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Bower 500mm f/8 Mirror lens



 
 
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  #21  
Old July 16th 15, 11:38 PM posted to rec.photo.digital
Me
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Posts: 470
Default Bower 500mm f/8 Mirror lens

On 17/07/2015 10:10 a.m., RichA wrote:
On Thursday, 16 July 2015 16:54:56 UTC-4, Me wrote:
On 16/07/2015 1:17 p.m., RichA wrote:
On Tuesday, 14 July 2015 04:11:48 UTC-4, Eric Stevens wrote:
Does anyone have knowledge or experience of these? How good are they?
(For a particular value of 'good')
--

Regards,

Eric Stevens

Even with focusing aids, 500mm with an APS or m4/3rds camera is a pain to focus absolutely accurately. Also, mirror lenses were never really meant for daytime use. They started out as telescopes. Reason is that light can (even with a baffle tube coming from the primary mirror centre hole) fall in the camera sensor from outside the light cone, reducing contrast. This can be dealt with pretty well in post-processing. Also, mirror lenses of that size are susceptible to heat waves, much more so than smaller aperture camera lenses. This means close-in (100ft or less) objects are generally ok to shoot, but anything beyond that that isn't high above the ground tends to blur. Ironically, the lenses are optimized to focus at infinity which means you get a small amount of spherical aberration when focusing on things close-in, but it's better than heat waves.

Why would atmospheric refraction/shimmer affect a mirror lens more than
a normal telephoto?


Not unless the normal telephoto had a larger front lens diameter. It's all predicated on that. Over 4" diameter, heat waves are very bad, but even less than that, if you are looking through a significant atmospheric blanket, it's bad.

Still doesn't make sense to me, as at the distances where shimmer is a
common problem (more than 50m or so - but of course it depends) then the
difference in angle between subject and the perimeter of the front
element (or mirror) is so insignificant, I doubt you'd see it.

Here's an example of some shimmer at only 300mm or so (100% crop), taken
on cool clear calm winter morning before sunrise:
http://i.imgur.com/NZBRL2w.jpg
The small island is about 40km, already degraded by atmospherics, the
low hills about 80km, the mountains about 150km.
  #22  
Old July 16th 15, 11:53 PM posted to rec.photo.digital
Alan Browne
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Posts: 12,640
Default Bower 500mm f/8 Mirror lens

On 2015-07-16 18:24, Me wrote:
On 17/07/2015 10:11 a.m., Alan Browne wrote:
On 2015-07-16 18:05, Me wrote:
On 17/07/2015 9:11 a.m., Alan Browne wrote:
On 2015-07-15 18:57, Eric Stevens wrote:
On Wed, 15 Jul 2015 18:52:55 -0400, Alan Browne
wrote:

On 2015-07-14 19:18, PeterN wrote:
On 7/14/2015 10:52 AM, Alan Browne wrote:
On 2015-07-14 04:11, Eric Stevens wrote:
Does anyone have knowledge or experience of these? How good are
they?
(For a particular value of 'good')

With some exceptions most people who buy reflex's relegate them
to a
shelf at some point after the initial blush of pleasure wears off.
They
are usually (always?) manual focus too ... which can be a challenge
for
the wearier eyes out there.

Some cameras, at least my Nikons, have some type of in focus
indicators
available in the viewfinder.

That doesn't always make focusing easy - esp. off centre.

Modern higher level Nikons have the ability to select an off-centre
focus point.

Does that work with a manual focus lens?

Yes.
The accuracy of those focus points when using small apertures may not be
very good.
OTOH there's always CDAF in LV mode. 500mm f8 with no stabilisation is
going to be a stretch unless in full daylight, unless high ISO is used,
you're probably on a tripod anyway.


How do you know it's in focus? What is the indication?

Left or right arrow either side of a dot - o . Arrows light up to tell
you which way to move the focus ring, dot lights up and arrows are off
when focus is achieved.


Fantastic.

I have such indicators on my Sony a900 (dot on when the focus point is
in focus - no indicator of which way however) - but I'm not sure if they
will work with a cat lens.

  #23  
Old July 17th 15, 07:20 PM posted to rec.photo.digital
Fred McKenzie
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Posts: 214
Default Bower 500mm f/8 Mirror lens

In article ,
Alan Browne wrote:

I have such indicators on my Sony a900 (dot on when the focus point is
in focus - no indicator of which way however) - but I'm not sure if they
will work with a cat lens.


Alan-

I think it depends on f/stop. I have the Minolta 500mm f/8 AF mirror
lens, which auto-focuses on the Alpha 100 and 900. I expect the focus
indicator would work with a 500mm f/8 manual focus lens.

I'm not sure what determines the critical f/stop. I recall that the
split image rangefinder on my old film cameras, could not be used with a
Celestron C-90 mirror telescope (1000mm f/11 as a lens). One half of
the split image would always be dark. I used the ground glass to focus,
but it was extremely critical.

Fred
  #24  
Old July 18th 15, 01:43 PM posted to rec.photo.digital
Alan Browne
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Posts: 12,640
Default Bower 500mm f/8 Mirror lens

On 2015-07-17 14:20, Fred McKenzie wrote:
In article ,
Alan Browne wrote:

I have such indicators on my Sony a900 (dot on when the focus point is
in focus - no indicator of which way however) - but I'm not sure if they
will work with a cat lens.


Alan-

I think it depends on f/stop. I have the Minolta 500mm f/8 AF mirror
lens, which auto-focuses on the Alpha 100 and 900. I expect the focus
indicator would work with a 500mm f/8 manual focus lens.


To be clear I'm referring to the off centre focus points, not the
central point.

blush I'd forgotten that the Minolta cat was AF.



I'm not sure what determines the critical f/stop. I recall that the
split image rangefinder on my old film cameras, could not be used with a
Celestron C-90 mirror telescope (1000mm f/11 as a lens). One half of
the split image would always be dark. I used the ground glass to focus,
but it was extremely critical.


I'm pretty sure contrast (ambient light) is the deciding factor. So an
f/11 lens will have a 1 stop disadvantage. That's the difference
between a sunny-16 day and moderate overcast.

  #25  
Old July 19th 15, 08:54 PM posted to rec.photo.digital
PeterN[_6_]
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Posts: 4,254
Default Bower 500mm f/8 Mirror lens

On 7/16/2015 5:12 PM, Alan Browne wrote:
On 2015-07-16 06:46, PeterN wrote:


snip



Nope, not always. Manual Off center focusing can be a challenge.
However, if off center focusing with MF is needed, the next best thing
is to use a tripod and move the camera. Tt is rare that off center MF is
needed.


Unless you're into photography, I suppose that may be true.


I should have been more clear. I rarely move into left or right focus
mode. I do most of my MF focus for macro and landscape work, where I am
more concerned iwith composition, and apparent focus.



--
PeterN
  #26  
Old July 23rd 15, 07:05 PM posted to rec.photo.digital
Alan Browne
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Posts: 12,640
Default Bower 500mm f/8 Mirror lens

On 2015-07-19 15:54, PeterN wrote:
On 7/16/2015 5:12 PM, Alan Browne wrote:
On 2015-07-16 06:46, PeterN wrote:


snip



Nope, not always. Manual Off center focusing can be a challenge.
However, if off center focusing with MF is needed, the next best thing
is to use a tripod and move the camera. Tt is rare that off center MF is
needed.


Unless you're into photography, I suppose that may be true.


I should have been more clear. I rarely move into left or right focus
mode. I do most of my MF focus for macro and landscape work, where I am
more concerned iwith composition, and apparent focus.



Jeez, can't you see a cheap shot for a cheap shot and fire back. ;-)

 




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