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Yvar 36mm f/2.8 Visifocus lens - Bolex B8


Chris Smart

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Hello everyone,

First time posting on the forum. Thank you all for taking the time to read this.

I've been experimenting and testing various standard 8mm cameras for the past 2+ years now. I recently acquired a Bolex B8 equipped with 2 lenses. One of these lenses has the Visifocus depth of field scale. I'm having a hard time understanding how to shoot these types of lenses because up until now all of my other cameras have had fixed focus lenses. I've come to realize I still have lots to learn on how to shoot and focus successfully with analogue motion picture cameras, especially with subjects moving towards the camera. I use a lightmeter app on my phone to calculate the proper exposure depending on the shooting speed and this has greatly improved my results over the years.

My question is, if I'm shooting at 24 fps and the lightmeter app says i need an F-Stop of 2.8, but the Visifocus reading indicates the 'zone of sharp focus' is under 7 feet at 2.8, how am I going to focus on a subject that is 20 feet away?  If i adjust the Visifocus dial on the lens to F22, zone of focus says 4-30 feet, then wouldn't the scene be under exposed? 

I give this example because thats exactly what happened to my footage. All of the scenes I shot with the 36mm lens at 2.8 are extremely out of focus but the exposure is correct. All the footage i shot with the other lens on the B8 looks ok. That lens is the Yvar 12.5mm f/2.5 fixed focus lens.

Any tips on how to shoot and expose correctly with these lenses would be greatly appreciated.

Thank you!

Chris

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My lightmeter is telling me to set the fstop to 2.8 to get the correct exposure. If I set it to 20 feet away then I should be setting the fstop to F22 according to the Visifocus dial on the lens. I would assume my scene would be very under exposed if I did that.

Edited by Chris Smart
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Most of the Kern D mount lenses for 8mm have focus rings as well as aperture rings, definitely the 36mm lenses do. The Visifocus scale is just a guide to help you work out your depth of field, but you need to set the focus to the correct distance. 

The only fixed focus lenses, from memory are a 5.5mm Pizar  (which will have very deep depth of field because it’s such a wide angle) and a couple of Yvars in the normal range around 12.5mm. Fixed focus lenses are usually set to around 8 ft, with depth of field for a 12.5mm wide open giving you a focus range of maybe 6ft to 12 ft (I’d need to look up a DOF chart to be accurate). The more you stop down the more you can get some sort of focus on closer and further objects. But obviously you can only stop down if you have enough light, and with movie cameras you don’t have the luxury of adjusting shutter speed as a variable (without changing the filming speed which is usually not an option). A variable shutter only gives you a stop of adjustment.

One thing you can do is unscrew a fixed focus lens from the mount (say half a turn) to make the lens focus closer. Or place a thin shim between the lens mount and camera mount (0.02mm to 0.05mm is all you need) to jack the lens out a bit but still have it securely screwed in. It’s possible some fixed focus lenses are not properly adjusted, if you find the focus isn’t set to mid-range distances. A lens tech can adjust them for you. 

If you want more focus control, then it’s probably best to invest in focussing lenses.

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Thank you both for the reply and thank you very much Dom for all this great insight. It's very helpful. Could it be that my lens needs service? There is a ring just below the visifocus dial but it does not budge at all. Not even slightly. This B8 was in mint condition but in storage for decades. It needed a CLA to get the motor working again. I see multiple screws on the lens ring indicated below. 3 very small and 1 larger screw. This camera isn't a reflex camera so I'm not even sure how I would go about adjusting the focus without being able to see the results through the view finder. My Bolex P3 Zoom Reflex is definitely better when it comes to getting an accurate focus. 

I just love this B8 and really want to use this 36mm lens.

Screen Shot 2021-10-23 at 7.21.57 PM.png

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The grease Kern used on their lenses unfortunately hardens when it dries out to a very gummy consistency, and many of their lenses have frozen or very stiff focus rings. That sounds like what has happened to your lens. It will need to be dismantled, cleaned and regressed to be functional again. Sometimes warming the lens can soften the grease, but it’s no long term solution.
 

Because most 8mm cameras are non-reflex you really need to rely on the focus scale for focussing, so you want the scale to be accurate. If you remove those little screws around the focus ring you can pull the rear focussing section out (with effort if the grease is hard) in order to clean and re-grease it, but you will lose the focus mark setting. It’s possible to reset it if you have a reflex camera or a camera with a rack-over focus, but it’s tricky for most non-technicians so I think best bet would be to send it to a technician familiar with these lenses.

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Thank you very much Dom. I had no reason to believe the lens was frozen. I've never had a lens like this before and the design is so clean, I just assumed it was fine. That ring doesn't budge at all.

Now i need to find a lens technician.

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