Vivitar 500mm f/8 Mirror Lens

"howard" wrote in message
news:1111268056.90ea9ef6140ab2a853f3e4acc1a97203@t eranews...
How good are these mirror lens? On a digital SLR it would be a 750mm f/8
lens, which wouldn't be too bad.
Any info would be greatly appreciated.

Howard



I think most all the 500 mm f/8 cats are alike. I have a Nikkor, and I only
use it when I have to have that kind of focal length. I can't afford the
several thousand dollars it would take for me to buy a really good 500 mm
lens, so I picked up my Nikkor mirror lens used for around $200. A lot
depends on the type of photography you are doing. If you do your own B&W
darkroom work, you would be better off using a good quality 200 or 300 mm
lens, and blowing up your pictures in the darkroom. But if you are doing
color slides, as I do, that's impractical, so you go with what you can
afford.....These mirror lenses generally lack good contrast, and should only
be used if there is no way you can get closer to your subject.

How good are these mirror lens? On a digital SLR it would be a 750mm f/8
lens, which wouldn't be too bad.
Any info would be greatly appreciated.

Howard

William Graham wrote:

afford.....These mirror lenses generally lack good contrast, and should
only be used if there is no way you can get closer to your subject.

Any idea why they lack contrast?

Pete

--
http://www.petezilla.co.uk

David Dyer-Bennet wrote:
"William Graham" writes:


"Peter Chant" wrote in message

Well, I don't know for certain, but I will hazard a guess.....Contrast is
kind of a, "signal to noise ratio" in optics. IOW, the lack of contrast is
due to too much optical noise in the system. I believe this comes about
because of the mirrored surface in the lens. Mirrors generally only reflect
like 95% of the light that strikes them, so the real image (signal) is
dimmer and the signal to noise ratio is poorer as a result......


I doubt this theory a LOT. Astronomers use mirror-based optical
systems for all their most extreme observations; I doubt they're
inherently inferior to refractive systems.

I agree partially. But the real advantage of mirrors is gain. 95% of
the light from a 3, 5 or more meter across mirror is a lot of light.
Even if it were 50%, it would be a lot more than economically (and maybe
physically) practical with a refractor.

Cheers,
Alan


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"Alan Browne" wrote in message
...
David Dyer-Bennet wrote:
"William Graham" writes:


"Peter Chant" wrote in message

Well, I don't know for certain, but I will hazard a guess.....Contrast is
kind of a, "signal to noise ratio" in optics. IOW, the lack of contrast
is due to too much optical noise in the system. I believe this comes
about because of the mirrored surface in the lens. Mirrors generally only
reflect like 95% of the light that strikes them, so the real image
(signal) is dimmer and the signal to noise ratio is poorer as a
result......


I doubt this theory a LOT. Astronomers use mirror-based optical
systems for all their most extreme observations; I doubt they're
inherently inferior to refractive systems.

I agree partially. But the real advantage of mirrors is gain. 95% of the
light from a 3, 5 or more meter across mirror is a lot of light. Even if
it were 50%, it would be a lot more than economically (and maybe
physically) practical with a refractor.

Cheers,
Alan

Yes. Astronomers use mirrors because it would take forever to make and grind
100 inch refractors.....Even the reflectors take months to cool off before
the grinding process can begin......

William Graham wrote:

"Peter Chant" wrote in message
...
William Graham wrote:

afford.....These mirror lenses generally lack good contrast, and should
only be used if there is no way you can get closer to your subject.

Any idea why they lack contrast?

Pete

Well, I don't know for certain, but I will hazard a guess.....Contrast is
kind of a, "signal to noise ratio" in optics. IOW, the lack of contrast is
due to too much optical noise in the system. I believe this comes about
because of the mirrored surface in the lens. Mirrors generally only reflect
like 95% of the light that strikes them, so the real image (signal) is
dimmer and the signal to noise ratio is poorer as a result......

95% is as good or better than multi-element lenses, which typically lose
10% - 30% of light depending on the complexity of the design. This is
the reason 'T'-stops were invented, to take account of the light loss
from reflection and scatter within a lens system.

Colin

It is due to the large central obstruction i.e the secondary mirror which
is typically greater that 35% of the primary mirror. However there are some
very high end cadiotropic systems with about 20-25% central obstructions
used in astronomy and these have what is referred to as a 'refractor like
view' (an unobstructed lens system)with very little loss of contrast.

Michael

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Message posted via http://www.photokb.com

Alan Browne wrote:
David Dyer-Bennet wrote:
"William Graham" writes:


"Peter Chant" wrote in message

Well, I don't know for certain, but I will hazard a guess.....Contrast is
kind of a, "signal to noise ratio" in optics. IOW, the lack of contrast is
due to too much optical noise in the system. I believe this comes about
because of the mirrored surface in the lens. Mirrors generally only reflect
like 95% of the light that strikes them, so the real image (signal) is
dimmer and the signal to noise ratio is poorer as a result......


I doubt this theory a LOT. Astronomers use mirror-based optical
systems for all their most extreme observations; I doubt they're
inherently inferior to refractive systems.

I agree partially. But the real advantage of mirrors is gain. 95% of
the light from a 3, 5 or more meter across mirror is a lot of light.
Even if it were 50%, it would be a lot more than economically (and maybe
physically) practical with a refractor.

Reflectors passed the point where you can't make refractors of equal specs
a *LONG* time ago, never mind that you couldn't make even a lot smaller
ones to the same specs (diffraction limited optics). The largest refractor
in the world is ~ 1m in diameter.


Cheers,
Alan



--
Sander

+++ Out of cheese error +++