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Does a viewfinder attached to a lens effect exposure?


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#1 Brenton Lee

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Posted 23 April 2016 - 04:59 PM

Hi, long time casual browser, first time question asker.

 

I was wondering, when shooting film (in this case, regular 16) with a lens that has a view finder attached (in this case, a Som Berthiot 17.5-70)  ... is there some type of optical prism in the lens to allow this viewfinder to work and does it effect your exposure measurement / setting? Surely it's robbing a bit of light as it passes by?

 

Sorry if this has been discussed already on this forum but couldn't find a thread with the same question. Excuse me if I've doubled up.

 

Thanks for your help

Brenton. 


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#2 Gregg MacPherson

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Posted 23 April 2016 - 05:04 PM

There's a partially silvered prism in there that does take light for the eyepiece.  I have had a Som Berthio zoom with one of these finders and I think I allowed that it took 1/3 of a stop.  Long time ago.  Perhaps someone who knows this zoom can tell you.


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#3 Brenton Lee

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Posted 23 April 2016 - 05:22 PM

There's a partially silvered prism in there that does take light for the eyepiece.  I have had a Som Berthio zoom with one of these finders and I think I allowed that it took 1/3 of a stop.  Long time ago.  Perhaps someone who knows this zoom can tell you.

 

 

Okay, that sounds about right. So between different shutter angles and prisms on my two Bolex's, I have a lot of things to remember, haha. 

 

Thanks. 


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#4 Dom Jaeger

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Posted 24 April 2016 - 12:51 AM

I can't remember if that particular zoom has a red T stop (for transmission) index mark next to the f stop mark, but the total light loss of both reflex dogleg prism and internal reflections probably ends up being more than 2/3 of a stop. It's a very early zoom, later ones certainly had the T stop marked, since that much light loss was problematic for cinematography.

The different shutter angles of Bolexes can certainly be confusing, I haven't seen an online resource that accurately describes the
changes by serial number other than this old post:
http://www.cinematog...showtopic=55725

Watch out for fungus in those old Berthiot zooms, I've seen it in quite a few, must be the type of coating they used.
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