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Ye olde 144 degree shutter angle and lightmeter rant


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#1 Karl Andre Bru

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Posted 20 May 2010 - 08:40 PM

Hi guys!

Im sorry but here it probably comes again....to put it short:
1.shooting on old bolex h16 reflex
2. shooting on 500T Vision 3
3. camera only has 144 degree shutter angle.
4. the light meter is Minolta Auto Meter IV F
5. shooting at 24FPS (not worrying about incorrect speed from the old lady)
7. the light meter can choose between cine mode(choosing FPS) or shutter speed.

So with a 144 angle at 24 FPS, at 500ASA I should choose 1/60 as my shutter speed to get the correct reading on my meter, yes? (24x360)/144=60
As a test: About a foot from a non clear(kind of frosted) 40 watt bulb, being the only source in my room I get 5.6 and 3/10 with my meter at 24FPS,500ASA and 1/60 shutter speed.

If anyone can be bothered to respond that would be super swell, gosh darn it...

Cheerio!

Karl Andre Bru
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#2 Chris Millar

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Posted 20 May 2010 - 08:54 PM

Yes 1/60sec is the duration the shutter exposes any given point on the film plane

but you're forgetting ye even older 'bolex lightmeter adjustment for reflex prism' rant ...

Not sure nowadays but I guess we'll see - reckon within maybe 5 posts (?) we'll have someone copy something they found on the net about Bolex reflex prisms that is incorrect :P

Anyways, pulling old book of shelf, opening it and blowing away the dust of ancient swiss armpits:

Your reflex Bolex has a prism that soaks up a smidge of your light for the viewfinder, just open up your iris above your meter reading a corresponding smidge to account for this ... Search the forum archives for the exact amount and technical discussion ad infinitum regarding how many stops a smidge is - but just between you me and the internet, lets say 1/3 stop for shits and giggles :lol:
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#3 Chris Elardo

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Posted 21 May 2010 - 12:06 PM

You're saying you have a Bolex reflex with a 144 degree shutter. Are you sure? The only Bolex I know with a 144 degree shutter is the M series cameras, which are non-reflex. Reflex models have a 131 degree shutter (spring wound), while the EBM has a 170 degree shutter. Just that I'd throw that in...
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#4 Chris Millar

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Posted 21 May 2010 - 05:29 PM

HA HA,

wow - that was TWO posts before someone assed up a number (me!) - :lol: - yes they are 130-ish degrees (the variable shutters are a bit sloppy)...

poop, maybe that 144deg thing is someones attempt at accounting for the prism in terms of the shutter angle... (confusing to communicate to others, but ok once you can figure it out on your own terms)

So anyways everything I said was correct apart from the fact it should be calculated with 130something/360 * 1/24 = 1/66 - then open a smidge with a reading from that number...
:rolleyes:
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#5 Glenn Brady

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Posted 23 May 2010 - 01:00 PM

Filmmaking was a lot simpler when lightmeters didn't require users to specify shutter angles and filmmakers had instruction manuals at hand. Designers nowadays seem to stuff as many functions as possible into electronic gadgets, making them both unnecessarily complicated and failure prone. I've been using the same battery-independent Sekonic meter for thirty-five years with no problem, never having to specify shutter angles to properly expose film.
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#6 Karl Andre Bru

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Posted 23 May 2010 - 08:58 PM

I`m quite sure you have. I guess its ye old inexperience with shooting on film that makes us a bit anal when it comes to clinging to our light meter for dear life.
But im looking forward to see how we did. Should be pretty sweet though, im sure.
Thanks for all the replies!

Cheers

Karl Andre Bru
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#7 jogr

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Posted 24 May 2010 - 05:57 AM

Actually, I believe the early h16 reflex (the pre rexs' without the variable shutter) do have a 143 degree shutter. I have one, circa 1957, and have always used the 143 or 1/60 with rx lenses. When using non rx lens I add the light loss correction, ~1/80.
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#8 Chris Millar

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Posted 24 May 2010 - 06:39 AM

Actually, I believe the early h16 reflex (the pre rexs' without the variable shutter) do have a 143 degree shutter. I have one, circa 1957, and have always used the 143 or 1/60 with rx lenses. When using non rx lens I add the light loss correction, ~1/80.



This is the kind of incorrect info I talk about. RX or non-RX lenses do not affect anything to do with the light intensity calculations around the reflex prism - they are made to correct for optical issues the prism causes with wider angle lenses - nothing to do with the light loss ...

Glenn, most new 'cine' digital lightmeters are just appended versions of a stills paradigm meter and if you want can be used in that mode just as easily as your older model...
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#9 Glenn Brady

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Posted 24 May 2010 - 12:55 PM

Glenn, most new 'cine' digital lightmeters are just appended versions of a stills paradigm meter and if you want can be used in that mode just as easily as your older model...


I'm not surprised this is so, but this question about shutter angle comes up so frequently, it suggests many believe a proper exposure can't be gotten without inputting a shutter angle to a lightmeter.
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#10 John Sprung

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Posted 24 May 2010 - 01:44 PM

The thing about 144 degrees is that it's the magic number for shooting NTSC TV sets at 24 fps without a roll bar.




-- J.S.
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#11 Oliver Christoph Kochs

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Posted 24 May 2010 - 06:47 PM

My Bolex has a 133 degree shutter (Rex 4) that makes it a theoretical 1/65 (rounded) when filming at 24 fps. Now for the prism-light-loss...
If i substract the 25% light loss of the prism viewfinder i get something like 1/81,2 which is the correct exposure time. This matches to my Bolex filmtable (okay: that one states 1/80). Since the Bolex is not crystal speed it's always a estimate. All H16RX seem to have 133 degree shutter above the serial number 200000 which marks the introduction of the Rex 2. EL and EBM have a 170 degree shutter opening.
Regards Oliver
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#12 Richard Tuohy

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Posted 27 May 2010 - 06:39 PM

When using a reflex bolex, the simplest thing to do is to use the cine speeds on your light meter and make the exposure correction with the asa setting. This way, you can readily switch between shooting speeds without having to re-calculate anything.
The correct compensation is 2/3rds of a stop in total, which includes 1/3rd of a stop for the 130 degree shutter angle, and 1/3rd of a stop for the prism light loss. Thus, if you are using 100 asa film stock, set the light meter to 64 asa (which is 2/3rds of a stop slower than 100 asa) then simply set the meter to 24fps if you are shooting at 24, or 12 if shooting at 12 etc..
Here is a chart that puts this correction in terms of real and adapted shutter speed:
http://www.city-net....ex/shutter.html
But as I say, I prefer to adapt the asa rather than the shutter speed.
richard
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#13 Jim Hyslop

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Posted 30 May 2010 - 10:08 PM

So with a 144 angle at 24 FPS, at 500ASA I should choose 1/60 as my shutter speed to get the correct reading on my meter, yes? (24x360)/144=60

Well, sort of. Technically, you've got the formula upside-down - it's 144/(24x360), which is .01666... or 1/60. But you remembered to invert the answer (1/60 instead of just 60) so it all comes out in the wash anyway.

Not to mention that "60" is easier to understand than "0.01666".

--
Jim

Edited by Jim Hyslop, 30 May 2010 - 10:09 PM.

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#14 jogr

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Posted 01 June 2010 - 11:13 PM

I guess I was just trying to point out not all early H16 reflex cameras have a 130 degree shutter. I should have stopped there, sorry for any misinformation:)

Thanks Richard, that was useful info and website.
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#15 Oliver Christoph Kochs

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Posted 02 June 2010 - 09:54 AM

Nahh, please compensate 25-35% light loss for the prism. The prism is INSIDE / IN FRONT OF the film gate and not in the lens. Please see the following shots i took from Douglas Underdahls 16mm Camera Book.
I recommend it to anyone seriously filming with the Bolex as well as the Bolex Bible by Andrew Alden. Sorry for the lousy iPhone pics...

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#16 Harry Laos

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Posted 29 January 2013 - 04:25 AM

When using a reflex bolex, the simplest thing to do is to use the cine speeds on your light meter and make the exposure correction with the asa setting. This way, you can readily switch between shooting speeds without having to re-calculate anything.
The correct compensation is 2/3rds of a stop in total, which includes 1/3rd of a stop for the 130 degree shutter angle, and 1/3rd of a stop for the prism light loss. Thus, if you are using 100 asa film stock, set the light meter to 64 asa (which is 2/3rds of a stop slower than 100 asa) then simply set the meter to 24fps if you are shooting at 24, or 12 if shooting at 12 etc..
Here is a chart that puts this correction in terms of real and adapted shutter speed:
http://www.city-net....ex/shutter.html
But as I say, I prefer to adapt the asa rather than the shutter speed.
richard

Great post! Thanks! I searched high and low for ASA adjustments like you mentioned :)
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#17 Chris Millar

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Posted 29 January 2013 - 03:28 PM

Those tables if I recall (and they haven't been adjusted by the website) are a direct copy of Bolex tables originally supplied with the cameras.

They are *also* wrong - how do I know? Well if you calculate a ratio or two you'll see they are internally inconsistent.

But back to part of my original point, it don't matter! We are talking small amounts, only appreciable by folk shooting reversal with every other factor locked in exactly (lighting etc...).

Do you have that much control over your reflections and practicals in incident, or have you mastered the zone system in spot mode and push/pull processing?

If not, less stress is best - meter as per normal and then open up a squeak ;)
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#18 Chris Millar

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Posted 29 January 2013 - 03:31 PM

'%35' lends itself to confusion also as it infers %100 would be a stop where some might interpret that as losing *all* of the light...
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#19 Jean-Louis Seguin

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Posted 29 January 2013 - 04:25 PM

Please see the following shots i took from Douglas Underdahls 16mm Camera Book.


Looking at second page of the Underdahl book, the line where it says flange/focal distance (FFD) could be a bit clearer.

In the Bolex system, again because of the beam-splitter prism, one must differentiate between optical FFD and physical FFD.

In a H16RX camera, with the traditional 3-lens turret, the optical FFD is 17.52mm, the C-mount standard. The prism elongates the light path by a distance of 3.24mm. The flange now has to be positioned at 17.52mm + 3.24mm = 20.76mm from the film plane. This is the physical FFD.

Now for a Bolex camera with a single bayonet mount. The flange of the bayonet mount is designed to be 5.70mm ahead of the turret flange. So the optical FFD is now 17.52mm + 5.70mm = 23.22mm. The physical FFD is 17.52mm + 3.24mm + 5.70mm = 26.46mm

Jean-Louis

Edited by Jean-Louis Seguin, 29 January 2013 - 04:27 PM.

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