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Whats in focus on the viewfinder


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#1 Bryce Lansing

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Posted 09 November 2009 - 01:45 PM

Before getting my Bolex, I read a lot about the complications of the distance of the lens to the prism. I read that RX lenses will be fine, but any non-reflex lenses used shouldn't be opened up past f4, or they will get soft results.

So I got my Bolex and my Nikon to C-mount adapter, hooked up my nikon lenses, shot a test, and sure enough, problems only at 2.8, 2, and 1.4!

Today I noticed that when I focus the lens to infinity, everything is soft. I have to pull the focus back to get infinity in focus. So I unscrewed the C-mount to nikon adapter slightly, pushing the distance of the Nikon lens a few millimeters farther, and found that the infinity was now focused at infinity.

Now it seems like this is the answer, Just pushing the lens out a bit and changing the mounting distance is solving the problem. But I'm shooting 2500 feet of film on my friend's short (budgeted by him), and I cannot let any shots go soft, or I'm in big trouble.

I don't have time/money for another test roll, so I'm a little scared to just say "problem solved", and shoot his whole film with the mount slightly unscrewed based on what I see through the viewfinder.

With the mount screwed all the way in, I have sharp results at f4 and up (as shown by tests), should I just shoot the whole film with the mount screwed all the way, and stay at f4 and above to be safe? Or should I shoot the film with the lens mount out?
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#2 Rob Vogt

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Posted 09 November 2009 - 03:36 PM

There are 2 kinds of C-mount to nikon adapters, ones made for the Bolexes and ones made for other c-mount cameras with a slightly different FFD (Eclair ACLs and NPRs) You might have the wrong one. Also check your diopter. I would definitely do another test, try to get some tri-x or plus-x reversal and shoot 100' get it processed for like 20 bucks.
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#3 Bernie O'Doherty

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Posted 09 November 2009 - 05:43 PM

The problem with using non-rx lenses on your Bolex is that they are set for 17.52mm. Bolex flange focal distance is longer because the prism pulls the light further into the camera. Instead of being 17.52mm, it becomes 20.76mm. The lenses can be shimmed-out to that longer flange focal distance.
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#4 Chris Millar

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Posted 10 November 2009 - 01:59 AM

Also check your diopter.


If you can see the grain of the ground glass across the full frame then your viewfinder/diopter is fine for focusing ;)
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#5 Jean-Louis Seguin

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Posted 10 November 2009 - 08:24 AM

As has been mentioned many times before on this board, I strongly recommend that you read Mr. Dennis Couzin's analysis entitled: "THE TRUTH ABOUT THE BOLEX PRISM" available at http://city-net.com/...olex/truth.html

This is by far the most accurate explanation of the effects of the prism on image results.

Cheers,
Jean-Louis
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#6 Chris Millar

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Posted 10 November 2009 - 08:21 PM

Yup,

on top of your back focus/infinity issues you're still using non-RX optimized lenses - this is where the f4 or smaller carry on came from originally, it just also happens to have partially solved your infinity focus issue ...

Mind you dont get the two confused ;)
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#7 Chris Millar

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Posted 11 November 2009 - 03:19 AM

http://city-net.com/...olex/truth.html

This is by far the most accurate explanation of the effects of the prism on image results.

Have a look at this one too:

http://www.apecity.c...6mm_cameras.pdf
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#8 Cary Lee

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Posted 21 November 2009 - 08:10 PM

The problem with using non-rx lenses on your Bolex is that they are set for 17.52mm. Bolex flange focal distance is longer because the prism pulls the light further into the camera. Instead of being 17.52mm, it becomes 20.76mm. The lenses can be shimmed-out to that longer flange focal distance.


What do you mean shimmed-out? How can it be done? Do you do this type of work?
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#9 Bernie O'Doherty

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Posted 22 November 2009 - 08:18 AM

Very much so . Since 1974.
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#10 Alex Malm

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Posted 14 April 2010 - 12:42 AM

If you can see the grain of the ground glass across the full frame then your viewfinder/diopter is fine for focusing ;)


You should also set your focus marks with the lens wide open since that will provide the shallowest dof and allow you more precision. Testing the focus ring with a focus chart and a buddy with a tape measure is advisable as well. And sometimes a lens will behave differently in extreme temps so if you plan to shoot in such conditions that's where you should test the lens.
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#11 Jean-Louis Seguin

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Posted 14 April 2010 - 09:03 AM

You should also set your focus marks with the lens wide open since that will provide the shallowest dof and allow you more precision. Testing the focus ring with a focus chart and a buddy with a tape measure is advisable as well. And sometimes a lens will behave differently in extreme temps so if you plan to shoot in such conditions that's where you should test the lens.





I have heard this focus wide open rule many times and i used to believe it too but it is essentially a mistake since virtually all lenses, especially fast ones, have some degree of focus-shift.

For a good explanation of focus-shift, I recommend this article:

http://diglloyd.com/...FocusShift.html

Quoted from this article, the key rules are:

1) Shoot wide open — what you see is what you get.

2) Focus at the shooting aperture — focus and shoot at f/1.4, focus and shoot at f/2, focus at f/2.8 to shoot at f/2.8, f/4, etc (by f/2.8 spherical aberration is all but eliminated).

Focusing at the shooting aperture is the only option for optimal results.

Cheers,
Jean-Louis
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#12 Simon Wyss

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Posted 14 April 2010 - 10:57 AM

What do you mean shimmed-out?

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#13 Duran Levinson

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Posted 11 May 2010 - 03:39 PM

I am studying cinematography and for a trailer we had to shoot i risked shooting on out school's Bolex H16. I love the look it gave but the small viewfinder was a huge problem.
I measured focus for all shots and many of them looked soft to me in the viewfinder.
At the end of the day when the sun was dying I was getting rushed and manually focused the final shot with my eye. BAD IDEA.
But it was a good experience. I have already shot the film with lots more Bolex footage and it came out great.

Here's the trailer on Facebook. And it is only the 2nd time I have shot on film.

http://www.facebook.com/home.php?#!/video/video.php?v=380521311929&oid=324180968934

Duran
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