Canon lenses and astro camera focusing

[kirkster501]

Hi all,

Bought a Canon adapter for my QHY268C to connect to my Canon-fit Samyang 135mm.  I also have a nice 50mm Sigma Art.  Not tried that camera yet on those lenses due to clouds, clouds, clouds (and when they're not there, moon, moon, moon....🙈).

What is the effect on a camera lens focusing if other bits such as an OAG are installed between the lens and the sensor -  which will obviously push the camera sensor further backwards from the lens' prime focus point...?  Never tried this with a camera lens before.  Would it focus?  Would it change the effective FL ?

Thanks, Steve

[tooth_dr]

You need to maintain the correct backspacing of the lens, as if it was intended to have been used on a camera body.

The Canon is backspacing 44mm, .... QHY268C has a back focus of 23.5mm, so in other words you have 20.5mm to play with between the flange of the 268 and the rear of the lens.  Assuming the Canon adapter is about 10-12mm, you wont have much space for anything else.  If you go outside the 44mm you will not be able to focus at infinity.

[kirkster501]

Ahh, ok.  So if the camera and its compulsory adapter consume 23.5 and also the Canon adapter takes up 10 that I have to make up that 44 -(23.5+10) ?  If so that leaves 10.5mm???   I think some OAG can be as thin as that....

Interestingly  - same concept with my Samyang 135mm on my Moravian G2-8300, I just used the Moravian Canon adapter and never bothered with any spacers.  Field seems quite good except in the extreme corners.  I'd had put that down to the lens not being a perfect one.  Maybe my spacings aren't correct.

[tooth_dr]

1 minute ago, kirkster501 said:

Ahh, ok.  So if the camera and its compulsory adapter consume 23.5 and also the Canon adapter takes up 10 that I have to make up that 44 -(23.5+10) ?  If so that leaves 10.5mm???   I think some OAG can be as thin as that....

Interestingly  - same concept with my Samyang 135mm on my Moravian G2-8300, I just used the Moravian Canon adapter and never bothered with any spacers.  Field seems quite good except in the extreme corners.  I'd had put that down to the lens not being a perfect one.  Maybe my spacings aren't correct.

Yes you are right. There are some OAG that are thin, including the QHY version at 10mm, you’ll just have to be careful with adapters which may add 2-6mm depending.

You mustn’t be far too away with the KAF8309 though or else it wouldn’t have focused on infinity.

 

[geeklee]

@kirkster501 You've probably seen it already for the Samyang 135 but if not, there's some excellent information throughout this thread on spacing with various setups including the difference filters make, using the FLO M48 adapter etc.

Imaging with the Samyang 135mm f2 - Getting Started With Imaging - Stargazers Lounge

Edited November 30, 2020 by geeklee

[Alien 13]

Not sure about the Samyang but in general the spacing has to be as close to the manufacturers spec i.e. 44mm for Canon so that when focused the infinity mark on the lens lines up correctly. The reason for this is that some lenses move the whole assembly to gain focus while other designs move certain groups of elements only, if the spacing is wrong you will get coma etc in the corners.

Alan