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Measuring Focus / Back Focus Issue


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#1 Alexander McCarron

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Posted 13 October 2015 - 09:10 PM

Hi,

 

I’m trying to shoot something on Plus - X, I have a Bolex Rex-5 with a wonky viewfinder that kind of comes out of the camera body and has to be pushed back in if you knock it out of place. My lens is an Angenieux 12-120mm 1:2.2 Bolex H16 RX. 

 

I want to be able to focus with tape because the viewfinder is so dark and I’m shooting some interiors where I have to be all the way open. When I tried to set my camera to the difference between focal plane and subject was 7 ft, the image was soft in the viewfinder. I focused by eye to around 5 ft. I was at 30mm, f/4.

 

I checked to see if the viewfinder was focused by taking off the lens, pointing the camera at a bright light and fiddling with the viewfinder focus ring to make the texture of the ground glass visible. It seems to be fine.

 

What’s going on here? Is the back focus off? Is my lens not currently parfocal and I shouldn’t be using the zoom to focus? Am I going to have to take my camera to a camera tech or can I fix/deal with this myself? Is there some place or someone affordable in New York who can help?

 

Thanks guys,

Alex


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#2 evanwalsh

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Posted 13 October 2015 - 11:51 PM

I vaguely remember this from film school so forgive me if I am wrong but it may have to do with the prism the Bolex uses for its reflex system. I believe there are specific C-mount lenses that are supposed to be used with the Bolex. If you're using a lens that wasn't intended for use with the prism it could be the reason why you're witness marks are not sharp.


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#3 Dom Jaeger

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Posted 14 October 2015 - 06:44 AM

From the description that lens is made for a reflex Bolex, and even it wasn't, the Bolex prism tends to introduce aberrations rather than dramatically alter the focus mark, and only at fairly wide open apertures.

There's a few reasons why a lens might read off compared to the viewfinder. Most likely is that the lens itself needs a bit of adjustment. Or the camera flange depth is out, or the groundglass is out. Or it's a combination of these. A simple reason might be that the zoom is a bit heavy for the c mount turret and is pulling the turret plate away from the film plane ( but then that would cause the focus marks to be shifted beyond the measured distance not before it). Another thing is that it's important to set focus at the long end, then zoom out to the required focal length, rather than just try to focus at 30mm.

If you have another lens you could check whether its focus marks line up, which might give you an indication that the camera itself is correctly set up. The zoom should be parfocal, it was designed as a cine zoom, but if the back focus is off (or indeed the camera flange depth is out) the zoom will drift out of focus as you zoom from long to wide. If the zoom stays in focus (which admittedly can be hard to properly judge through a Bolex viewfinder) but reads the wrong distance it's likely the focus ring itself has slipped and needs adjusting.

Sending both camera and lens to a technician for checking is the best way to guarantee that your footage will look as good as it can, and a check of the basic settings shouldn't cost more than the cost of a wasted roll of film and its processing. Otherwise the safest bet is to rely on the viewfinder, which is less likely to be out of adjustment than the lens. But as mentioned it's sometimes hard to critically judge focus through a Bolex viewfinder..
Someone else may have a tech recommendation that's closer, but Jean-Louis Seguin is in Montreal.
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