# Bolex extension tubes

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### #1 Arnis Hagendorfs

Arnis Hagendorfs

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Posted 26 June 2008 - 10:38 AM

Hi,

Can anyone share exposure compensation tables for original Bolex extension tubes (5, 10, 20, 40mm).
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### #2 todd eacrett

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Posted 27 June 2008 - 01:44 PM

If you can't find a chart, the bellows extension formula will work.

BL squared / FL squared x Time = adjusted Time

or

Bellows Length squared divided by Focal Length squared = Exposure Factor.

Bellows Length in this case is the focal length of the lens PLUS the length of the ext tube(s).

For instance, using a 25mm lens with a 10mm tube --> 35 squared (1225) divided by 25 squared (625) = an exposure factor of 1.96 or basically 2. In which case you need to double your exposure (half your frame rate or open 1 stop). Using a 25mm lens with a 25mm tube (2500/625=4) requires 4 times the exposure, or 2 stops.

Rather than doing the math, an easy rule of thumb is for every 50% extension over the actual focal length you need to double your exposure (twice as long a shutter speed or open 1 stop).
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### #3 jake harris

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Posted 15 August 2008 - 08:48 AM

Rather than doing the math, an easy rule of thumb is for every 50% extension over the actual focal length you need to double your exposure (twice as long a shutter speed or open 1 stop).
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I've been trying to figure this out for a while and I eventually bought a leaflet of ebay with macro tables for d.o.f etc. Basically you open up 1/3 of a stop. Unless you extension is at the same length of the lens or over in which case its 2/3 of a stop. I'll try and scan the leaflet for you, its a photocopy though so quality might not be great. Does specify what d.o.f is with certain extensions and lens types, switar etc.
Good Luck!
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