Info on reduction/stepper for telephoto lens

[GigiG]

Hi Robert, first of all, many thanks for sharing your great project. I was thinking about making a DRV8825 version of your system to control the focusing tube of my old Sigma 400 f5.6 lens (Nikon screw driven autofocus). My concerns is about the stepper/reduction gear combination. This lens has a focusing tube that rotate for a very limited amount of degrees, about 50° total from infinite to the minimal focusing distance of 4m, so I suppose that a motor microstepping capability, togheter with a lot of gear reduction, is needed in this case. The focus tube has very little resistance (friction) on rotation so no heavy torque required. I've got a little stepper like this one (12V 18°/step, built-in 36:1 reduction gear, 720 steps per rot. final) and I'm thinking to use a simple rubber belt to transmit motion to the focus tube (that is about 60mm in diameter). I was wondering if this gear combination is suitable for focusing's sufficient accuracy or more reduction (28BYJ-48 stepper) is better. Thanks.
Gigi

[brownrb]

Hi
You need a lot of reduction on SLR cameras. Very minute changes make the difference between focus and not in focus, especially as one approaches infinity, as I have found with my Canon EOS. I drive the canon EOS with a GT belt, on a PG27 NEMA 17 stepper. Yes you can use the 28BYJ-48 stepper. The idea is to get a lot of reduction. I know using a small 15T pully on the stepper and a GT2 belt driving onto the rubber grip of the lens assembly works well. Its easy enough to set limits in a focuer - I have figured out what the steps are so I generally connect up, set the lens focus all the way in, connect the focuser and belt and stepper, set the position to 0, then set maxsteps and I am away focusing.

There is a belt calculator for working out reductions using a belt system - see
http://www.bbman.com/belt-length-calculator/

and
http://www.bearingboys.co.uk/Timing-Belt-Finder--Calculator-24-a

For belts and stepper pulley search on ebay for GT2 (belts are 6mm wide)

There is a project in development - beta testing at present only - for directly talinkg to the canon eos and doing the focusing automatically - under control of the myFocuserPro2 controller - but its not ready for release yet.

Cheers
Robert

[brownrb]

I forgot to mention that the most common lens I use is the EF70-200mm f2.8L USM. One awesome lens but very very sensitive to focusing being f2.8

Cheers
Robert