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Bolex lens focusing problem with eye-level focus (non-reflex)


Robert House

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I have a Bolex H-16 Supreme (non-reflex model) with eye-level finder. The camera has 3 lenses: a Switar 25mm, a Yvar 16mm AR, and a YVAR 75mm. 

The problem that I am having is that with the two Yvar lenses, when I get the image in focus on the ground glass of the eye-level finder, the distance on the focus ring does not match the actual distance of the subject.

I am focusing on an object that is measured at 8 feet from the camera. When the image appears in focus in the eye-level finder, the distance indicated on the focus ring is around 4 feet on the Yvar 16mm lens, and is at Infinity on the Yvar 75mm lens. (The 25mm switar seems to be about correct.)

I really don't know what is going on here. I can't imagine that the problem is with the eye-level finder... 

Can anyone help me understand what is going on here?

 

IMG_2873.jpg

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Have you looked at images in the gate ?  If you don't have the Bolex gate-focuser maybe some tracing paper held in with tape.  Shutter set on 'T'.   A magnifier should show if the Yvars are in focus or not.  Assuming your lenses are screwed in properly ?

Edited by Doug Palmer
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I’d say that the two lenses got opened at some time and not adjusted properly. If you want to improve on it yourself:

Loosen the three set screws in the focusing ring, just loosen each by about half a turn, then turn the focusing tube with your fingertips until image appears sharp in the finder (on whatever distance you like, infinity is best, the horizon for instance), and bring focusing ring with that distance on reference mark. Finish by tightening the screws back.

Shorter distances are a bit tricky due to the personal hypofocal or hyperfocal view. You need to see the ground surface tack sharp.

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Thanks for the replies so far. I have done some additional measurements, so I will add more info.

I set up the camera aimed at my computer screen ( a bright subject) at a distance of 5' 3" from the film plane measured with a tape measure. Then I focused all 3 lenses, and I now think all 3 seem pretty off ?

The readings on the 3 lenses when focused at this distance are as follows:

 

Switar 25mm — 6’10”

Yvar 16mm — 2’7”

Yvar 75mm— 13’

Also... I recently had the Bolex given a CLA (just the camera itself). I had been thinking that the ground glass prism element inside the camera was somehow improperly located. But it is a bit disconcerting that NONE of the lenses seem to be focusing properly... and two are indicating distances further than the actual measured distance, while one (the 16mm yvar) is indicating a closer distance.

Thank you Simon and Doug for the feedback. Simon... do you suppose that all 3 lenses are adjusted incorrectly? I bought all 3 recently on ebay. Is this that common of a problem?

Rob

 

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I am going to try what you suggested Simon. One follow up question... all three lenses have the 3 (very small) screws on the focus ring. But there is also an additional slightly larger screw just above the 3 tiny ones. In the Switar it is in the silver focus ring. On the Yvars it is in the black band that has the numbers. Do you know what that screw is for? Do I need to loosen that one as well?

Also... as far as focusing... I assume that regardless of what I am seeing indicated on the actual focus ring, what I am seeing as focused on the ground glass in the Eye-level focuser is what I should get in theory on the actual film... am I thinking correctly on this? 

In other words... I am currently thinking that the focus rings are not correctly lined up on any of the 3 lenses.

IMG_2874.jpg

Edited by Robert House
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The large screw acts as an end stop, you don't need to undo it.

The eye-level focuser ground glass should be at the same depth as the film plane, but that's something whoever serviced your Bolex should have checked. As Doug mentioned, you can try to check the focus at the gate with a prismatic focuser or a piece of tracing paper, and compare it to the viewfinder focus, or just shoot a test. One test method is to film a sheet of newspaper at an angle, focussed on a particular word that you circle, then check if the processed footage is also focussed on the same word, or at a distance closer or further away. Adjust your lighting to shoot with the lens set at maximum aperture (to get the shallowest depth of field) to pinpoint where the focus plane is.

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  • 2 weeks later...

If you can get hold of a mirrorless camera and a C-mount mount adaptor you can get pretty accurate calibration of the lens's distance scale using that and a measuring tape to ensure the distance from film/sensor plane to the subject is accurate. It's nice to have at least one lens that you know has a correctly calibrated distance scale to check your C-mount mount adaptor/camera combination is set to the correct flange distance, not essential but gives you some peace of mind.

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