Bolex extension tubes

[Arnis Hagendorfs]

Hi,

 

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

[todd eacrett]

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).

[jake harris]

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).

 

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!