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Varable Shutter Strobing


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#1 andrew parrish

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Posted 07 February 2012 - 03:11 PM

Hi everybody,

I was trying to figure out when I can expect strobing when using the Variable shutter. One of my books says past 90 degrees. I found this formula on the net: (fps x 360) / exposure time. I plugged in the numbers from my manual for the 1st position, 1/4 closed at 18fps-
( 18X360) / 87, and got the answer 74.48. Is this right? Will I get strobing at 1/4 closed.
( More likely I have messed this up some how)

Thanks for reading,

Andrew
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#2 Chris Millar

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Posted 07 February 2012 - 03:46 PM

to get angle use:

exposure * fps * 360

~first positon on the variable shutter is half a stop

Therefore simply take the open shutter angle and halve it - depending on how you like to think about fully open bolex shutters - it's either 144/2 or 135/2 or 133/2 (see here for more confusion: http://www.cinematog...showtopic=11912)

~fps is redundant when it comes to shutter angle - but since you're calculating it via exposure it's a bit of a tangle Posted Image

Remember strobing is a perceptual effect, not something set in stone - the subjective response to it depends on the shooting frame rate, the shooting exposure relative to the frame rate and the same again in the projector.

To answer your question, need some info: you're shooting at 18fps - to project/persent at 24, 30 ? and your variable shutter is set to what number on the dial?
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#3 andrew parrish

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Posted 07 February 2012 - 08:08 PM

Hi Chris

Thanks for the quick response. One of the reasons why I am asking this question, is because I got the formula from a website that has wrong data on it.

to get angle use:

I have read some of the other stuff, and I was hoping to to get a "kiss" solution, because of the whole 144-135 argument seemed to get bogged down after awhile....

exposure * fps * 360

~first positon on the variable shutter is half a stop

I thought that they were one and the same- to stop the shutter down a half a stop, the shutter had to be closed down a quarter of it's anglepublic/style_emoticons/default/huh.gif

Therefore simply take the open shutter angle and halve it - depending on how you like to think about fully open bolex shutters - it's either 144/2 or 135/2 or 133/2 (see here for more confusion: http://www.cinematog...showtopic=11912)

I had a look at this thread previously, and I was hoping to avoid it by going straight to the manual.

~fps is redundant when it comes to shutter angle - but since you're calculating it via exposure it's a bit of a tangle Posted Image

Remember strobing is a perceptual effect, not something set in stone - the subjective response to it depends on the shooting frame rate, the shooting exposure relative to the frame rate and the same again in the projector.

To answer your question, need some info: you're shooting at 18fps - to project/persent at 24, 30 ? and your variable shutter is set to what number on the dial?

I just got my camera back with a new transport mechanism in it, and I was hoping to test it for the sharpest possible. I get you point about the transfer, so I'll change shooting at 25fps, ( to capture at 24p) at between f5.6 and f 11. I will project at 25fps, +/-, because I have a variable speed projector.

As to the shutter dial, I will set it to the smallest opening possible, that will not have a chance of strobing. I was hoping to use F-1 ( the second position)

Thanks for your answers, and your time,

Andrewpublic/style_emoticons/default/smile.gif
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#4 andrew parrish

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Posted 07 February 2012 - 08:42 PM

Sorry to repost, but I thought my new additions would be in a different colour. That last post was unreadable.



Thanks for the quick response. One of the reasons why I am asking this question, is because I got the formula from a website that has wrong data on it.

I have read some of the other stuff, and I was hoping to to get a "kiss" solution, because of the whole 144-135 argument seemed to get bogged down after awhile....I had a look at this thread previously, and I was hoping to avoid it by going straight to the manual.

Interms of the mechanics of the thing, I thought that -F1/2 and closing down 1/4 were one and the same- to stop the shutter down a half a stop, the shutter had to be closed down a quarter of it's full opening?

My goal is this:

I just got my camera back with a new transport mechanism in it, and I was hoping to test it for the sharpest possible. I get you point about the transfer, so I'll change shooting at 25fps, ( to capture at 24p) at between f5.6 and f 11. I will project at 25fps, +/-, because I have a variable speed projector.

As to the shutter dial, I will set it to the smallest opening possible, that will not have a chance of strobing. I was hoping to use F-1 ( the second position)

Thanks for your answers, and your time,

Andrewpublic/style_emoticons/default/smile.gif
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#5 Chris Millar

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Posted 07 February 2012 - 08:43 PM

A bit confusing not putting qoute marks around what I typed..

when you say "I thought that they were one and the same" - what are the 'they' you are refering to ?

oh DAMNIT

I've stuffed up...

man :o

First positon on the variable shutter is ONE stop

Much apologies for this - here I am trying to cut down on misinformation on the net pertaining to Bolexes, and I just added to it :(

But then to add to the confusing for anyone following this, to get it to half a stop...


Ahhh goddammit -

you know what


I GIVE UP - I know it, but am incapabale of communicating it

All I can say is learn log2 calculations - without understanding them, no one will get anywhere correct - You'll get close, but not correct results - these close results will be close enough, and they will make you think you are right.

And to a major extent if it looks right, it is right (but just dont do off trying to develop a new camera system or bridge or space shuttle thinking that way because unless you learn log2 you are wrong.

I reserve the right to return to this topic sometime I'm less grumpy :lol:

(and the goddamn f&$king %$^$t f$#kf^&e input screen stops glitching when I use subscript text)

(+ please don't take it personally)
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#6 andrew parrish

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Posted 08 February 2012 - 03:36 PM

No problems Chris,

I real appreciate the time you have taken t help me.

I have very basic math skills, any ways. When someone talks to me about logs, I think of a saw mill.

I did have a look again at my camera, a h8 rex 4, and the manual I have for it. The camera has four settings on the levers mounting socket. They are listed in f stops, according to the manual. They read as follows, from top to bottom: blank ( wide open), 1/2 ( the manual says that the aperture must be opened 1/2 and f stop to compensate in this position, because the variable shutter has closed down by 1/4 of it's opening) The third position reads 1 ( the manual say to correct 1 f stop, because the shutter is now down to 1/2 of it's full aperture.) The final setting is blank, and is for fully closed.

The manual also talks about " jerkiness" when using the Variable shutter. I'm thinking that what they are talking about is strobing. The manual stays with the lever set to 1/2 ( a f stop), the film speed should be more that 25fps. I think that the formula I found might be right, as a rule of thumb.

In his book, Cinematography;theory and Practice, Blain brown points to two causes for strobing- a shuttler angle of less than 90 degrees, and panning too quickly ( a subject crossing the view finder faster than 5 seconds.

I guess I will just have to shut up and shoot.

Thanks Again,

Andrew

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#7 Chris Millar

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Posted 08 February 2012 - 04:18 PM

heh,

thanks for putting up with my little breakdown there...

The entry screen was glitching up and down after I had entered the subscipt for the log base 2 doohickey (a trap I wont fall into again) and I've been studying for an applied math paper here at uni and am in the middle of moving countries (lots to organize).

Just be aware that if the manual is saying that "the aperture must be opened 1/2 an f stop to compensate in this position, because the variable shutter has closed down by 1/4 of it's opening" - the manual is wrong.

it has closed 1-1/sqrt(2) - which is:

0.293

Which in an unrounded state is an irrational number

(Irony)

That's a %17 error from 0.25

And who am I to say that the Bolex manual is wrong ?

erm,

yeh

me :lol:

Anyhoo - yes best to test, but by some hack calcs 90deg is around:

log2(90/135) = 0.585

Which in english means:

Slightly past the 1/2 setting on the dial (the half setting equates to a 95.445 deg shutter angle)
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#8 andrew parrish

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Posted 08 February 2012 - 11:34 PM

Interesting Chris. I don't mind playing Lewis to your Martin, so I will ask a potentially dumb question. Does this %17 equate to a 1/6 of a stop? If we factor in the disputed difference in shutter angle - 135 ( about 1/60th of a second ) and 144 ( about 1/64th of a second), and this %17, will this all add up to a 3rd of a stop or more? Did Bolex do this on purpose, just to make the numbers easier to use? I was suprised by how non-tech-ee the manual is, considering how complex the camera can be.

You seem very passionate about the subject. Have you made a replacement chart, based upon these more refined numbers? If you have, I would love to see it, and shoot some test footage.

Thanks,

Andrew
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#9 Chris Millar

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Posted 09 February 2012 - 12:19 AM

this is where modern life gets me down

see I wrote a post for about 20minutes just then in reply

then IE crashes


game over
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#10 Dom Jaeger

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Posted 10 February 2012 - 03:03 AM

Well this thread sure is a head scratcher..

Maybe we can work through some of the confusion.

First off, Bolex shutter angles are different depending on the model. Very early ones (before serial no 100401 for H16 and 97801 for H8) had a 192º opening. In 1954 they changed it to 143º. The early variable shutter models (introduced in 1959) still retained the 143º opening, but in 1962 Bolex reduced the angle further to 130º. The modification, which also included changing the shape of the variable sector and removing a third (2-stop) setting, was done "in order to reduce the flicker of the variable shutter". The quote comes from an official modification notice in one of my Bolex service manuals that specifically mentions reducing the angle from 143º to 130º. A quick measure of the 2 shutters in my parts inventory confirmed those figures. A lot of sites and literature (including the Bolex manual exposure times, if you convert them to a shutter angle), give a figure of 133º for reflex Bolexes, and until I dug up that modification sheet and measured the damn thing, I thought it was 133º too, but there you go. Serial numbers from 195801 have the 130º shutter, though some earlier variable shutter ones may have been retro-fitted. EL and EBM cameras have 170º shutters.

Whew!

Now, the variable shutter is marked in stops, rather than angle (though the manual does describe the settings as quarter- and half-closed). As we discussed in that other thread, there is some discrepancy between the exposure times listed in the H16 manual, a true 1/2 and full stop reduction in light, and the idea of a quarter and half-closed shutter. The printed exposure times for the "quarter-closed" setting are pretty close to a 1/2 stop light reduction (but equate to a shutter angle of 96º, which is smaller than 1/4 closed - as it should be for a half stop), and for the 1 stop setting they equate to a shutter angle of 58º, which is neither 1 stop less nor half closed. But lets not get too bogged down in all that, the printed exposure times are probably the more accurate figures, and it's all close enough not to be a concern. When you consider all the variables involved - the meter used, the age or condition of the stock, the play in the variable shutter, the accuracy of the particular lens aperture scale and the 'approximately' 25% light loss to the prism, we're not dealing with precision exposure here. Shooting a test is always the best indicator anyway.

Deep breath.

Anything past the 1/2-stop (quarter closed) mark on the variable shutter (let's call it 96º) will probably give you stuttery motion, depends on how and what you're filming really. You can get strobing at normal shutter angles if you pan too quick. Until you shoot it and watch it as you intend it to be shown you won't really know how unacceptable it might be.
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#11 Chris Millar

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Posted 10 February 2012 - 03:44 AM

The post I lost the other day was about half as literate and about a third of a stop as helpful as Doms...

In my defense there were appeals to post-historic animalism, the recipe for my mothers self saucing chocolate pudding and a draft complaint to the local tertiary education commission.
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#12 andrew parrish

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Posted 10 February 2012 - 11:01 AM

Hey Guys, thanks for a very stimulating discussion: it really got me thinking about how the mechanic of the camera translate into film. Thanks again for all your responses.

Cheers,
Andrew
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