From: Wolfgang Weisselberg on
David Kilpatrick <iconmags3(a)btconnect.com> wrote:

> The main problem with the 250mm f5.6 on full frame was very strong
> vignetting, which you couldn't do anything about.

Graduated ND (center)?
(Yes, yes, it makes everything even darker ...)

-Wolfgang
From: Alan Browne on
David Kilpatrick wrote:
> 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.

C'mon David. Give us details!

Reading your post prompted me to dig out an old French 1995 Minolta lens
brochure, and there on p.11 is the AF v., but with a tiny lens diagram.
Nonetheless it helped me follow your expl. above (not that it isn't
clear).

I guess what surprises me most is where you say they reduced the surface
of the secondary mirror. I would see this as a benefit (assuming a very
high quality mirror material and accurate shaping) as then the rays into
the back optics would be more close to parallel to the lens axis and
hence less CA from those lenses. OTOH what I know about optics at that
level fills a very small notebook.

If the MTF (ref for curves?) is as good as you say, I'm getting itchy.
(However there is a 24mpix camera and a couple other lenses in the
nearer future...)

If the new Sony sensor is much better in noise, then using the 500 f/8
with it at ISO 400 or 800 could result in some great images.

It could also be that your past underwhelming performance with this lens
was due to movement (hand holding/mirror bang).

Cheers,
Alan.

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From: David Kilpatrick on
Alan Browne wrote:

> If the MTF (ref for curves?) is as good as you say, I'm getting itchy.
> (However there is a 24mpix camera and a couple other lenses in the
> nearer future...)

Let's just say that every other lens listed in the book has half a dozen
wavy lines for MTF with separate dotted and solid ones for
radial/tangential. The cat has two lines - a straight one just below the
very top 100 per cent mark, running right across to the edge without
even a dip (10 lppm) and a single one just below it with a slight
decline to the edge (30 lppm). All the other lines are concurrent - they
are hidden by the principal solid lines.
>
> If the new Sony sensor is much better in noise, then using the 500 f/8
> with it at ISO 400 or 800 could result in some great images.
>
> It could also be that your past underwhelming performance with this lens
> was due to movement (hand holding/mirror bang).

Or that I only ever use it close-up on bad subjects in difficult light.
I maybe need to try it on middle distance subjects in excellent light.

David
From: Alan Browne on
David Kilpatrick wrote:
> Alan Browne wrote:
>
>> If the MTF (ref for curves?) is as good as you say, I'm getting itchy.
>> (However there is a 24mpix camera and a couple other lenses in the
>> nearer future...)
>
> Let's just say that every other lens listed in the book has half a dozen
> wavy lines for MTF with separate dotted and solid ones for
> radial/tangential. The cat has two lines - a straight one just below the
> very top 100 per cent mark, running right across to the edge without
> even a dip (10 lppm) and a single one just below it with a slight
> decline to the edge (30 lppm). All the other lines are concurrent - they
> are hidden by the principal solid lines.

! Sounds too good to be true... Hmm, I wonder who know that might have
one...

>>
>> If the new Sony sensor is much better in noise, then using the 500 f/8
>> with it at ISO 400 or 800 could result in some great images.
>>
>> It could also be that your past underwhelming performance with this
>> lens was due to movement (hand holding/mirror bang).
>
> Or that I only ever use it close-up on bad subjects in difficult light.
> I maybe need to try it on middle distance subjects in excellent light.

With a limited sweet spot... well at least on a good digital camera one
can ISO up a bit...


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From: Richard on
One thing is important for a mirror, it has to be good to work well and by
that I don't mean being able to produce a sharp image, I mean being able to
properly focus it. A moving mirror focuses very fast, if the lens stinks,
you'll blow right by what passes for the focus without really seeing it. If
the optics are top notch, it'll "snap" into focus, which is absolutely
needed in a lens with a shallow DOF.


"Alan Browne" <alan.browne(a)Freelunchvideotron.ca> wrote in message
news:DMednW54i_hRQDzVnZ2dnUVZ_i2dnZ2d(a)giganews.com...
> 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.


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