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Bolex Prism - putting filters *behind* it


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#1 Nick Mulder

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Posted 22 April 2007 - 07:33 PM

Hi,

I've used red filters for B+W stills photography for the effect (especially outdoors) and reduced DOF from the three stop reduction in the particular filter I was using - I've attached the same similar filter again but I'm getting next to nothing through my viewfinder as a result. This wasn't an issue with stills as I could simply remove it to frame up the composition - but with moving pictures or should I say moving camera its a problem ...

I was thinking of getting some wratten/gel style red filter and putting it behind the prism so I would still get a full brightness finder image - I'd make a little mount or at a pinch just use a little glue (theres a few ways to go about this depending on how much time I have, but I'm confident I'll get something going well) ... Could even put it on the shutter itself ...

My question is with the filter now even closer to the film plane will the image be affected 'locally' by any defects in the filter ? (that would otherwise be diffused out into a general loss of sharpness)

I know the simple answer is to test it, and the logic says well yeh depends on the quality of the filter and how parallel you get it - My query is more along the lines of > "has anyone tried this and do they have some qualitative info as opposed to quantitative"

I dont want to waste $$$ buying a rare wratten filter not available locally if its going to be a waste of time and my standard resin filter will do the job as per normally (and soak up my valuable finder lemons)

cheers!
Nick
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#2 Clive Tobin

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Posted 22 April 2007 - 08:06 PM

...I was thinking of getting some wratten/gel style red filter and putting it behind the prism...

Putting a gel filter either in front of or behind the prism will throw off the focus a little. The focus should be changed equally regardless of exactly where the filter is placed, either in front or behind the prism. If in front of the prism you can see the effect in the reflex finder.

Bolex used to publish charts of how much to fudge the focus to allow for the gel filter, but I don't think I have these any more. It is not much and can be disregarded if using a telephoto lens or if stopping down the lens.

Maybe stick a clear gelatin filter or gel skylight filter in front of the prism to set focus through the finder. Then remove it and film through the red filter behind the prism. If the thicknesses are equal the focus should then be correct on the film though slightly incorrect through the finder.

After removing the front gel filter, be sure to insert an empty filter holder or equivalent where the original holder was, or you will get a light leak.

Edited by Clive Tobin, 22 April 2007 - 08:08 PM.

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#3 Nick Mulder

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Posted 22 April 2007 - 08:22 PM

Maybe stick a clear gelatin filter or gel skylight filter in front of the prism to set focus through the finder. Then remove it and film through the red filter behind the prism. If the thicknesses are equal the focus should then be correct on the film though slightly incorrect through the finder.

After removing the front gel filter, be sure to insert an empty filter holder or equivalent where the original holder was, or you will get a light leak.

Smart! - I'll try it with some ND 0.1 wratten gels I picked up a while back for free - I'll test the back end with the same stuff scratched up a little on one half to see how local the effect becomes, thanks for indirectly reminding me I had this gel packed away doing nothing otherwise

It will be on an EL in most cases so I dont need to worry about the filter holder - but I've seen enough of a friends RX5 footage sans holder to remember that ol' bean ;)
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