CameraMods question: I just can’t sent up focus point. Any advice how to measure distance between lens and film back?

 Piotr is in the UK.

After seeing my post and videos, he started building his own Instax Wide 300 camera mod.

Now he is setting up the lens. He wrote to me at vancouverbydesign gmail with this question:
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Q: Hi JJ Lee:

Hope you are enjoying your weekend.

I’ve put the camera together.

I’ve got Mamiya Universal 100mm f3.5 lens.

Colour rendering is amazing, I just can’t sent up (the) focus point (sic). 

Any advice how to measure distance between lens and film back?

Thanks a million.

A: Hi Piotr:

To set the lens. Create a rough ground glass.
Use a piece of Plexi or clear plastic like the cover of a cd jewel case and sand it.

This acts as the ground glass.

Then, insert it into a cannibalized Instax film package. Now you have a ground glass that can work with your set up.

Use it to adjust for infinity focus. The rig should have an adjustment. Though on a Mamiya Universal I don't know how to do that. (Actually, all Piotr has to do now is set the correct barrel length).

JJ

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And here's what he sent back.

The view through the ground glass is so terrific. A magic window.

Looking forward to Piotr's finished product. Thanks for writing.