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Vario Switar POE - 250 film speed exposure compensation?


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#1 Richard Ashrowan

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Posted 25 September 2012 - 06:47 PM

I have a Vario Switar POE on a Bolex. The film speed dial on the lens only goes to 200 ASA, but I am shooting using Kodak 7222 daylight stock at 250 ASA. Does anyone know what kind of exposure compensation I might need to apply to get a correct exposure using the over-fast 7222 film for this lens?
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#2 Chris Millar

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Posted 25 September 2012 - 07:58 PM

Stop down an extra 1/3 stop

To be exact:

Log2(250/200)


...or more generally Log2(filmSpeed/meterSetting)

positive results infer you need to stop down - negative that you need to open up




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#3 Richard Ashrowan

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Posted 26 September 2012 - 04:53 AM

Thanks Chris - a perfect reply with a useful math tool! Much appreciated.
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#4 Chris Millar

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Posted 26 September 2012 - 06:21 PM

Actually, the math is a bit upside down.

If you use:

Log2(meterSetting/filmSpeed)


the results might be a bit more intuitive - positive means open up, negative = stop down


Whichever way your head works ;)
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#5 Robert Lewis

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Posted 27 September 2012 - 11:48 AM

I have a Vario Switar POE on a Bolex. The film speed dial on the lens only goes to 200 ASA, but I am shooting using Kodak 7222 daylight stock at 250 ASA. Does anyone know what kind of exposure compensation I might need to apply to get a correct exposure using the over-fast 7222 film for this lens?


This is the Mark I POE lens by the sounds of it. The highest ASA setting it could take was, as you say, 200ASA. The Mark II version of the lens could be set to 400ASA.

So, if you wish to use the auto-exposure function of the lens you can't really use manual exposure settings unless you are shooting a picture which has a steady light reading because the auto exposure will contually vary the exposure as the light reading changes. You can't do that in any event unless you have an exposure meter and unless you know how to take into account the Bolex prism factor which one has to have regard to when using a conventional exposure meter. A Bolex exposure meter takes the prism factor into account, and so if you had one of those you could lock the exposure control on the lens and use manual settings.

To use the lens you have on auto setting, you need to reduce the light going through to the film by 1/3 of a stop. You could use a 1/3 of a stop filter behind the lens. The lens will, on its 200ASA setting, allow 1/3 of a stop over exposure. So you use your POE lens on 200ASA setting and use a 1/3 of a stop filter behind the lens. That will then reduce the light going through to the film by 1/3 of a stop.

That having been said, however, having used EK 7222 film myself, I am not so sure that 1/3 of a stop over-exposure would be significant.

I hope this is helpful.
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#6 Chris Millar

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Posted 27 September 2012 - 05:57 PM

As you've pointed out, if we choose to he concerned about 1/3rd stops with neg film stocks the prism will be a factor. Funny thing is the 250 to 200 offset would work out kind of well, it would over-expose about what the prism under-exposes
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