From: Richard on

"David Kilpatrick" <iconmags3(a)btconnect.com> wrote in message
news:A4mdnVvpzdR2_TzVnZ2dneKdnZydnZ2d(a)bt.com...
> 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.

But they don't even need extra elements to focus closer. Nothing can have
the focus range of a mirror lens, all you need is enough inter-mirror
travel. You can focus a 2000mm mirror lens (telescope) down to 20ft to
infinity with only 1" or so of mirror travel. Extra elements defeat the
purpose, which is as pure an optical system as possible. They could
conceivably do it with just the front corrector and a central mirror spot
with a different curve on it, essentially, 2 elements since the front
corrector does very little magnifying, if any. Some of the Russian ones
were like this. But most of them use a negative relay lens in the baffle
tube (at the back) to bring the image out far enough to reach the film
plane.

> 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.

T-stops in mirror lenses are odd. I've measured units marked as f10 to be
f8 in use. There isn't much happening in the photographic world, but in the
astronomy world, light throughput is up to around 84% thanks to
high-efficiency coatings, even with the central mirror obstruction. A
10-element lens probably has less, given it probably has at least 14
surfaces.


From: R. Mark Clayton on

"John O'Flaherty" <quiasmox(a)yeeha.com> wrote in message
news:25g3a4laoaq5aafrr1h0m9f5o7nju7h5et(a)4ax.com...
> On Tue, 12 Aug 2008 16:56:52 +0100, "R. Mark Clayton"
> <nospamclayton(a)btinternet.com> wrote:

SNIP

>
> Why is it to the fourth power?
> --
> John

1. Distance to the film surface.

If the distance from the lens to the central point is r then the
distance to a point away from the centre is r*sec(t) or r/cos(t) and the
relative illumination is 1/r**2 / 1 / (r/cos(t))**2. Eliminating r gives
1/cos2(t).

2. Area of film illuminated.

If the film surface is flat then at the limit the area illuminated is also
larger in both dimensions by 1/cos(t) (= 1/cos2(t) in area) so the fall off
is 1/cos4(t).

Plotted on my graphical calculator it gives a drastic fall off between 30
and 60 degrees off centre.


Interestingly one of the benefits of the negative film system is that it
automatically compensates for natural vignetting to a certain degree during
the print process - certainly on "normal" focal lengths (like 50mm on 35mm
film).

OTOH if you "print positive" the prints flare out in the centre (because of
natural vignetting in the enlarger).


From: John O'Flaherty on
On Wed, 13 Aug 2008 11:54:45 +0100, "R. Mark Clayton"
<nospamclayton(a)btinternet.com> wrote:

>
>"John O'Flaherty" <quiasmox(a)yeeha.com> wrote in message
>news:25g3a4laoaq5aafrr1h0m9f5o7nju7h5et(a)4ax.com...
>> On Tue, 12 Aug 2008 16:56:52 +0100, "R. Mark Clayton"
>> <nospamclayton(a)btinternet.com> wrote:
>
>SNIP
>
>>
>> Why is it to the fourth power?
>> --
>> John
>
>1. Distance to the film surface.
>
> If the distance from the lens to the central point is r then the
>distance to a point away from the centre is r*sec(t) or r/cos(t) and the
>relative illumination is 1/r**2 / 1 / (r/cos(t))**2. Eliminating r gives
>1/cos2(t).
>
>2. Area of film illuminated.
>
>If the film surface is flat then at the limit the area illuminated is also
>larger in both dimensions by 1/cos(t) (= 1/cos2(t) in area) so the fall off
>is 1/cos4(t).
>
>Plotted on my graphical calculator it gives a drastic fall off between 30
>and 60 degrees off centre.

Thanks. That's a lot more concise than what I found by googling.

>Interestingly one of the benefits of the negative film system is that it
>automatically compensates for natural vignetting to a certain degree during
>the print process - certainly on "normal" focal lengths (like 50mm on 35mm
>film).
>
>OTOH if you "print positive" the prints flare out in the centre (because of
>natural vignetting in the enlarger).

That is interesting.
--
John
From: Helpful person on
On Aug 12, 9:48 pm, "Richard" <rander3...(a)gmail.com> wrote:
>
> But they don't even need extra elements to focus closer.  Nothing can have
> the focus range of a mirror lens, all you need is enough inter-mirror
> travel.  You can focus a 2000mm mirror lens (telescope) down to 20ft to
> infinity with only 1" or so of mirror travel.  Extra elements defeat the
> purpose, which is as pure anoptical systemas possible.  They could
> conceivably do it with just the front corrector and a central mirror spot
> with a different curve on it, essentially, 2 elements since the front
> corrector does very little magnifying, if any.  Some of the Russian ones
> were like this.  But most of them use a negative relay lens in the baffle
> tube (at the back) to bring the image out far enough to reach the film
> plane.
>

The reason to have refracing lenses to perform focus is to maintain
image quality. As you change the mirror conjugates the image quality
drops fairly quickly.
From: Fred McKenzie on
In article <zC1ok.275$T16.103(a)read4.inet.fi>,
"Mulperi" <juha.heinonen(a)pp2.inet.fi> wrote:

> 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.

Mulperi-

I have no experience with the Tamron, but do have a Minolta 500mm f/8 AF
mirror lens. I bought it for use with the old 8000i film body, and it
was one of the reasons I purchased a Sony Alpha 100 camera.

The lens is fairly sharp in my estimation. However, it is a mirror
lens. As others mentioned, bright spots become doughnuts. It also has
slightly less (barely noticeable) contrast than a standard lens. The
contrast is significantly better than that of my Celestron C90 mirror
telescope using a T-Mount adapter.

I live about 15 miles from the Space Shuttle launch pad, and often use
this lens for launch pictures. Even though the Shuttle's image is
somewhat small, it shows good detail when enlarged. I even have been
able to capture the Solid Rocket Boosters being jettisoned from about 80
miles down range!

Fred