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Super 16 Conversion Exposure?


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#1 Craig Mieritz

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Posted 25 November 2007 - 04:47 PM

Hi-

I recently purchased a Bolex EBM that was converted to Super 16. My question is: is there any further exposure compensation required (in addition to the standard prism/shutter correction) because of the larger aperture? I ask because the first roll of film I shot with the camera was seriously underexposed (maybe 2 stops).

I corrected for the 170 shutter (about 10%) and the 25% prism light loss (25%) by opening the aperture approx 1/3-1/2 stop. I had just shot a couple of rolls of regular 16 on a different camera using my ambient light meter and they were exposed perfectly, so I think the meter is fine. I used two different lens as well, a prime (Switar 10mm) and a zoom (Kern 16-100), so it should rule out a defect there. The camera was definitely at 24 fps.

Any ideas about what I'm missing will be well appreciated.
Thanks.

Craig

www.ubu-roi.com
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#2 Brian Drysdale

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Posted 25 November 2007 - 05:04 PM

There are no exposure adjustments required when shooting Super 16.

I'd check to see if the camera's shutter is set/working correctly and if you're using different lenses, the iris. Also check if the correct gel filter is in your internal holder, could be there's some ND.

Edited by Brian Drysdale, 25 November 2007 - 05:05 PM.

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#3 Craig Mieritz

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Posted 25 November 2007 - 05:20 PM

There are no exposure adjustments required when shooting Super 16.

I'd check to see if the camera's shutter is set/working correctly and if you're using different lenses, the iris. Also check if the correct gel filter is in your internal holder, could be there's some ND.


Thanks for the reply. The filter in the internal holder was not in the "aperture covering" position. It seems like shutter speed would have to be it. I sent the box to Dieter in AZ to service it and check the shutter speeds. Just wanted a reality check, that I wasn't overlooking something obvious.
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#4 Nick Mulder

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Posted 25 November 2007 - 05:56 PM

You're confusing the gate with the aperture...

The aperture is in the lens ... The gate will not affect the light falling on the film aside from the extra area in super16 - which may or may not vignette (which again is actually a function of the lens)
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#5 Craig Mieritz

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Posted 25 November 2007 - 08:11 PM

You're confusing the gate with the aperture...

The aperture is in the lens ... The gate will not affect the light falling on the film aside from the extra area in super16 - which may or may not vignette (which again is actually a function of the lens)


Oh dear, I'm more tired than I thought. Yes, I know the difference between the gate and aperture. And both lenses are good to go with Super 16 surface area. Well, the 10mm is iffy but it seems ok on this conversion. The underexposure was consistent between both lens and equal in the entire frame.

Wouldn't having a larger gate area mean that the light, at a constant aperture, would have a larger area to expose than in a standard 16mm? Wouldn't that mean that a larger gate area would require more light than a smaller gate (to get an equivalent exposure) at the same lens aperture? Same light/more exposure area=underexposure? I was also wondering if the amount of light being diverted by the prism would remain constant, or because of the larger prism area that receives light that there might be some variation there.

Edited by Craig Mieritz, 25 November 2007 - 08:13 PM.

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#6 Nick Mulder

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Posted 25 November 2007 - 10:46 PM

Wouldn't having a larger gate area mean that the light, at a constant aperture, would have a larger area to expose than in a standard 16mm? Wouldn't that mean that a larger gate area would require more light than a smaller gate (to get an equivalent exposure) at the same lens aperture? Same light/more exposure area=underexposure? I was also wondering if the amount of light being diverted by the prism would remain constant, or because of the larger prism area that receives light that there might be some variation there.


no

... but good try

You are correct in your 'fluxy' description of light being spread out more and therefore requiring more power, when you focus your trusty AA maglight you see happening before your eyes - wide, not so bright - spot, brighter ...

righto...

The lens coverage is basically the same as the wideness of a maglight - thing is, its already set ... and in all cases except the Switar 10mm, 12.5-100 and POE (to a lesser extent) the coverage or 'wideness' is already good enough for the super16 frame... Lucky huh! - it really is a case of the lenses having more spec/leeway than they required in the first case for reg16 - some might say that even though the lenses give adequate coverage/even illumination at super16 the edge areas have much poorer optical qualities, color aberration, blah-ness, zimzam, floobum etc...

Summing up> A super16 mod in effect has nothing to do with your lenses (unless you've sent your 12.5-100 off to Switzerland for modification) so yes your physics are correct but the reality aint quite as exciting ;) - it should, but thats a total redesign of the lens - you'd end up with something like this:
Posted Image


I was also wondering if the amount of light being diverted by the prism would remain constant, or because of the larger prism area that receives light that there might be some variation there.


In terms of photons, yes the prism diverts more - but the extra amount is spread over a correspondingly larger area so it balances out...

(we wont mention the fact that the portal where the periscope prism is isn't quite large enough and soaks up a little of the image, which at that stage of the 'beam' is still relatively incoherent so nobody notices - shhhh! )
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#7 Craig Mieritz

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Posted 26 November 2007 - 12:56 PM

no

... but good try

You are correct in your 'fluxy' description of light being spread out more and therefore requiring more power, when you focus your trusty AA maglight you see happening before your eyes - wide, not so bright - spot, brighter ...

righto...

The lens coverage is basically the same as the wideness of a maglight - thing is, its already set ... and in all cases except the Switar 10mm, 12.5-100 and POE (to a lesser extent) the coverage or 'wideness' is already good enough for the super16 frame... Lucky huh! - it really is a case of the lenses having more spec/leeway than they required in the first case for reg16 - some might say that even though the lenses give adequate coverage/even illumination at super16 the edge areas have much poorer optical qualities, color aberration, blah-ness, zimzam, floobum etc...

Summing up> A super16 mod in effect has nothing to do with your lenses (unless you've sent your 12.5-100 off to Switzerland for modification) so yes your physics are correct but the reality aint quite as exciting ;) - it should, but thats a total redesign of the lens - you'd end up with something like this:
Posted Image




In terms of photons, yes the prism diverts more - but the extra amount is spread over a correspondingly larger area so it balances out...

(we wont mention the fact that the portal where the periscope prism is isn't quite large enough and soaks up a little of the image, which at that stage of the 'beam' is still relatively incoherent so nobody notices - shhhh! )



condescension

Main Entry:
Pronunciation:
\ˌkän-di-ˈsen(t)-shən\
Function:
noun
Etymology:
Late Latin condescension-, condescensio, from condescendere
Date:
1647

1 : voluntary descent from one's rank or dignity in relations with an inferior 2 : patronizing attitude or behavior
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#8 Nick Mulder

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Posted 26 November 2007 - 02:21 PM

condescension

Main Entry:
Pronunciation:
\ˌkän-di-ˈsen(t)-shən\
Function:
noun
Etymology:
Late Latin condescension-, condescensio, from condescendere
Date:
1647

1 : voluntary descent from one's rank or dignity in relations with an inferior 2 : patronizing attitude or behavior


eh ??

I'll take a guess that you didn't like my manner ?

You'd get used to me, and understand my personality if say we were working together for a while ;) ... above all else though, I do hope it was helpful and made sense to you...
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#9 chuck colburn

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Posted 26 November 2007 - 02:59 PM

The terms aperture plate and gate are interchangeable.
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#10 Craig Mieritz

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Posted 26 November 2007 - 10:56 PM

eh ??

I'll take a guess that you didn't like my manner ?

You'd get used to me, and understand my personality if say we were working together for a while ;) ... above all else though, I do hope it was helpful and made sense to you...



If that's a job offer, I can be there on Thursday and I don't have any idea how someone hacked into my account and posted that shameful posting.
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