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Bolex intervalometer exposure help

Erren Franklin

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Hello! I've recently purchased an intervalometer for use with my Bolex H8REX4. It's the kind that uses the 1:1 drive shaft. The maker says that the exposure time is .75 sec but says that Bolex light loss from the prism means that exposure should be set to a .5 sec exposure time. I'm having a difficult time working out the math for how I should compensate for this exposure time when I set my aperture. I assume I'll need ND filters. Does anyone have any guidance here? I'd love some simple help, ie: tell me how many stops I need to compensate for, but I'm also into finally trying to understand the math so I can figure this stuff out myself in the future. Any help will be much appreciated! Thank you!

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  • 1 month later...

It would help if we knew which make and model device you are planning to use.  Yes, there is some light loss due to the beam-splitter prism in the BOLEX H-8 Rex, and this should also be addressed in the Bolex Instruction Manual, which I HOPE you have and refer to!   Careful reading, and making notes for yourself will help avoid various problems.   The fractions  you cite out of reference make me wonder if they mean a Half Second or 5/10ths second exposure?


The light loss due to the prism is about 20% according to BOLEX.  That is a small amount, so less than 1/3rd of an F-Stop.  BOLEX has exposure factors printed to compensate for that loss, as well as for when using the Variable Shutter. But if you don't have those, then just factor in that you will need to exposure an additional 20% more.  Also, importantly is the film stock you plan to use.  If it's REVERSAL Film, then that slight under-exposure could be helpful.  If it's NEGATIVE Film [either Color Negative or B&W Negative or B&W Reversal to be shot & processed as Negative], that small amount is still insignificant in many situations, but you could benefit to expose extra for to compensate. 

I recommend conducting a very short test of just a couple feet and processing that to see where your results are, and what adjustments to make.  You could include a Color Chart/Density Chart in the image scene to help make fine tuning adjustments to your notes for when you tackle your project.

Some Intervalometers I have seen for the BOLEX have the exposure determined by the Single Frame setting [I for Instant].....or the actual timing of the unit when the Exposure Setting on the camera is set to T [for Time].  I hope this helps you somewhat in setting up your camera.

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