Thoughts on how to compute step size for Rokinon 135mm?

[Scott Mitchell]

Hi everyone, I finally finished my setup for my Rokinon Cine DS 135mm lens. For my refractor and SCT I could follow the instructions to come up with a pretty good figure to use for step size, but I'm not sure how to do that for a camera lens. I'm using a GT2 20 tooth pulley and a 350mm GT2 belt to connect to the lens. I can compute that ratio, but I don't understand how to relate that to the idea expressed in the directions I used for my refractor where we measured the travel of the focuser for 1 full rotation of the focuser knob.

I did get first light with this rig a couple of nights ago. For that session, I just used the step size I had for my other setup and it mostly worked. However, the final focus position was often just off from the exactly right spot. Not too forgiving at f/2.8.

Any suggestions would be welcome!

[brownrb]

Hi Scott
I have a Canon EOS 50D which I use with an USMII f2.8 zoom lens.
Even when you focus visually with the camera in terrestial use, you realize quickly that the focus occurs over such a very small range.

For astrophotography then focus control is set to manaul. I try to use a NEMA with looped belt and either micro step the NEMA at 1/4, failing that I use a PG14.

There is no real way to translate how you calculate step size on a "refractor" to a camera like an EOSD. But my personal view is the lower the f ratio, the more critical it is to have small step sizes.

I think your own experience bears this out with the focus not being quite correct. This is quite common when you do not have a small enough step size.

Regards
Robert

[Scott Mitchell]

Thanks for the response, Robert. I'm currently half-stepping with a step size around 8 I believe (I'm not at the right computer at the moment for an exact answer). I'll experiment with 1/4 stepping to see if that helps.

And yes, having been focusing this lens manually up till now, I agree with how challenging getting correct focus is! That's with it stopped down to f/2.8. I can't imagine how fiddly f/2 must be.

I'll try to remember to update this thread when I come up with a working set of parameters.

[Chuck Perrygo]

See my post of 16-Feb-2021 for a method to calculate camera lens step size.

[Scott Mitchell]

Thanks Chuck, I will absolutely give that a try!