WB: Bolex Prismatic Focuser

[Bjorn Koling]

Hi

 

Im looking for a Bolex prismatic focuser prism for checking the focus at the plane of the gate in my Bolex EBM.

Thanks!

 

Björn Köling

[Simon Wyss]

If I may draw your attention to something important. These focusing aids are dangerous to the polished film rails. Glass is harder than steel, the rough prism can scratch the film supports which is highly unwanted.

 

It is not easy to understand how Paillard engineers wouldn’t have grabbed that. They’d better had had polished prisms and suggested the use of a piece of matted film. If you pick a Standard model you can use the built-in focusing prism together with the rack-over accessory.

Edited September 8, 2018 by Simon Wyss

[Doug Palmer]

I agree. It needs to be used with great care so that it doesn't scratch the gate. Also the unit tends to come off quite easily as it's only magnetic. And this increases the risk of scratching by mistake. Paillard's earlier model was better I think. You can get away with just using some matted film as Simon says, as long as it fits tightly against the front. But the prism does make life easier and I used one here.

https://filmisfine.com/blog/ultra-16-viewfinder-for-bolex-h16m-super-16-similar/

Edited September 8, 2018 by Doug Palmer

[Bjorn Koling]

I'll only use the prism once to adjust the beamsplitter prism in my Bolex EBM. But I'll be careful thanks! Please let me know if you see one for sale!

[Simon Wyss]

To adjust its position vertically, I presume. I hope it works well. If you have only the mount screws loose, you can do it so. Once you have the set screws loose you will need a collimator for readjusting the double prism perpendicular to the optical axis.

[Bjorn Koling]

To adjust its position vertically, I presume. I hope it works well. If you have only the mount screws loose, you can do it so. Once you have the set screws loose you will need a collimator for readjusting the double prism perpendicular to the optical axis.

 

Here is the short version: Bought a super16 converted Bolex EBM from Mr.Bolex. I get the camera, do a film test and the focus in the captured image is a few feet behind the subject in focus in the viewfinder. Mr.Bolex tells me to adjust the small set screw to the camera-left of the beamsplitter prism. As I understand it that screw only swings the beamsplitter prism on its hinges and does not adjust the focus of the prism/ground glass.

So I guess I need to move the prism either backward/forward in relation to the gate or as you said vertically.

Could I adjust the prism vertically by just loosening the mount screws?

 

One option is too do this myself with the help of a prismatic focuser. The other alternativ is sending the camera back to the seller.

 

What are your thoughts Simon?

[Simon Wyss]

Quite clear, adjustment of optical systems is a technician’s job and it takes special instruments, too. A focuser won’t do.

 

You’re right, the threaded pin you mention locates the double prism around its vertical axis. I’m sure you’ve noticed that it is additionally blocked by a set screw. But there is more to the story, the distance between double prism and image plane, the position of the prisms in all three dimensions, and the axes of the finder prisms must be also be set.

 

What you can do yourself is check the finder, first of all the eyepiece. If you can’t achieve a perfectly sharp view of the matte surface, everything else needs to wait. I don’t know anything about your vision. Many people have eye defects, keratoconus, myopia, hyperopia, astigmatism. I am a myopist and hope your eyes are good. After loosening the ocular blocking ring you adjust the eyepiece by turning the ring the eyepiece sits in. No lens on the camera, camera pointed to the sky or any well-lit uniform surface. When sharp hold the ring and tighten the backing one back.

 

Om du vil ha en undersökning ge mig ett meddelande, var så god.