Achieving Focus - How to work out?

[Z3roCool]

Hi there, I wonder if someone could help with making sure I will achieve focus with the following setup?

I will be using a 400mm Canon lens with a Geoptik EOS adapter. Camera will be an ASI 1600MM Pro with EFW (x5).

I have worked out that I go over the 44mm needed for the 400mm Canon lens but not really sure what all this means and how to get around it, if I am even on the right track!

I think the sizes of the train are:

6.5mm - (ASI1600 distance to sensor from housing)

19mm - EOS Adapter

20mm - EFW

If using the ZWO adapter (17.5mm) the calculation comes out at 44mm, which is on the money. Does this mean the Geoptik adapter is just too big, or is there a way around this.

Can anyone with a bit more understanding guide me on the right path?

Many thanks,

 

Tom.

[Guest]

Hello Tom,

As far as I can see, The GEOPTIK EOS ADAPTER is also 19mm short Geoptik Adaptor for Canon EOS Lenses to T2 for CCD cameras and the adapters you use with the ZWO EFW, then you are already at 45,45mm which means, you are 1.45mm away from the optimal focus point..

But !! you could add a FILTER between the ZWO and the LENS path ( which will the case since you will use a FILTER WHEEL ) then the Filter you will use like a Ha Filter or IDAS filter.. and with the filter in the light path you will prolong the optical focus path with 1mm.

hence, prolonging the focus path is possible, but would you be happy with the result ?? is something else !

 

The best would be if you would also wait for other members to write other answers or opinions.... 

See the explanation from ASI below..

Regards

Martin

 

 

[Guest]

Or Read the Thread here.. 

 

Regards

 

Martin

 

[michael8554]

Hi Tom

If you're looking to get infinity focus using the lens's manual focus ring, then you will achieve that by either over, or under, spacing the lens - I forget which way, best to try both if you have a range of spacer rings available.

You don't need to get it exactly right so long as the lens focus ring doesn't hit the end stop. If you go the wrong way with the spacing, you will hit the stop before getting focus on infinity.

Michael

[Z3roCool]

20 hours ago, artem said:

Or Read the Thread here.. 

 

Regards

 

Martin

 

Thanks for the info Martin  Much appreciated.

[Guest]

Hi Tom,

You are mostly welcome.. 

Regards

Martin

 

[Z3roCool]

10 hours ago, michael8554 said:

Hi Tom

If you're looking to get infinity focus using the lens's manual focus ring, then you will achieve that by either over, or under, spacing the lens - I forget which way, best to try both if you have a range of spacer rings available.

You don't need to get it exactly right so long as the lens focus ring doesn't hit the end stop. If you go the wrong way with the spacing, you will hit the stop before getting focus on infinity.

Michael

Thanks Michael, sadly I have no rings available yet so will just have the camera and gear that comes with that. I am really new to this part and clueless on it

I am getting terms mixed up I think...some places I read that you need to get the distance exact to get focus and others either way is okay...sure it is me just not understanding properly each of the terms.

[michael8554]

Hi again Tom

I've been back over my old posts.

If you have the lens closer than the precise back focus of 44mm, the lens will go through infinity focus.

Sounds like you are spot on, or 1.45mm too much according to Martin, which might come good due to the refraction of the filters.

Michael

[Z3roCool]

On 11/12/2018 at 15:26, michael8554 said:

Hi again Tom

I've been back over my old posts.

If you have the lens closer than the precise back focus of 44mm, the lens will go through infinity focus.

Sounds like you are spot on, or 1.45mm too much according to Martin, which might come good due to the refraction of the filters.

Michael

Thanks Michael