Are some mirror lenses better than others?

You don't know what you are talking about. I've owned all the gear, I
know exactly how they perform. I've also owned over 200 telescopes
ranging from $50 cheapos to $10,000 SCT's so I'm pretty familiar with
them. Vivitar Series 1 stuff is vastly over-rated, like many lenses
old, misty-eyed men fondly remember from days of yore. Modern lenses
of good quality walk all over them.

 

I would only be using a telephoto outdoors, for wildlife, landscapes,
and some of the larger astrophotography targets such as the moon.

 

This is probably obvious but the trade is always between having a faster
physically big refractive lens that will work well in low light and
allow faster shutter speeds for wildlife (and controlled depth of field)
against the compact but slow mirror lens which is hard to focus.

You should be able to pick reasonably priced manual focus long focal
length prime lenses by watching the secondhand lists for a while.

If you can force your camera to preview at f11 that will give you a
taste of what it is like focussing with an f10 mirror lens.

In astronomy everything is effectively at infinity so depth of field
issues do not matter so much but vibration and exposure time do.

 

After shopping around for a bit, it looks like the best ones I am
going to be able to find are the Nikon, the Tamron, and the Minolta,
all 500mm F/8. Wayne Grundy's review claims that the Tamron is
sharper than the Nikon; he did not review the Minolta. So, between
those three, does anybody have any strong feelings? The autofocus
of the Minolta/Sony will probably be useless to me, but if it is a
good lens otherwise, I will consider it. Sharpness/detail is my
primary consideration; I like to use the moon as a test subject,
if that is any help.

 

Possibly a little careless in post.

 

Nikon.

Here is Ken Rockwill's review.
Note: he states it is not great for photography. Note also it is manual
focus.

It is fairly inexpensive and I would not jump though hoops to sell it.
If someone is in the NY area, and they want to try before they buy,
fine. I have not made any effort to sell, nor will I, subject to the
above.

 

(Paul Ciszek) wrote in

No, any light source, specular reflection that is out of focus will
likely show them because what you are seeing is the central obstruction
(secondary mirror) that's in the centre of the mirror lens.

 

Where does infinity begin?

 

I have gone ahead and purchased the Tamron. The good news is, the
Adaptall adapter for micro-four-thirds supposedly guarantees the
ability to focus at infinity.

 

Out there.

 

Wow! Amazing that you have the time to researcg what to buy or to test
what you've bought at that kind of rate!

 

Hyperfocal infinity? Well, one of my standard tests for long lenses is
to photograph a large tower clock that's about two miles away. When my
500mm f8 reflex is focused on that the stuff behind it is clearly out
of focus. So it's certainly miles away.

 

Wow. Apparently only for astrophotography can I safely assume that things
are at infinity.

 

I'd suggest a more terrestrial daytime target like a pylon or church
spire. The moon won't show you the out of focus point spread function. A
picture of a bird in a tree almost certainly will.

I'd probably suggest looking for a used Tamron 500mm f8 then.

 

Especially whilst residing under a bridge.

He is in my kill file. I only see his tripe when someone replies.

 

For a given camera/lens/sensor working at the limit it scales with
(focal length)^2/(focal ratio)/(circle of confusion)

So 500^2/8/0.02 = 25/16 x 10^6 mm = 1500m

If the lens is close to diffraction limited and the sensor more than
6Mpixels it could easily be 2-3x longer.

I'm surprised it is quite that bad (but the hyperfocal infinity is
certainly of the order of a km for long lenses). I'd have expected it to
be just possible to get infinity and the church spire in focus on film
(OTOH for a digital SLR the circle of confusion is smaller).

http://en.wikipedia.org/wiki/Depth_of_field#Lens_DOF_scales

My back of the envelope numbers based on classic 35mm film gives about
3km as hyperfocal infinity for my old 1000mm f10. My favourite high
contrast test targets are a weather vane at 500m and a pylon at 2km.

I confess I have never really looked carefully at depth of field at long
range with these long lenses.

 

In my 35mm film shooting days my longest focal length was 300mm. I
printed up to A4ish in size, sometimes from a slightly cropped
image. I examined my best negs and slides with a low power microscope
made for that purpose.

When I started looking carefully at pixel level sharpness on 14MP crop
sensor images of long lenses (a zoom at 250mm, a 200mm prime
teleconverted to 400mm, and a 500mm f8 mirror lens) I was startled to
discover how far out was the conventional wisdom from my 35mm film
days about DoF, mirror slap vibration, hand-holdable shutter speeds
(no, my hands don't shake more than they used to , etc..

Now I sometimes print A2, almost always from a slightly cropped
image. In terms of the care I now have to take to get a really sharp
image at modern digital resolutions it's like the difference between
washing up dinner in the kitchen sink, and washing up the glassware in
a biochemistry lab.

 

Already ordered. ;-) This is the picture that convinced me:

http://www.flickr.com/photos/rain0975/3083568658/sizes/o/in/photostream/
(Be sure to view the original resolution.)

Now, this guy was using a teleconverter behind the Tamron, but I figure
the teleconverter can't add detail--the fact that he was able to get an
image with that much detail means that the Tamron can produce that much
detail, and all the teleconverter does is scale the available detail to
his sensor. Is my reasoning correct?

 

In a sense that could add some new detail in practice. A well made 500mm
f8 mirror lens operating at its aperture diffraction limit will need a
sensor with pixels smaller than 5um to be adequately sampled. Most times
atmospheric seeing and air currents will limit you first.

Allow for perfect conditions with a bit of additional oversampling and
3um pixels would be ideal. Adding a multiplier effectively matches the
lens to the sensor and prevents jaggies on edges. There is a lot of info
around about choice of oversampling for astronomical imaging.

Real astro CCDs don't have anything by way of anti-aliasing as the aim
is to capture every last possible bit of signal.

 

Paul-

I have the Minolta 500/F8 AF lens, as well as a Celestron C-90 (1000mm
F/11) with T-Mount adapter and a TeleSor 500/F8 with PK mount. They all
are reasonably sharp but have noticeably less contrast than the
non-mirror lenses I've tried with about the same focal lengths. They
represent a trade-off between image quality and lens size/weight.

Do not sell the Sony/Minolta Autofocus short. I found focusing to be a
major problem with those with no AF, since it could be quite critical.
The longer the lens, the harder it was to focus manually. (Back in the
day of split image focusing, a lens of F/8 or smaller could only be used
with ground glass focusing.)

Fred