Canon Lens and Hsfilter back focus

[lensman57]

Hi,

The back focus of a Canon EF lens is 45 mm from the lens flange to the sensor, if using an Ha filter between the lens and the sensor would this distance needs to be compensated for as in the case of a reducer? A  Baader filter has a thickness of 2mm and I normally allow 0.7mm extra back focus,  would I need to do this for the Canon lens too?

Thanks for your help in advance,

Regards,

A.G

[Psychobilly]

I would have thought that the infinity focus position on the lens changes when you add in the filter , but depending on the lens may still be able to get infinity focus - I'm probably wrong but adding In glass will move the infinity focus "nearer"  as when you took out the thicker filters and didnt repalce them with anything  in front of the sensor provide you had an L series lens - or another where you could focus "beyond" infinty  you could still get infinity focus...

I used to focus with the filter in place when I was trying Ha  with my modified DSLR's. At the time I was using  a Bahtinov mask  - which would produce broken rather than solid lines with a bandpass filter in the optical train.... 

Peter...

[lensman57]

I would have thought that the infinity focus position on the lens changes when you add in the filter , but depending on the lens may still be able to get infinity focus

I used to focus with the filter in place when I was trying Ha  with my modified DSLR's. At the time I was using  a Bahtinov mask  - which would produce broken rather than solid lines with a bandpass filter in the optical train.... 

Peter...

Thank you Peter for your prompt reply, please  accept my apology as I did not make myself clear, the lens is not connected to a DSLR but to CCD via adapter. At the moment I have set the distance to 45.65mm with the Ha filter in place.. Am I good to go with this?

Regards,

A.G

[Psychobilly]

You should just need to refocus the lens  slightly with the filter in place... (I added additional info to my reply)

Peter...

[lensman57]

I would have thought that the infinity focus position on the lens changes when you add in the filter , but depending on the lens may still be able to get infinity focus - I'm probably wrong but adding In glass will move the infinity focus "nearer"  as when you took out the thicker filters and didnt repalce them with anything  in front of the sensor provide you had an L series lens - or another where you could focus "beyond" infinty  you could still get infinity focus...

I used to focus with the filter in place when I was trying Ha  with my modified DSLR's. At the time I was using  a Bahtinov mask  - which would produce broken rather than solid lines with a bandpass filter in the optical train.... 

Peter...

Thank you Peter,

I have reset to 45mm, well 44.85. Hope this works.

Regards,

A.G

[Psychobilly]

I would only adjust the "register" as a last resort I would have just  tried adjusting the focus on the lens  first...

Fingers crossed you can get it sorted

Peter...