From: John O'Flaherty on
On Tue, 12 Aug 2008 16:56:52 +0100, "R. Mark Clayton"
<nospamclayton(a)btinternet.com> wrote:

>
>"David Kilpatrick" <iconmags3(a)btconnect.com> wrote in message
>news:UJGdnX6EGdMpPjzVnZ2dnUVZ8vydnZ2d(a)bt.com...
>> Chris Malcolm wrote:
>>> R. Mark Clayton <nospamclayton(a)btinternet.com> wrote:
>>>
>>>> PS back in the manual days Minolta did a 250mm reflex. Scarcely any
>>>> bigger than standard prime lens and very light. Lovely if one didn't
>>>> want to take a full kit.
>>>
>>> Plus with enough aperture to autofocus without special internal aid,
>>> and a useful telephoto length of 375mm on a 1.5 crop sensor. Shame it
>>> needs a lensed convertor to work on a Sony alpha mount.
>>>
>>
>>
>> The main problem with the 250mm f5.6 on full frame was very strong
>> vignetting, which you couldn't do anything about. I had this lens but in
>> the end, the gain over the 75-200mm conventional zoom or the 200mm f4.5
>> was not enough to compensate for the restrictions.
>
>I knocked back [various brand] 19-35 zoom lenses until recently after I
>realised that the dark corners at short focal lenght were a natural effect
>not a defect in the lens*, however why did the [long] lens above suffer from
>it - was it because of the way the secondary mirror worked or got in the
>way?
>
>>
>> However, on APS-C the vignetting would not be an issue and the aperture,
>> size and length would make it a great design to re-issue in AF form.
>
>Oh yes.
>
>And a zoomable multiplier?
>
>>
>> David
>
>
>
>* at 19mm on to 35mm film, about 45degrees in the corner, so (cos 45)**4 =
>0.25!

Why is it to the fourth power?
--
John
From: David Kilpatrick on
R. Mark Clayton wrote:

>
> I knocked back [various brand] 19-35 zoom lenses until recently after I
> realised that the dark corners at short focal lenght were a natural effect
> not a defect in the lens*, however why did the [long] lens above suffer from
> it - was it because of the way the secondary mirror worked or got in the
> way?


The 500mm f8 also suffers from it - it seems to be a constant issue with
mirror lenses, and the size the central spot has something to do with
it. In the 1970s, Vivitar UK (before producing the Solid Cat)
demonstrated an imported mirror telephoto which used a folded lightpath
- the Katoptar. When looking for a reference to this, I happened to find
this page:

http://home.att.net/~infraredvideo/CollectableRareTelephotoLenses.html

Which shows, after the big Canon lens, a Vivitar 800mm Solid Cat and the
Katoptar. I handled and looked through the Katoptar at the time its
prototype, or first production run, was around - it may have been 1974
or 1976, perhaps 1978 but it would have been a photokina year. I can't
remember ever having the lens to test properly, but it's possible one my
colleages on Photo Technique like the editor, Jack Schofield, may have
taken test shots with it.

The Katoptar is correctly listed on this site as having zero
illumination loss across the field, and diffraction limited levels of
correction. While this example for sale is in a Canon FD mount, I think
for someone wanting an extreme quality of totally apochromatic 500mm f8,
it might be a worthwhile purchase and conversion to a modern mount.

David
From: Alan Browne on
David Kilpatrick wrote:
> Richard wrote:
>> "Mulperi" <juha.heinonen(a)pp2.inet.fi> wrote in message
>> news:zC1ok.275$T16.103(a)read4.inet.fi...
>>> Which one is better. Yes I know that Tamron AF 200-500 F5-6,3 Di LD
>>> IF is a zoom lens and SONY 500/8 REFLEX is not but which one gives
>>> better photos.
>>
>> I've rarely seen a zoom in that range that is any good, outside of
>> Nikon's 200-400 $5000+ monster. If the Sony is a good mirror lens, it
>> can produce excellent images, often completely free of colour
>> aberration that effects all but the most apochromatic of the
>> refractive lenses.
>> I shot this with a Tamron 350mm mirror lens, it's a 50% reduction from
>> actual size.
>>
>> http://www.pbase.com/andersonrm/image/99552245
>>
>
> I've owned three examples of the 50mm AF mirror over the years -
> Minolta, but these are exactly what Sony is now rebranding - and while
> they are a convenient lens, I've never found the sharpness all that
> stunning compared to the earlier non-AF mirror lens from Minolta. I
> guess the compromise of putting in some more glass elements, to enable
> the AF and closer focusing, takes the edge off a pure mirror design.
>
> I still have one but it is lens for special purposes, while the Tamron
> 200-500mm is a fairly versatile all round sports and wildlife lens and
> will (at f9, which closer to the true T-stop of the mirror lens) produce
> better results most of the time.

I would still bet the Sony mirror over the Tamron at 500mm and f/8 (and
give another stop down to the Tamron as well) for sharpness.
The CA shown in the photo.net article is a good indicator that the
Tamron is not that hot. The mirror lens should not have any CA

(although you mention the additonal glass for AF... where does that sit?).

--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: Remove FreeLunch.
-- usenet posts from gmail.com and googlemail.com are filtered out.
From: David Kilpatrick on
Alan Browne wrote:

>
> (although you mention the additonal glass for AF... where does that sit?).
>


Pure mirror lenses don't use a correcting front element - the glass in
the front of the Minolta design is a lens, not a plain cover glass. It
has corrections both front and rear, so that it is able to focus using
the front group only - and I don't think the action is 100 per cent down
to the curved mirror face in the inside of the front glass. With a
classic mirror lens design moving the secondary mirror forward is equal
to double the movement of a front-group focusing telephoto. With the
Minolta/Sony design it will be better than double.

The focusing action required is an extension of just 2.75mm by the front
element/secondary mirror assembly, to focus down to 4m. I have next to
me a conventional 500mm f8 simple telephoto, with front unit focusing.
This requires an extension of 17mm just to focus to 12.5m!

The front element is a fairly thick plano-convex with the convex
outward, to which is attached a mirror coated on the inside of a
concave-convex. This mirror is made smaller in the Minolta design to
allow exit paths which can work with rthe AF system. The overall
diameter of the lens is smaller than most 500mm cats, and the central
mirror is much smaller. This also means that the front element and the
primary mirror must be slightly more powerful, to focus the rays on a
smaller circle at the point where they hit the secondary mirror. The
glass elements - a group of four including the thick glass of the
primary mirror, all with air to glass surfaces - at the rear of the lens
serve almost entirely to increase the telephoto factor and push the
focal plane back to match the flangeback distance.

The design is very well explained, and shown in diagrammatic form, in
the Sony Alpha Lens book. This also shows that the lens has amazingly
high MTF at 10 lppm, very respectable at 30lppm, zero astigmatim.
Technically it should be matching either the CZ 85mm or 135mm used at
f8. My experience is that it doesn't but having just studied the MTF
functions as shown, I will be giving this lens another serious bout of
use - maybe even on the Alpha 350, to see what it does on 14 megapixels.

Of course other modern catadioptric lenses may use dioptric front glass,
but I am not aware of anything using the same principles as the Minolta
design.

David
From: John O'Flaherty on
On Tue, 12 Aug 2008 12:01:27 -0500, John O'Flaherty
<quiasmox(a)yeeha.com> wrote:

>On Tue, 12 Aug 2008 16:56:52 +0100, "R. Mark Clayton"
><nospamclayton(a)btinternet.com> wrote:
>
>>
>>"David Kilpatrick" <iconmags3(a)btconnect.com> wrote in message
>>news:UJGdnX6EGdMpPjzVnZ2dnUVZ8vydnZ2d(a)bt.com...
>>> Chris Malcolm wrote:
>>>> R. Mark Clayton <nospamclayton(a)btinternet.com> wrote:
>>>>
>>>>> PS back in the manual days Minolta did a 250mm reflex. Scarcely any
>>>>> bigger than standard prime lens and very light. Lovely if one didn't
>>>>> want to take a full kit.
>>>>
>>>> Plus with enough aperture to autofocus without special internal aid,
>>>> and a useful telephoto length of 375mm on a 1.5 crop sensor. Shame it
>>>> needs a lensed convertor to work on a Sony alpha mount.
>>>>
>>>
>>>
>>> The main problem with the 250mm f5.6 on full frame was very strong
>>> vignetting, which you couldn't do anything about. I had this lens but in
>>> the end, the gain over the 75-200mm conventional zoom or the 200mm f4.5
>>> was not enough to compensate for the restrictions.
>>
>>I knocked back [various brand] 19-35 zoom lenses until recently after I
>>realised that the dark corners at short focal lenght were a natural effect
>>not a defect in the lens*, however why did the [long] lens above suffer from
>>it - was it because of the way the secondary mirror worked or got in the
>>way?
>>
>>>
>>> However, on APS-C the vignetting would not be an issue and the aperture,
>>> size and length would make it a great design to re-issue in AF form.
>>
>>Oh yes.
>>
>>And a zoomable multiplier?
>>
>>>
>>> David
>>
>>
>>
>>* at 19mm on to 35mm film, about 45degrees in the corner, so (cos 45)**4 =
>>0.25!
>
>Why is it to the fourth power?

Never mind, found it on the net.
--
John
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