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mirror lens focusing question.

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9 replies to this topic

#1 Tragusa113

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Posted 16 July 2020 - 06:03 AM

So I obtained a mirror lens from a wonderfully nice gentleman from here and was able to test it out last night.  Its a 500mm f8.  When I tried to focus on Jupiter I noticed that Infinity focus was not quite there.  I needed to go a little bit further to get it perfect.  My question is, is there anything like a reducer that might help me get it in focus 100%.  Thank you everyone in advance.



#2 StarBurger

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Posted 16 July 2020 - 06:54 AM

If the lens comes to focus BEYOND infinity this is not really a problem. Is the lens marked with the infinity symbol? Looks like a figure 8 on it's side.

Most lenses these days do not even have the mark and most of them go through focus and a little beyond to allow for the addition of filters etc.

No reducers necessary and anyway you may not even be able to fit one.

If the view is "perfect" then all well and good. 

Enjoy it.



#3 sharkmelley

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Posted 16 July 2020 - 07:08 AM

So you are unable to achieve focus?

 

Is your camera modified?  Many legacy lenses cannot focus on very distant objects (including astro targets) when used on a modified camera.

 

Mark


Edited by sharkmelley, 16 July 2020 - 07:12 AM.


#4 Tragusa113

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Posted 16 July 2020 - 08:06 AM

I can not achieve focus, My 70D is not modified.  It is an older mirror lens, Im at work right now and could not tell you the brand.  I am using a M42 T ring to attach to my camera



#5 Alen K

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Posted 16 July 2020 - 08:17 AM

Some people have been able to adjust where certain mirror lens models reach infinity focus by accessing screws on the body and doing an adjustment before tightening them down again. Wish I could find the link I saw about that but the same might be possible with yours, depending on the brand. Certainly, such an adjustment was done in the factory, so it all depends on the accessibility of the relevant screws.

Edited by Alen K, 16 July 2020 - 10:15 AM.


#6 kathyastro

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Posted 16 July 2020 - 08:30 AM

Adding an extension should allow you to focus at infinity.  If you can find a 1mm M42 extension, that would probably do it.  Or, if your adapter has enough thread, you could slip a 1mm shim between the lens and the T-ring.


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#7 SteveInNZ

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Posted 16 July 2020 - 03:42 PM

You said that you are using an M42 adapter. Some of the Rokinon (etc) lenses have a T-mount which is also 42mm but a different thread pitch. If you have the wrong thread, it may not screw on completely, which would prevent you from reaching infinity.

Another issue can be some models of the Tamron and the Tokina (IIRC) have a rear mount filter. If that's missing, you may not be able to reach focus.

 

Steve.



#8 Tragusa113

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Posted 16 July 2020 - 05:58 PM

Its a Lentar 500mm f8 mirror with built in filter wheel f8 f11 f16



#9 DanH.264

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Posted 17 July 2020 - 12:27 AM

It may require a filter in the optical path to achieve infinity focus.  It it close, but not quite in focus without one?

Otherwise, there might be a stop screw to adjust the infinity stop.


Edited by DanH.264, 17 July 2020 - 12:40 AM.


#10 Tragusa113

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Posted 17 July 2020 - 05:35 AM

I will take a look thanks everyone.




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