>
>Notice how it's already struggling to pprovide the sensor with a decently
>sharp image, even though the D30 was only a 3 megapixel camera. Notice also
>the lack of decent contrast. Notice also the utterly vile way it's rendered
>the slightly out-of-focus eyebrows.
>
>This was a Sigma 600mm f/8 mirror lens.

I notice that it will focus fairly close. Mine won't.
August 13, 2005 7:53:26 PM

Archived from groups: rec.photo.digital (More info?)

On Sat, 13 Aug 2005 08:25:28 -0400, Scott Schuckert <not@aol.com>
wrote:

>In article <42FD5E37.4000001@qwest.net>, change username to rnclark
><username@qwest.net> wrote:
>
>> The concept that they are fixed f/ratio is bogus. You can add
>> a diaphragm that is off-center so as you close down, you
>> crop out the central obstruction.
>
>
>Is this theoretical, or are you thinking of a production lens I'm not
>aware of? I've sold and/or owned a lot of mirror lenses over the years,
>and they were all fixed aperture.


This is familiar, I've read about it. Take the production lens, put a
hole in the lens cap, off center, but circular. It reduces the
effective aperature. Never tried it, though.
August 13, 2005 10:16:06 PM

Archived from groups: rec.photo.digital (More info?)

On Sat, 13 Aug 2005 11:38:58 -0600, "Roger N. Clark (change username
to rnclark)" <username@qwest.net> wrote:

>redbelly wrote:
>
>> Telescopes differ from cameras in that the field of view required is
>> very tiny, just a few degrees at most and usually less than 1 degree.
>
>1) We are talking telephotos here, so this is not relevant.
>2) It is wrong. There are telescope designs that give wide field that
> are diffraction limited. Perhaps not as wide as a wide angle
> lens, but equivalent to a 100mm lens on a 35mm camera.
>
>> Compare that to a camera, where the field of view is much larger. With
>> a multi-lens system, those many curved surfaces are needed to
>> compensate for aberations and the fact that much of the image is not
>> being viewed straight-on.
>>
>> A mirrored system is limited to just 2 curved surfaces. That's fine
>> when most of the image is being viewed from straight ahead, as in
>> astronomical work, but for large fields of view needed for cameras,
>> photos would be blurred away from the center of the picture.
>
>No, this too is incorrect. There are single mirror as well as
>multiple mirror designs.
>
>Mirror systems in moderate to long focal lengths generally outperform
>lenses (e.g. that is why big astronomical telescopes are mirror systems).

Are you sure about this part? For a given cost, yes, mirrors are
cheaperand above a certain size, refractors are difficult/impossible
to build, but for smaller stuff, say below six inches diameter, I had
heard that refracotor were better in terms of image quality. I expect
most people here know more about this than I do.



>
>Some designs are not straight through designs, so people might find them
>awkward for photography (e.g. a Newtonian). But it could also
>be an advantage: for wildlife, you would not be facing the animal,
>so it might have greater comfort in you being close, is it would
>perceive you were looking at something else.
>
>Roger
Anonymous
August 13, 2005 11:29:47 PM

Archived from groups: rec.photo.digital (More info?)

In article <3o5sf1did2g9g68s6eq6bjmnqvmsn2arm4@4ax.com>,
Charles <ckraft@SAMTRAP.west.net> wrote:
>On Sat, 13 Aug 2005 11:28:37 GMT, Chris Brown
><cpbrown@ntlworld.no_uce_please.com> wrote:
>>
>>http://homepage.ntlworld.com/narcissus/mirrorlens.jpg
>>
>>Notice how it's already struggling to pprovide the sensor with a decently
>>sharp image, even though the D30 was only a 3 megapixel camera. Notice also
>>the lack of decent contrast. Notice also the utterly vile way it's rendered
>>the slightly out-of-focus eyebrows.
>>
>>This was a Sigma 600mm f/8 mirror lens.
>
>I notice that it will focus fairly close. Mine won't.

2 metres is the closest focusing distance on that lens, for some definitions
of "focus" which don't seem to be too fussy about resolving things
especially sharply.
August 14, 2005 1:59:54 AM

Archived from groups: rec.photo.digital (More info?)

On Sat, 13 Aug 2005 19:29:47 GMT, Chris Brown
<cpbrown@ntlworld.no_uce_please.com> wrote:

>In article <3o5sf1did2g9g68s6eq6bjmnqvmsn2arm4@4ax.com>,
>Charles <ckraft@SAMTRAP.west.net> wrote:
>>On Sat, 13 Aug 2005 11:28:37 GMT, Chris Brown
>><cpbrown@ntlworld.no_uce_please.com> wrote:
>>>
>>>http://homepage.ntlworld.com/narcissus/mirrorlens.jpg
>>>
>>>Notice how it's already struggling to pprovide the sensor with a decently
>>>sharp image, even though the D30 was only a 3 megapixel camera. Notice also
>>>the lack of decent contrast. Notice also the utterly vile way it's rendered
>>>the slightly out-of-focus eyebrows.
>>>
>>>This was a Sigma 600mm f/8 mirror lens.
>>
>>I notice that it will focus fairly close. Mine won't.
>
>2 metres is the closest focusing distance on that lens, for some definitions
>of "focus" which don't seem to be too fussy about resolving things
>especially sharply.


Mine will only go down to about 23 feet. That was a limitation I
never liked.
Anonymous
August 14, 2005 12:01:00 PM

Archived from groups: rec.photo.digital (More info?)

Roger N. Clark (change username to rnclark) wrote:

> 1) We are talking telephotos here, so this is not relevant.

Wrong. The OP brought up telescopes, and has even discussed them
further in a followup post.

> 2) It is wrong. There are telescope designs that give wide field that
> are diffraction limited. Perhaps not as wide as a wide angle
> lens, but equivalent to a 100mm lens on a 35mm camera.

Could you provide an example? I am curious.

> There are single mirror as well as
> multiple mirror designs.

Wouldn't the more-than-two mirror designs need to have the extra
mirrors off-axis, giving astigmatism issues that must be compensated
for? If you could provide a counterexample, instead of a blanket "No,
you're wrong", it would be appreciated.

> Mirror systems in moderate to long focal lengths generally outperform
> lenses (e.g. that is why big astronomical telescopes are mirror systems).

And the big astronomical scopes often employ aspheric corrector lenses,
in which case they are not strictly mirror-only systems. (For example,
the telescope used for the Sloan Digital Sky Survey)

Mark
Anonymous
August 14, 2005 12:12:57 PM

Archived from groups: rec.photo.digital (More info?)

"Charles" <ckraft@SAMTRAP.west.net> wrote in message
news:nudsf15qitel9rv3v1ivvcfa2c9phtmuet@4ax.com...
> On Sat, 13 Aug 2005 11:38:58 -0600, "Roger N. Clark (change username
> to rnclark)" <username@qwest.net> wrote:

>>Mirror systems in moderate to long focal lengths generally outperform
>>lenses (e.g. that is why big astronomical telescopes are mirror systems).
>
> Are you sure about this part? For a given cost, yes, mirrors are
> cheaperand above a certain size, refractors are difficult/impossible
> to build, but for smaller stuff, say below six inches diameter, I had
> heard that refracotor were better in terms of image quality. I expect
> most people here know more about this than I do.
Yes for long focal lengths and wide apertures, reflector telescopes
significantly outperform refractor designs. There are some good refractors
made, but they are dreadfully expensive. The chromatic aberations caused by
lenses is the big problem. The only glass in a reflector is the eyepiece,
which reduces aberations significantly. The Cassegrain style reflectors
aren't quite as good as Newtonian reflectors, but not significantly worse.
When using it as a big camera lens you don't even have that. Thats why I
would have thought a mirror would be an improvement over a lens. Practically
every astronomical telescope facility mostly uses reflector telescopes,
which i'm sure is for reasons other than cost alone. These days most
high-end telescopes are used with CCD camera gear, effectively making the
telescope a long camera lens.
>
>
>
>>
>>Some designs are not straight through designs, so people might find them
>>awkward for photography (e.g. a Newtonian). But it could also
>>be an advantage: for wildlife, you would not be facing the animal,
>>so it might have greater comfort in you being close, is it would
>>perceive you were looking at something else.
>>
>>Roger
>
Anonymous
August 14, 2005 12:12:58 PM

Archived from groups: rec.photo.digital (More info?)

Justin Thyme wrote:

> "Charles" <ckraft@SAMTRAP.west.net> wrote in message
> news:nudsf15qitel9rv3v1ivvcfa2c9phtmuet@4ax.com...
>
>>On Sat, 13 Aug 2005 11:38:58 -0600, "Roger N. Clark (change username
>>to rnclark)" <username@qwest.net> wrote:
>
>
>>>Mirror systems in moderate to long focal lengths generally outperform
>>>lenses (e.g. that is why big astronomical telescopes are mirror systems).
>>
>>Are you sure about this part?

Yes, but note I did say long focal lengths. You can make specific designs
for wider field but the designs generally are restrictive for general
use photography (e.g. poor close focusing range).

> The Cassegrain style reflectors
> aren't quite as good as Newtonian reflectors, but not significantly worse.

The major aberration in Cassegrains and Newtonians is coma.
For a given f/ratio, the coma in the two are identical.
Cassegrains are generally harder to baffle, but not excessively so.

> When using it as a big camera lens you don't even have that. Thats why I
> would have thought a mirror would be an improvement over a lens. Practically
> every astronomical telescope facility mostly uses reflector telescopes,
> which i'm sure is for reasons other than cost alone. These days most
> high-end telescopes are used with CCD camera gear, effectively making the
> telescope a long camera lens.

A big reason is wavelength coverage. A lens with chromatic aberration just
couldn't cut it in any modern astronomical observatory.

Roger
Anonymous
August 14, 2005 2:21:42 PM

Archived from groups: rec.photo.digital (More info?)

Roger N. Clark (change username to rnclark) wrote:
> Scott Schuckert wrote:
>
>> In article <42FD5E37.4000001@qwest.net>, change username to rnclark
>> <username@qwest.net> wrote:
>>
>>
>>> The concept that they are fixed f/ratio is bogus. You can add
>>> a diaphragm that is off-center so as you close down, you
>>> crop out the central obstruction.
>>
>>
>>
>>
>> Is this theoretical, or are you thinking of a production lens I'm not
>> aware of? I've sold and/or owned a lot of mirror lenses over the years,
>> and they were all fixed aperture.
>
>
> I've not seen a production lens with it, but it is not hard.
> I've done it on my telescopes.
>
> Roger
A problem is that such a lens would be normally operating off-axis, with
a big increase in aberrations that are field angle dependent. You may
not notice it in a telescope, but you would in a normal image.
Anonymous
August 14, 2005 2:29:52 PM

Archived from groups: rec.photo.digital (More info?)

Don Stauffer wrote:

> Roger N. Clark (change username to rnclark) wrote:
>
>> Scott Schuckert wrote:
>>
>>> In article <42FD5E37.4000001@qwest.net>, change username to rnclark
>>> <username@qwest.net> wrote:
>>>
>>>
>>>> The concept that they are fixed f/ratio is bogus. You can add
>>>> a diaphragm that is off-center so as you close down, you
>>>> crop out the central obstruction.
>>>
>>>
>>>
>>>
>>>
>>> Is this theoretical, or are you thinking of a production lens I'm not
>>> aware of? I've sold and/or owned a lot of mirror lenses over the years,
>>> and they were all fixed aperture.
>>
>>
>>
>> I've not seen a production lens with it, but it is not hard.
>> I've done it on my telescopes.
>>
>> Roger
>
> A problem is that such a lens would be normally operating off-axis, with
> a big increase in aberrations that are field angle dependent. You may
> not notice it in a telescope, but you would in a normal image.

Actually, in a diffraction limited system, you would notice it in
a telescope. Good telescopes are diffraction limited. If they
are diffraction limited full aperture, putting in an aperture stop
does not introduce aberrations that make them not diffraction limited.
In fact adding an aperture stop reduces aberrations at the expense of
increased diffraction.

Roger
Anonymous
August 14, 2005 3:04:17 PM

Archived from groups: rec.photo.digital (More info?)

redbelly wrote:

> Roger N. Clark (change username to rnclark) wrote:
>
>
>>1) We are talking telephotos here, so this is not relevant.
>
>
> Wrong. The OP brought up telescopes, and has even discussed them
> further in a followup post.
>
>
>>2) It is wrong. There are telescope designs that give wide field that
>> are diffraction limited. Perhaps not as wide as a wide angle
>> lens, but equivalent to a 100mm lens on a 35mm camera.
>
>
> Could you provide an example? I am curious.

e.g. Schmidt cameras:
http://www.answers.com/topic/schmidt-camera-1

Cameras with 20+ degree fields of view
with f/1 speed have been built. E.g., check the
48-inch (aperture) Schmidt camera at Palomar Observatory
is f/2.5 and can take 14-inch square film covers
7 degrees.

http://www.astro.caltech.edu/palomarnew/18images.html


> Wouldn't the more-than-two mirror designs need to have the extra
> mirrors off-axis, giving astigmatism issues that must be compensated
> for? If you could provide a counterexample, instead of a blanket "No,
> you're wrong", it would be appreciated.

Not necessarily off axis, but there are designs that are.
Examples:
on axis:
http://bobmay.astronomy.net/mersenne/mersntop.htm
off axis:
http://www.seds.org/~spider/scopes/schiefi.html

> And the big astronomical scopes often employ aspheric corrector lenses,
> in which case they are not strictly mirror-only systems. (For example,
> the telescope used for the Sloan Digital Sky Survey)

While true, as with the Schmidt design above, most observatory
telescopes have no lenses. Those with lenses are limited to
optical wavelengths. Mirrors are used because 1) quality,
no chromatic aberrations, and no absorption from glass.
The vast majority of observatory instruments are mirror only
systems. The Schmidt telescopes have a very thin corrector
plate that does no focusing, only controls aberrations
like spherical (its main function) and has virtually no
chromatic aberration.

Roger
Anonymous
August 14, 2005 5:23:32 PM

Archived from groups: rec.photo.digital (More info?)

[A complimentary Cc of this posting was sent to
Roger N. Clark (change username to rnclark)
<username@qwest.net>], who wrote in article <42FD5E37.4000001@qwest.net>:
> With a lens, you have to have multiple elements (10 elements are common),
> each with 2 surfaces. Optical glass is very expensive as it can have
> no bubbles or shear (stress). You must then grind about 20 surfaces,
> but they are typically spherical (with perhaps a couple of spherics,
> and tolerance on the surfaces is about 1/2 wavelength.

[AFAIK, an "excellent" lens should have difference of optical paths of
order of magnitude 1/40 wavelength (for green light). But this is
applicable equally to refrax and reflex designs, so this 20x
difference may be factored out of the discussion.]

With refraction coefficient 1.6, this translates to 1/25 wavelength of
tolerance of SUM of glass thickness (over all 10 elements). So I
would guess the tolerance of a surface should be close to 1/110 of
wavelength. [Sounds silly, right?] This is compensated by cemented
elements having much easier to achieve tolerance (due to small
difference in refraction coefficients).

[I did not take into account obliqueness of rays coming into an
optical surface - but the contribution is the same in reflex and
refrax designs.]

> A mirror system can have as little as 2 surfaces, the glass can have bubbles,
> and you have only 2 mounts and 2 coatings. Optical tolerance is 1/2
> that of a lens, about 1/4 wavelength (or better)

You forget about refraction coefficient; the actual number turns out
to be closer to 1/3.5. But I have no idea how these tolerances
translate to cost...

> Still simpler. For a given
> aperture, a mirror system is many times less costly to produce.

Russian reflex designs were dirt cheap 15 years ago. IIRC, now, when
the market can bear more, they became as expensive as the Western
ones.

Hope this helps,
Ilya
Anonymous
August 14, 2005 5:23:33 PM

Archived from groups: rec.photo.digital (More info?)

Ilya Zakharevich wrote:

> [A complimentary Cc of this posting was sent to
> Roger N. Clark (change username to rnclark)
> <username@qwest.net>], who wrote in article <42FD5E37.4000001@qwest.net>:
>
>>With a lens, you have to have multiple elements (10 elements are common),
>>each with 2 surfaces. Optical glass is very expensive as it can have
>>no bubbles or shear (stress). You must then grind about 20 surfaces,
>>but they are typically spherical (with perhaps a couple of spherics,
>>and tolerance on the surfaces is about 1/2 wavelength.
>
>
> [AFAIK, an "excellent" lens should have difference of optical paths of
> order of magnitude 1/40 wavelength (for green light). But this is
> applicable equally to refrax and reflex designs, so this 20x
> difference may be factored out of the discussion.]
>
> With refraction coefficient 1.6, this translates to 1/25 wavelength of
> tolerance of SUM of glass thickness (over all 10 elements). So I
> would guess the tolerance of a surface should be close to 1/110 of
> wavelength. [Sounds silly, right?] This is compensated by cemented
> elements having much easier to achieve tolerance (due to small
> difference in refraction coefficients).
>
> [I did not take into account obliqueness of rays coming into an
> optical surface - but the contribution is the same in reflex and
> refrax designs.]
>
>
>>A mirror system can have as little as 2 surfaces, the glass can have bubbles,
>>and you have only 2 mounts and 2 coatings. Optical tolerance is 1/2
>>that of a lens, about 1/4 wavelength (or better)
>
>
> You forget about refraction coefficient; the actual number turns out
> to be closer to 1/3.5. But I have no idea how these tolerances
> translate to cost...
>
>
>>Still simpler. For a given
>>aperture, a mirror system is many times less costly to produce.
>
>
> Russian reflex designs were dirt cheap 15 years ago. IIRC, now, when
> the market can bear more, they became as expensive as the Western
> ones.
>
> Hope this helps,
> Ilya

Uh-OH. This is like the 1960 HI-FI stereo specifications.
I suggest you read "Star Testing Astronomical Telescopes" by
H. R. Suiter, Willmann-Bell, Richmond, 1994. In particular
read Chapter 1. There are many ways of stating accuracy
requirements, many meaning the same thing. And there are
many factors affecting the final image, including alignment.
1/110 wavelength is 5000 angstroms / 110 = 45 angstroms
which is a ridiculous requirement, and paging through Suiter's
book has perhaps never been achieved. The common standard for
superb optics seems to be 1/20 wave for mirrors. But again it
depends on how you define it. For example, the 1/27 wave
RMS Murechal's criterion is equal to 1/4 wavelength of
spherical aberration (Suiter, page 7).

Roger
Anonymous
August 14, 2005 7:04:14 PM

Archived from groups: rec.photo.digital (More info?)

In article <1401t2-o7d.ln1@narcissus.dyndns.org>,
Chris Brown <cpbrown@ntlworld.no_uce_please.com> wrote:
>In article <d8cLe.1843$vj.1781@pd7tw1no>,
>Dirty Harry <nopsam@nojust.com> wrote:
>>
>>Could someone please post some large images from one of these things so we
>>can actually see what the heck we are dealing with?
>
>I didn't keep many of the shots I took with mine, but here's a test shot.
>Full res JPEG, taken on a Canon EOS D30:
>
>http://homepage.ntlworld.com/narcissus/mirrorlens.jpg
>
>Notice how it's already struggling to pprovide the sensor with a decently
>sharp image, even though the D30 was only a 3 megapixel camera. Notice also
>the lack of decent contrast. Notice also the utterly vile way it's rendered
>the slightly out-of-focus eyebrows.
>
>This was a Sigma 600mm f/8 mirror lens.

Just for fun, I took a couple of pictures with an ancient Nikkor 50cm/5.0,
a D1 and a TC-14:

<http://misc.hq.phicoh.net/wester/w1.jpg&gt;
<http://misc.hq.phicoh.net/wester/w2.jpg&gt;
<http://misc.hq.phicoh.net/wester/w3.jpg&gt;

The last one is without the TC-14.

Focussing is very hard with that lens, and my tripod is not really stable
enough, but the lack of weight is quite nice.


--
That was it. Done. The faulty Monk was turned out into the desert where it
could believe what it liked, including the idea that it had been hard done
by. It was allowed to keep its horse, since horses were so cheap to make.
-- Douglas Adams in Dirk Gently's Holistic Detective Agency
Anonymous
August 15, 2005 12:33:55 AM

Archived from groups: rec.photo.digital (More info?)

In article <tf9qifs7dv7u4ptuo8bamlcbv0@inews_id.stereo.hq.phicoh.net>,
Philip Homburg <philip@pch.home.cs.vu.nl> wrote:
>
>Just for fun, I took a couple of pictures with an ancient Nikkor 50cm/5.0,
>a D1 and a TC-14:
>
><http://misc.hq.phicoh.net/wester/w1.jpg&gt;
><http://misc.hq.phicoh.net/wester/w2.jpg&gt;
><http://misc.hq.phicoh.net/wester/w3.jpg&gt;
>
>The last one is without the TC-14.
>
>Focussing is very hard with that lens, and my tripod is not really stable
>enough, but the lack of weight is quite nice.

Do you recall if the air was somewhat hazy, or is the slight "fuzziness" due
to the lens?
Anonymous
August 15, 2005 4:02:31 AM

Archived from groups: rec.photo.digital (More info?)

In article <tpk4t2-vn6.ln1@narcissus.dyndns.org>,
Chris Brown <cpbrown@ntlworld.no_uce_please.com> wrote:
>In article <tf9qifs7dv7u4ptuo8bamlcbv0@inews_id.stereo.hq.phicoh.net>,
>Philip Homburg <philip@pch.home.cs.vu.nl> wrote:
>>Focussing is very hard with that lens, and my tripod is not really stable
>>enough, but the lack of weight is quite nice.
>
>Do you recall if the air was somewhat hazy, or is the slight "fuzziness" due
>to the lens?

Probably a bit of both. Remember, this lens was produced in the 1960s.
I doubt that any (long/Nikon) lens from those days lives up to today's
standards. But, it did rain a bit from time to time during the time I was
taking these pictures.


--
That was it. Done. The faulty Monk was turned out into the desert where it
could believe what it liked, including the idea that it had been hard done
by. It was allowed to keep its horse, since horses were so cheap to make.
-- Douglas Adams in Dirk Gently's Holistic Detective Agency
!