Backfocus plug for 35mm adapters

[Bob Gruen]

I built a backfocus plug for my HVX-200 / hard mounted RR Micro 35 system, and you can too!

What does it do? Simply jack the plug into the remote focus jack and it locks focus on to the ground glass, every time. It disables the focus ring so you can quit putting gaffers tape on your expensive camera. The remote focus jack works by testing the resistance between the base and center node of a 1/8th inch stereo plug. From Radio Shack You will need:

• A pack of 1K resistors (271-1118)
• 1K Ohm, 15 turn Cermet Pot (271-342)
• 1/8 inch stereo plug (274-284 )

To start, I did some testing with a 20K pot and a 1/8th jack and found that the resistance value I needed was about 4.25K Ohms (this sets the focus value at 15/16 juncture). I then wired four 1K resistors and the 1K pot in series (end to end) so that I can adjust the load between 4K Ohms and 5K Ohms. My exact value turned out to be 4.32K Ohms, but yours may be different due to the GG position and hardmount / softmount issues.

Bob














For more info...

http://www.dvxuser.com/V6/showthread.php?t=104801

[Bob Hill]

Hiya Bob,

I made one of these up right after you mentioned it on the other thread. This is must have for folks with 35mm adapters.

[mikkowilson]

This is an awsome invention Bob. I've bounced it through the 35mm section to leave a redirect, before making it one of the few DIY stickys.

I'd love to see a couple of pictures of your unit.

- Mikko

[MarcMalkowski]

So if I need a 21juncture, what will I need?

[Bob Hill]

I need to finish off a unit that lets you set a fixed focus and still have the iris adjustment available. I should have that done this week and I will post up pics and plans. Again, great idea Bob.

[Noah]

For me, ideally, I'd love to have fixed focus and zoom while still having iris control. Is this possible?

And thanks for doing this much already. The slipping focus issue has become a big problem, especially when doing handheld stuff.

[JasonFox]

Very, very cool! But I'll definitely need to see some pics before heading down to The Shack. But this will be such a handy thing to have.

[Bob Hill]

Noah,

The basics of the focus control are the same for the zoom. I think ideally everyone is going to want to have fixed focus and zoom but still have control of the iris and rec/pause functions remotely. Basically the difference between the two units would be the size of the plugs. I'll post up details as soon as I have something sorted out for an off the shelf case for the trim pot and female plug.

[snodart]

This is going on my list of DIY things build. Looking forward to those details

[Noah]

Gotcha. It would be amazing to be able to customize what you need vs the size of the unit. For instance, I have no need for the rec/pause, iris, focus, or zoom remote capabilities. All I would really want is to lock the focus and zoom where they are and still have control over the iris and record buttons on the camera. This would then be a much smaller unit than having everything with remote capabilities (which if I'm not mistaken already exists - just not in the cheaper diy world).

[Bob Gruen]

For me, ideally, I'd love to have fixed focus and zoom while still having iris control. Is this possible?

Zoom maps the resistance to the rate of change of the zoom value. At 10K Ohms the zoom does not move. At 9K or 11K Ohm is zooms slowly in one direction or the other. At 15K or 5K Ohms it is zooming very quickly in one direction or the other. Think of it like the zoom rockers on the handle, it will control the zoom servo, but bumping the zoom ring (which is purely mechanical) will override or damage it.

To effectively lock down the zoom, simply set it where you want it, engage the servo switch, and unscrew the micro-knob on the zoom ring and store it in a safe place. The reason I remove the knob is that snagging it on something with the servo gears engaged could damage the gears. I have yet to mess up a shot by hitting one of the zoom rocker switch. I'm not sure if inserting a plug disables the toggle switches or not.

Bob

[Bob Hill]

Bob,

I was thinking that the plug disables the toggle switch but I could be wrong. I'll check it out. I am still waiting on some tiny cases to make up a final piece that lets you use an iris controller and rec/pause button in conjunction with the focus lock. I'll post pics and plans once I get it all sorted out.

[Jason Ramsey]

Wild. Thanks for sharing this, Bob.

[Bob Hill]

Just wanted to let everyone know that I am still working on getting a good set of plans with a parts list put together for this. I have the parts list mostly worked out but finding small cases is a real PITA! I should have something for everyone this weekend though.

[Avenger007]

hows this coming along? Any updates?