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Eye vs. Tape Focus


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#1 Justin Schroepfer

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Posted 28 March 2012 - 06:26 PM

I was doing an eye vs. tape focus on my Bolex and I noticed the measurements weren't matching. It was a difference of about 4-5 inches or so on my 16mm and 25mm lenses.
I decided to try adjusting the lenses by not having them screwed into the C-mount turret as much (just a very slight adjustment). This got the eye and tape focus measurements to match because of the change in flange distance.

But is it possible that even with eye and tape focus measurements matching, that something could still be off?

I will do a test soon, but I'm just wondering if anyone here might know anything. Thanks.
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#2 Dom Jaeger

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Posted 28 March 2012 - 08:50 PM

Well there's 3 factors involved - the camera flange depth, the ground glass setting and the lens back-focus.

If the ground glass is set to the same depth as the film plane (which is how it should be), then what you see through the viewfinder is what will be on the film. In that case, if the lens marks aren't lining up either the lens back-focus is out, or the flange depth is out (and the ground glass out by the same amount). The lens may not reach infinity (or it will pass through it) but what's in the viewfinder will be what's on the film.

If the ground glass isn't set to the same depth as the film plane, then what you see won't be what's on the film.

So if your lens marks aren't matching the distance measured, it could be that the ground glass is out, or it could be that the lenses are out. Until you get the film back you can't say whether the camera flange depth is out as well. If you find that all your lenses are out in the same direction, and getting worse with wider focal lengths, then chances are the lenses are OK but there's an issue with the camera.

Unless the gate has been removed (for a DIY conversion, for example), or the reflex prism fiddled with, generally the ground glass should match the film plane. It's quite possible for the camera flange depth to be out, but still match the ground glass (if the turret plate is bowed out for example, both flange depth and ground glass will be out by the same amount). So going by eye-focus is probably more reliable than trusting the lens marks and a tape. But if the camera and lenses have been professionally checked or serviced, it's better to tape out distances for lenses up to about 50mm focal length - you'll get sharper results, and judging best focus on wider lenses by eye can be difficult.

The only way to check whether all 3 critical settings are correct is to have the camera and lenses professionally checked. You could shoot chart tests for each lens, but if something is out of tolerance and the tests are soft, you may not know whether it's the ground glass, the flange depth or the lens.
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#3 Bernie O'Doherty

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Posted 28 March 2012 - 09:04 PM

It could be so many things :
! : The camera's flange focal distance may not be set at 17.52mm (non-reflex Bolex) or 20.76mm (Reflex or Rx Bolex).
2 : The groundglass may not be in the correct position.
3 : The lens/lenses may not be properly collimated to the proper mount distance regarding infinity setting, and
4 : The lens rear thread may be touching and thus moving the prism block, throwing focus off.
Another factor to consider is the stability of the film in motion, and this would involve the claw transport and the pressure plate setting. Claw length and film gear alignment also play an important role in providing a focused image.
A further consideration would be to ensure that you're using Rx lenses on a Rx Bolex , and "c" mount lenses on an older non-Rx Bolex.
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