Bolex Rex 5 prism reflection in viewfinder

[Jon O'Brien]

I've got some strange reflection I can't account for happening in the viewfinder of a Bolex Rex 5. I've been so absorbed in digital cinematography and other things that I haven't filmed with a reflex Bolex for a while.

What happens is that a thin reflection, about 1/10 height of the viewfinder and almost the full width of the frame, appears towards the top of the viewfinder image when filming at certain angles in brighter light. It is actually a reflection of the top edge of the lens image circle. It appears as a dimmer but still very noticeable image on the ground glass surface of the reflex prism. If you move the camera you can see it is the edge of the image circle. One side of the reflection is curved as you would expect. The reflection only appears when the light entering the lens is at a certain angle, but it appears often. It happens with both lenses currently fitted to the camera: 26mm and 16mm RX Switars.

It can be eliminated with my hand being lowered until it is almost visible in the top of frame. I could use a matte box top flag to hide the reflection but I don't currently have a matte box that fits.

Is this reflection likely to also appear on the film? I suspect not as I've never noticed a problem with the footage from this camera in the past. But I don't want to risk wasting some footage.

Also, when I look directly at the prism, with the lenses moved aside, I can see a dull brownish/grey horizontal line, about 1 or 2mm high, towards the bottom edge of the prism. It almost seems like glue separation or something similar. It seems to be on the front of the prism or just beneath. Do the prisms have a thin coating at the front surface, that can start to discolour with age? I tried polishing the mark off with lens cleaner and lens tissue. It's smaller and less noticeable now but still there. Any ideas? Thanks!

 

Edited May 24, 2025 by Jon O'Brien

[Dom Jaeger]

Separation of the beamsplitter prism can occur, but you can also see the prism join at certain angles, so it might be totally normal. A photo or two would certainly help.

The ultimate test is what you get on the film. I’ve seen all sorts of issues with optics that seem awful but barely affect the image. 

Is the viewfinder image centered and complete? Sometimes if the right angle prism at the end of the viewfinder tube is not properly aligned the groundglass image can be offset and you might see reflections from outside the groundglass mask. 

But again, it’s hard to diagnose anything just from a verbal description.

[Jon O'Brien]

Thanks for your help Dom. The viewfinder image is centered and complete. I will see if I can get a good shot of the front of the prism. Maybe I can get a shot of the viewfinder image too. Also I will shoot a part of a roll with the camera, inducing the effect while I film, and see if anything records on the film itself. It might take a while.

It seems to me that the horizontal mark along the bottom part of the front of the prism is likely the culprit, as if the light is reflecting off there. My best guess is some kind of separation of a coating that is on the front of the prism. I came to this conclusion because it looks like it's on the front surface, plus I was able to polish off part of the mark, but not all of it. Bottom part of front of prism, and top part of viewfinder image, in approximately same part of frame, suggests they are related.

However ... how would the edge of the lens image circle get reflected off the front of the prism? The image circle would extend well beyond the edges of the prism. It's a mystery.

Edited May 25, 2025 by Jon O'Brien