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Can't get focus with Rokinon 135 lens + filter drawer + OSC camera

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#1 RickSpeedyGonzales

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Posted 13 October 2021 - 07:07 PM

Don't know what I may be doing wrong, but I can not get focus with Rokinon 135 lens filter drawer and OSC camera all together - I can only get it to work WITHOUT the filter drawer. Any ideas? I'm using the ZWO camera mount / base plate with the ASI533MC Pro, then Starizona filter drawer, then lens adapter, then the lens. It seems the back focus is to far, but I don't know how to get everything to work. Thanks in advance.



#2 DJL

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Posted 13 October 2021 - 07:16 PM

Don't know what I may be doing wrong, but I can not get focus with Rokinon 135 lens filter drawer and OSC camera all together - I can only get it to work WITHOUT the filter drawer. Any ideas? I'm using the ZWO camera mount / base plate with the ASI533MC Pro, then Starizona filter drawer, then lens adapter, then the lens. It seems the back focus is to far, but I don't know how to get everything to work. Thanks in advance.

Hi - I had a similar issue with Canon mount Rokinon lenses. You need to add up everything in the back focus. The thread here documents what did and didn't work for me.

 

https://www.cloudyni...-lens-no-focus/


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#3 photoracer18

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Posted 13 October 2021 - 08:25 PM

The total room you have for things in the optical path is equal to the registration distance of that particular camera company. Most DSLRs are in the 42-45mm range and that is it. Never obviously buy the lens for a MILC camera because that one is only around 20mm range. Telescopes don't have that issue.


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#4 jkcolli

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Posted 13 October 2021 - 11:45 PM

I believe this is what you need

https://astronomy-im...er-for-eos-lens

 

(This is assuming that the Rokinon lens is for Canon cameras.)

 

It has a thickness 26.5mm and a filter drawer.

So the total back focus is

 

Filter Drawer for EOS lens.  26.5mm

ASI 533MC backfocus.        17.5mm

Total                                      44.0mm

 

44mm is the back focus for the lens.

 

I am not sure which camera mount you currently have.  ZWO has two. One is 26.5mm thick and the other is 17.5mn thick.  Both are too thick to add the Filter Drawer from Starizona which is 17.5mm thick as that would exceed the 44mm back focus distance of your lens.

 

The 26.5mn thick camera mount is intended to be the only adapter between the lens and your camera.

 

The ZWO camera mount you are using is 26.5mm thick and that with the ASI533MC backfous of 17.5mm equals 44.0mm which is the back focus of your lens.

 

The ZWO website says that with the ZWO EOS-T2 Adapter you can use a 2 inch filter.  I think that means that you could screw in a 2 inch filter on the back of the adapter.

 

Jack

 

 

 

Jack


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#5 calypsob

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Posted 14 October 2021 - 10:49 AM

The thickness of your filter needs consideration. 
It reduces backfocus. Someone had a very good explanation that I cannot find now but you need to add spacing to compensate filter thickness.


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#6 jkcolli

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Posted 14 October 2021 - 08:37 PM

The equation is:

Filter glass thickness*(Refractive index of glass-Refractive index of air)/(Refactive index of glass)

 

The refractive index of glass is ~1.5
The refractive index of air is 1.0

 

So the equation becomes:
     Filter glass thickness*(1.5-1)/1.5= Filter glass thickness * 1/3

 

For a Filter glass thickness of 3mm, the optical path is shorter by 1mm.

 

Therefore in this case, a spacer of 1mm needs to be added to the optical path.

 

Jack


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#7 calypsob

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Posted 15 October 2021 - 11:22 AM

The equation is:

Filter glass thickness*(Refractive index of glass-Refractive index of air)/(Refactive index of glass)

 

The refractive index of glass is ~1.5
The refractive index of air is 1.0

 

So the equation becomes:
     Filter glass thickness*(1.5-1)/1.5= Filter glass thickness * 1/3

 

For a Filter glass thickness of 3mm, the optical path is shorter by 1mm.

 

Therefore in this case, a spacer of 1mm needs to be added to the optical path.

 

Jack

Perfect thanks for sharing jack. 1mm is plenty of tolerance to prevent focus at F2


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#8 RickSpeedyGonzales

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Posted 18 October 2021 - 05:08 PM

Thank you all for the insight. I found the problem - if I use a focus wheel it brings the back focus to where it needs to be in the 44-45mm range. Good to go, clear sky's


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