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Are some mirror lenses better than others?

 
 
PeterN
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      07-13-2012
On 7/11/2012 11:16 PM, Paul Ciszek wrote:
> In article <4ffded33$0$5702$(E-Mail Removed)-secrets.com>,
> PeterN <(E-Mail Removed)> wrote:
>>>

>>
>> 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.

>
> 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.
>
>


Enjoy

--
Peter


 
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nospam
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      07-13-2012
In article <4fff7ea0$0$5718$(E-Mail Removed)-secrets.com>, PeterN
<(E-Mail Removed)> wrote:

> > Where does infinity begin?

>
> At its ending.


no, because there's always a bigger infinity that extends beyond it.
 
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Chris Malcolm
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      07-13-2012
Alan Browne <(E-Mail Removed)> wrote:
> On 2012-07-12 18:56 , Chris Malcolm wrote:
>> Alan Browne <(E-Mail Removed)> wrote:


>>> Everyone I know who bought a 500 reflex ended up using it less and less
>>> and it eventually kept time in its case, in the closet.

>>
>>> Alone. In the dark.

>>
>> That would have been me too, if I'd have had to focus mine
>> manually. Desperately tricky and slow to get good sharp focus
>> manually. But I have the Sony 500mm reflex which autofocuses. That
>> makes a huge difference to how useful the lens is.
>>
>> The remaining very tricky problem once focus has been solved is
>> aim. Like a powerful telescope a 500mm lens is difficult to aim,
>> especially on a crop sensor DSLR. That made catching moving wildlife
>> difficult, and birds in flight verging on the impossible.
>>
>> So I added a red dot gunsight to the lens. Amazing improvement! I can
>> instantly point the lens at what I want to shoot so easily that I can
>> track birds in flight accurately enough for the central spot autofocus
>> to lock and shoot -- the central spot focus sensor is the only one
>> which can focus this lens. Catching birds in flight or footballers in
>> mid kick has become easy and fun.
>>
>> It's become so easy to use it's now one of my often-carry-just-in-case
>> lenses. I've adapted my bag to have the right shaped pocket to carry
>> it with mounted gunsight.


> Interesting adaptation. One of those I spoke about who lost faith in
> the lens had the Minolta AF version as well. I'll mention it to him if
> I run into him (haven't seen him in a few years).


> Do you have a photo of your setup?


http://www.flickr.com/photos/chris_malcolm/6988604951/

That's a photo of the first trial implementation, looked pretty
scruffy, and wobbled enough that the sight often hd to be
recalibrated. I tightened it up with more glue and a pair of rubber
bands to clamp the tube more firmly on the lens barrel. That worked
well and reliably enough not to require calibration of the sight after
every removal and replacement on the lens. So I've painted it all
black to make it look neater until I get around to devising a metal
version, which I'll probably base on the cheap long metal lens hoods
you can get for the lens.

Here's a photo inlcuding that long lens hood on the end.

http://www.flickr.com/photos/chris_malcolm/6842482858/

That hood BTW is supposed to reduce flare and improve contrast. Well,
mirror lenses are inherently low in contrast, and also seem to be
unusually prone to flare. But adding that hood didn't noticeably
reduce flare or improve contrast in the few simple tests I did. So
while flare might be present and the longer hood be a slight
improvement, perhaps a big improvement if shooting near the sun, it's
not generally speaking an obvious problem with this latest version of
the lens. It's clear from careful scrutiny of the existing hood and
what can be seen of the lens interior that a lot of care has been
taken to minimise internal reflections, more than in any other lens
I've seen.

The existing hood BTW is thought by many to be fixed. It isn't. it's
just scarily hard to remove unless you know the antideformation filter
ring removal tricks which makes it very easy. That photo shows the
existing hood remounted as an extension on the end of the metal
exension hood, which is threaded at both ends.

--
Chris Malcolm
 
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Chris Malcolm
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      07-13-2012
Chemiker <(E-Mail Removed)> wrote:
> On Thu, 12 Jul 2012 13:44:42 -0400, Fred McKenzie <(E-Mail Removed)>
> wrote:


>>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


> Makes sense. Wife has a Meade that calculates at 2000MM FF, but
> contrast really sucks. I never use it because of that.


Out of camera jpegs of my Sony 500mm reflex are noticeably flatter in
contrast than from a refractive lens. But a simple boost of contrast
in post processing fixes it. Many editors have a simple "fix contrast"
button which does it instantly and is usually all that's needed. If
I'm preparing a shot for an exhibition I'll carefully crop off both
ends of the tone curve and improve microcontrast with some
sharpening. The end result looks as good as a good refractive lens
shot, except of course for the rather fierce bokeh.

Of course that "as good as" assumes good enough conditions to give
enough latitude for the editing boosts. In marginal conditions the
refractor will easily and obviously win.

If that is you had that big heavy 500mm refractor lens with
you. There's no 500mm refractor small and light enough to carry around
in the bag just in case and easily use hand held.

--
Chris Malcolm
 
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PeterN
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      07-13-2012
On 7/12/2012 10:06 PM, nospam wrote:
> In article <4fff7ea0$0$5718$(E-Mail Removed)-secrets.com>, PeterN
> <(E-Mail Removed)> wrote:
>
>>> Where does infinity begin?

>>
>> At its ending.

>
> no, because there's always a bigger infinity that extends beyond it.
>


And curves back to the point of beginning.

--
Peter


 
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PeterN
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      07-13-2012
On 7/12/2012 10:24 PM, Savageduck wrote:
> On 2012-07-12 18:48:54 -0700, PeterN <(E-Mail Removed)> said:
>
>> On 7/11/2012 11:15 PM, Paul Ciszek wrote:
>>
>> <snip>
>>>
>>> Where does infinity begin?
>>>

>>
>> At its ending.

>
> That's just loopy!
>


Like many comments on Usenet.

--
Peter


 
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PeterN
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      07-13-2012
On 7/13/2012 11:29 AM, PeterN wrote:
> On 7/12/2012 10:06 PM, nospam wrote:
>> In article <4fff7ea0$0$5718$(E-Mail Removed)-secrets.com>, PeterN
>> <(E-Mail Removed)> wrote:
>>
>>>> Where does infinity begin?
>>>
>>> At its ending.

>>
>> no, because there's always a bigger infinity that extends beyond it.
>>

>
> And curves back to the point of beginning.
>


Left off my cite.

<http://arxiv.org/pdf/1009.6203.pdf>

--
Peter


 
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Wolfgang Weisselberg
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      07-13-2012
Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote:

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


They also don't have a Bayer filter.
And the airy disk there is usually a bit larger than a pixel, IIRC.

-Wolfgang
 
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Paul J Gans
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      07-13-2012
Chris Malcolm <(E-Mail Removed)> wrote:
>Alan Browne <(E-Mail Removed)> wrote:
>> On 2012-07-12 18:56 , Chris Malcolm wrote:
>>> Alan Browne <(E-Mail Removed)> wrote:


>>>> Everyone I know who bought a 500 reflex ended up using it less and less
>>>> and it eventually kept time in its case, in the closet.
>>>
>>>> Alone. In the dark.
>>>
>>> That would have been me too, if I'd have had to focus mine
>>> manually. Desperately tricky and slow to get good sharp focus
>>> manually. But I have the Sony 500mm reflex which autofocuses. That
>>> makes a huge difference to how useful the lens is.
>>>
>>> The remaining very tricky problem once focus has been solved is
>>> aim. Like a powerful telescope a 500mm lens is difficult to aim,
>>> especially on a crop sensor DSLR. That made catching moving wildlife
>>> difficult, and birds in flight verging on the impossible.
>>>
>>> So I added a red dot gunsight to the lens. Amazing improvement! I can
>>> instantly point the lens at what I want to shoot so easily that I can
>>> track birds in flight accurately enough for the central spot autofocus
>>> to lock and shoot -- the central spot focus sensor is the only one
>>> which can focus this lens. Catching birds in flight or footballers in
>>> mid kick has become easy and fun.
>>>
>>> It's become so easy to use it's now one of my often-carry-just-in-case
>>> lenses. I've adapted my bag to have the right shaped pocket to carry
>>> it with mounted gunsight.


>> Interesting adaptation. One of those I spoke about who lost faith in
>> the lens had the Minolta AF version as well. I'll mention it to him if
>> I run into him (haven't seen him in a few years).


>> Do you have a photo of your setup?


>http://www.flickr.com/photos/chris_malcolm/6988604951/


>That's a photo of the first trial implementation, looked pretty
>scruffy, and wobbled enough that the sight often hd to be
>recalibrated. I tightened it up with more glue and a pair of rubber
>bands to clamp the tube more firmly on the lens barrel. That worked
>well and reliably enough not to require calibration of the sight after
>every removal and replacement on the lens. So I've painted it all
>black to make it look neater until I get around to devising a metal
>version, which I'll probably base on the cheap long metal lens hoods
>you can get for the lens.


>Here's a photo inlcuding that long lens hood on the end.


>http://www.flickr.com/photos/chris_malcolm/6842482858/


>That hood BTW is supposed to reduce flare and improve contrast. Well,
>mirror lenses are inherently low in contrast, and also seem to be
>unusually prone to flare. But adding that hood didn't noticeably
>reduce flare or improve contrast in the few simple tests I did. So
>while flare might be present and the longer hood be a slight
>improvement, perhaps a big improvement if shooting near the sun, it's
>not generally speaking an obvious problem with this latest version of
>the lens. It's clear from careful scrutiny of the existing hood and
>what can be seen of the lens interior that a lot of care has been
>taken to minimise internal reflections, more than in any other lens
>I've seen.


>The existing hood BTW is thought by many to be fixed. It isn't. it's
>just scarily hard to remove unless you know the antideformation filter
>ring removal tricks which makes it very easy. That photo shows the
>existing hood remounted as an extension on the end of the metal
>exension hood, which is threaded at both ends.


Those are amazing shots, simply amazing. And the red-dot sight
is a great idea! I'm surprised that the cardboard "clamp" provides
enough rigidity.

--
--- Paul J. Gans
 
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Chris Malcolm
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      08-15-2012
Rich <(E-Mail Removed)> wrote:
> Chris Malcolm <(E-Mail Removed)> wrote in news:a67c2hFj90U3
> @mid.individual.net:


>> Paul Ciszek <(E-Mail Removed)> wrote:
>>
>>> In article <(E-Mail Removed)>,
>>> Rich <(E-Mail Removed)> wrote:
>>>>(E-Mail Removed) (Paul Ciszek) wrote in
>>>>>
>>>>> Now, that is a very nice shot. As for the "donuts", I figure if the
>>>>> focal length of the mirror lens is half a meter, and everything I am
>>>>> taking telephoto pictures of is, say, a hundred meters away, that
>>>>> should not be a problem, should it?
>>>>>
>>>>
>>>>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?

>>
>> 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.


> It should be in focus about 3500ft to infinity. The clock could be
> closer


At long last I got round to measuring the distance between my vantage
point and the clocktower using google maps. It's not two miles away,
almost exactly a mile in fact. So you're right -- it is closer!

I've also photographed that clock tower from the roof of my house,
which is about 3,700 feet away. From my roof there is a nearby hill a
few miles further away which is out of focus if the clock is in focus,
and vice versa, however. So I'm clearly being a bit fussier about
sharp focus than the assumptions behind your DoF calculation.

The image BTW is 14MP taken with a 500mm lens on a 1.5 crop sensor.

--
Chris Malcolm
 
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