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Bolex intervalometer exposure help

Erren Franklin

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Hello! Posted this originally to the super8 forum, but there are many  Bolex users here, so I'm sharing here too.

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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You shouldn't have to compensate anything.

The maker made it so that the shutter is open for .75 seconds. The prism will take 1/3rd of the light, leaving 2/3rds of the light through to the film, so your effective exposure time as seen by the film is 0.75sec * 2/3 = 0.5 seconds.

1/2 second, conveniently, is an easy number to work with photographically, and probably why the builder chose that weird .75 sec mechanical shutter time in the first place.

So treat your Bolex as if it is a still camera with a fixed 1/2 second exposure time. Set you meter to sill mode, dial in 1/2 second exposure, and adjust your aperture and ND accordingly.

All this assumes that your lenses so not have iris rings marked to pre-compensate for the prism loss (Bolexes are not my specialty, so maybe someone can chime in)  if the lenses are marked to take the prism into account, then set your meter to the speed  between 1/2 sec and 1 sec, which is technically .707s but may be listed as .7 or 3/4, either way it's close enough to make no difference.

they key thing is that your movie camera is now a still camera with a fixed exposure time, and you have to calculate aperture and ND as if you're taking stills

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  • 4 weeks later...

If you're shooting on negative film, you don't have to be so precise with your exposure calculations. Actually, some overexposure would likely be beneficial. If you've settled on 1/2 a second, you could select 1 second or a little slower on your light meter. 

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