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Bolex light loss


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#1 Ralph Tabith

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Posted 25 August 2007 - 11:56 AM

Now I am still a little confused here, so if you could please bear with me..

The lost from the an EBM viewfinder as it states in the manual is 25% so it gives a chart of "exposure times" and corrected "photometric exposure times" something like this -

10 fps 1/21 - 1/30
18 fps 1/33 - 1/50
24 fps 1/50 - 1/67

I have just bought my new sekonic 508 cine light meter. How do I set these "photometric" values into my meter? I have set the angle at 170 degrees and 24 fps. How do I compensate for these exposure times, please help!
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#2 Steve Wallace

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Posted 25 August 2007 - 01:40 PM

Now I am still a little confused here, so if you could please bear with me..

The lost from the an EBM viewfinder as it states in the manual is 25% so it gives a chart of "exposure times" and corrected "photometric exposure times" something like this -

10 fps 1/21 - 1/30
18 fps 1/33 - 1/50
24 fps 1/50 - 1/67

I have just bought my new sekonic 508 cine light meter. How do I set these "photometric" values into my meter? I have set the angle at 170 degrees and 24 fps. How do I compensate for these exposure times, please help!


If you're shooting negative, I would overexpose 1.5 stop. 2/3rd for the reflex viewfinder and 2/3rd to tighten the grain. Others experience may vary.
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#3 Nick Mulder

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Posted 25 August 2007 - 03:17 PM

Now I am still a little confused here, so if you could please bear with me..

The lost from the an EBM viewfinder as it states in the manual is 25% so it gives a chart of "exposure times" and corrected "photometric exposure times" something like this -

10 fps 1/21 - 1/30
18 fps 1/33 - 1/50
24 fps 1/50 - 1/67

I have just bought my new sekonic 508 cine light meter. How do I set these "photometric" values into my meter? I have set the angle at 170 degrees and 24 fps. How do I compensate for these exposure times, please help!

From actual measurement I think Bolex prism light loss is %17 - On my Sekonic meter I can enter in values as stops in increments of 0.1 ('1' is a whole stop) - so... a bit of log math (that I cant remember how to do, but there is a post here already about it... i.e. search) and the closest value is '0.3' which I got by looking at my meter ...
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#4 Ralph Tabith

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Posted 25 August 2007 - 05:56 PM

From actual measurement I think Bolex prism light loss is %17 - On my Sekonic meter I can enter in values as stops in increments of 0.1 ('1' is a whole stop) - so... a bit of log math (that I cant remember how to do, but there is a post here already about it... i.e. search) and the closest value is '0.3' which I got by looking at my meter ...


Thanks, I will play around with the exposure compensation with my sekonic. by setting it to 0.3 that should take care of it..?

BTW I havent seen Mr.Bumpy is it?> in a long long time..
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#5 Nick Mulder

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Posted 25 August 2007 - 06:56 PM

Thanks, I will play around with the exposure compensation with my sekonic. by setting it to 0.3 that should take care of it..?

BTW I havent seen Mr.Bumpy is it?> in a long long time..

Mr Bump yep ...

It certainly takes care of it for me on my meter - to be sure yours isn't set up to take filter factors where 0.3 would = 1 stop - take a reading, note the time or stop then set the compensation to 1.0 - if your meter now shows a compensation of 1 stop from the earlier reading then 0.3 is the correct input for the prism light loss (change it again) - if it shows a 3 stop deviation then your meter compensates in filter-factors and I'd say even an input of 0.1 might be too much of a compensation for the light loss and I'd say dont enter anything, just think 'open up a smidge' instead ;)
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#6 Ralph Tabith

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Posted 26 August 2007 - 05:39 AM

Mr Bump yep ...

It certainly takes care of it for me on my meter - to be sure yours isn't set up to take filter factors where 0.3 would = 1 stop - take a reading, note the time or stop then set the compensation to 1.0 - if your meter now shows a compensation of 1 stop from the earlier reading then 0.3 is the correct input for the prism light loss (change it again) -



Hi just took a reading in the shade - 5.6 and then notched it up to 1.0 and it showed a reading of f 8.0

So I guess that 0.3 is the correct compensation.

This is going to sound woefully stupid but what is the compensation measured in? (0.1,0.2 etc)

Is there any books you know of that deals/explains/teaches with these kinds of issues? And why does the bolex manual state its 25 % loss when it's 17 %? Thanks for your insight!

Edited by Ralph Tabith, 26 August 2007 - 05:40 AM.

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#7 Nick Mulder

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Posted 26 August 2007 - 09:24 AM

Hi just took a reading in the shade - 5.6 and then notched it up to 1.0 and it showed a reading of f 8.0

So I guess that 0.3 is the correct compensation.

This is going to sound woefully stupid but what is the compensation measured in? (0.1,0.2 etc)

Is there any books you know of that deals/explains/teaches with these kinds of issues? And why does the bolex manual state its 25 % loss when it's 17 %? Thanks for your insight!



The problem is one of Log/linear terminology I think ...

Have a read of this thread here: http://www.cinematog...n...44&hl=nerds

Warning though >>strictly for the nerds<< - lot of firey hoops to jump through :lol:

p.s. what Bolex manual ? there is a heap of incorrect info floating around the net re. Bolex cameras - some of it posing as the real deal even ... and to top it off I do think that Bolex itself for the sake of simplifying things has rounded up facts and figures to nice numbers - eg. %25 = 1/4 .... Anyway, testing it for yourself is simple>> just use a spot meter, one normal reading of a surface, then another through the prism of the same surface - compare... You'll have to use a setting that you know you can work with mathematically - nasty log2 vs. log10 vs. linear problems at every step, very easy to assume its all linear like every other thing in life :blink: (aside from dB in sound)

If its all a bit much then just know this - '0.263' is the actual number you want in your meter, and thats the conversion of %17 (not %26.3 as you'd expect)
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#8 Andy_Alderslade

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Posted 26 August 2007 - 10:52 AM

Hi just took a reading in the shade - 5.6 and then notched it up to 1.0 and it showed a reading of f 8.0

So I guess that 0.3 is the correct compensation.


Hi Ralph

It sounds like you're getting a little confused, you maybe getting lost in the detail of the conversation here.

Think of every stop as 100%, if the bolex viewfinder is stealing 25% or 17% (either way it doesn't matter because they are very similar figures) then you need to give your film that percentage back.

Now with dealing with metering and setting the stop we often work in thirds, (though many digital light meters will give you the stop down to 1/10th which is perhaps getting lost in the detail again!) so just round up your light loss from the viewfinder to 1/3rd which is essentially 33.3%.

So all you have to do is take a normal light metering (24fps, 180 degree shutter = 1/50 roughly) and then use the given f-stop, then open up an extra third to compensate.

So if the light meter says f4 then set the lens at f2.8 and 2/3rds.

Again if the light meter says f8 and 1/3rd set the lens on your camera to f5.6.


As already mentioned in this discussion with colour negative film (not reversal or b/w neg) it is often beneficial to over-expose by 1/3 to 2/3 of a stop. So you have the choice with your bolex, you can go out and set the stop 2/3 to 1 stop higher than the reading.

Regarding books, 'Cinematography' by Kris Makiewicz (and our very own David Mullen) is one of the best guides through shooting 16mm.

Best of luck,
Andy
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#9 Ralph Tabith

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Posted 26 August 2007 - 02:06 PM

Thank you Nick and Andy, I am absorbing as much as I can from your posts...I have bought that cinematography book by malkiewicz (sp?) although somehow i find the layout a bit confusing, I bought a great photography book the other day that had a very visual layout to problems and I think I understand things better in that way rather than lots of words, maybe I will try out David Mullens book...

and the thought of doing maths I find abit scary after all these years! :)
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#10 Nick Mulder

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Posted 26 August 2007 - 03:53 PM

Andy - please read this post here: http://www.cinematog...n...44&hl=nerds

There is good reason for the confusion that I've outlined - not to mention I've given him the exact figure he needs already ...

One thing I've learned is that Thirds is another simplifying rounding up of not 'nice' numbers (well, I'm about %95 sure of it and was going to start another thread on it, but then realized/asked>> who cares anyway...)

Anyhoo, maybe I do care after all >> have a read of that thread and get out your log2 calculators before perpetuating a simplification as solid fact.
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#11 Andy_Alderslade

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Posted 27 August 2007 - 08:24 AM

Anyhoo, maybe I do care after all >> have a read of that thread and get out your log2 calculators before perpetuating a simplification as solid fact.


I don't believe I was perpetuating a simplification as solid fact, I was simply trying to simplify things into a workable method that would ensure Ralph a good thick exposure - just as thousands of bolex shooters, professional and amateur have done before.

I'm sure Peter Sellers didn't consult his Log2 Calculator before shooting some of his excellent home movies. Did Andy Warhol perhaps, or many of the 100s of british animators who produced television commercials on them during the 60s and 70s? - I doubt it.

You seem to be implying that thirds is a simplification - when its actually the building block of film photography. Its accurate and simple enough at the same time - film speeds 50,64,80,100...320,400,500 are all step increase f a 1/3 of a stop.

How much extra stop does a Wrattan 85B filter require? = easy 2/3 of a stop (Well actually if you test it with a meter they often absorb a little more, but anything less than a 1/3 isn't worth worrying about - as i've been told on numerous occasions)

Now apologies if you thought I was disregarding you research but if Ralph simply wants to shoot with a bolex, than it felt like the extra information was over complicating the matter. In fact in his original post he mentions his sekonic 508 light meter - somebody could have said set the angle at 130 degrees and it will correct it for you - but again that would have been over complicating the matter.
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#12 Nick Mulder

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Posted 27 August 2007 - 02:27 PM

I'll start up another thread on it one day ...

It was just something I noticed in my math that the thirds we all (me included) refer to were actually rounded numbers - thing is I cant remember the actual math right now and am heading overseas for a week - If I recall the 1/3 rounding was a bit more blatant than the rounding for 2/3rd's so much so that if people compounded the error by some logic of 'hmmm, ok if we add up thirds like this' ... then the results would be waaay off - maybe I'll get bored on the plane and nut it out again :lol:

Re-reading my post I think I may have directed a little too much negativity towards you regarding my concern - you are quite correct on the reality of the situation, I'm just a tad more interested in the science and math behind cinematography than the average shooter, and a pet peeve of mine (as vain as it may well be) is people who read some 'fact' on the net and then on-quote it without ever having attempted to test it themselves ...

A good example is the bolex RX lenses f-stops being adjusted for the prism ... (this is not true in case anyone reading this is wondering, would be helpful if it was tho huh, sort of an extended T-stop if you will)

Regarding the lightmeter though - I did give him the right answer straight off the cuff - '0.3' is the compensation, and that is on top of the 130/135 shutter angle ... (the real angle from my measurements is somewhere between the two)
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#13 Andy_Alderslade

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Posted 27 August 2007 - 05:30 PM

A good example is the bolex RX lenses f-stops being adjusted for the prism ... (this is not true in case anyone reading this is wondering, would be helpful if it was tho huh, sort of an extended T-stop if you will)


Ah yes, I forgot that about Bolex's and RX lenses its been several months since i've used one, maybe someone should mention that to Ralph just in case he didn't already know that.

Regarding the light meter though, I can't help feeling that setting the shutter angle in the light meter is a bit risky; in the past i've adjusted it down to 95 degrees for my Bealeau Super 8 camera and then loaned it to a friend who was shooting my sisters wedding on an SRII. Because I had forgotten to adjust it back to 170 or 180, I didn't sleep all night terrified I would forget to tell him the following day.

In reality the negative film would probably have coped easily with a one-stop over exposure - but from now on i still keep it around 170 or 180 and correct manually with relevant cameras.

I guess the lesson is: the more features a light meter has, the more likely it can go wrong!
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#14 Nick Mulder

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Posted 27 August 2007 - 05:38 PM

Quote from this current eBay auction: http://cgi.ebay.com/...1QQcmdZViewItem

The Spectra is a compact, easy to use "industry standard" digital light meter for film/video/photography - it's won many awards.

How can it help you?

Example: I wanted to use a Switar "RX" prime lens on my Eclair ACL. "RX" Switars are made for exclusive use with Bolex reflex cameras - they have a prism (not mirror based) reflex (through the lens) viewfinder system. Problem: "RX" Switars let in more light at each f stop than other lenses. That's to compensate for the light the prism takes away from the film. This means they are a little too bright when used on anything other than a Bolex. I have to compensate by at least 1/3 of an f stop when using these lenses on my ACL. Not a problem as long as I've got a Spectra IV-A.


aaaaaarrrrrgghhh!!!!! :lol:
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#15 Andy_Alderslade

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Posted 27 August 2007 - 06:02 PM

Nick, you've found a good example why some people shouldn't ever be given a key-board, let alone an ebay account!
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#16 Hugo Alexandre

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Posted 17 January 2008 - 01:19 AM

Ok. I was told that if I use a lens labelled 'RX' on a Bolex RX-5 (a Switar Zoom lens for example), then I have to take my reading at 1/50th instead of at 1/80th because the f-stops on the lens already compensate for the viewfinder. But this thread seems to state otherwise...

Anybody care to comment?
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#17 Nick Mulder

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Posted 17 January 2008 - 02:14 AM

Ok. I was told that if I use a lens labelled 'RX' on a Bolex RX-5 (a Switar Zoom lens for example), then I have to take my reading at 1/50th instead of at 1/80th because the f-stops on the lens already compensate for the viewfinder. But this thread seems to state otherwise...

Anybody care to comment?


We are correct - they are wrong.

1/80th is close enough, which is to say 'correct' for 30fps - THEN you adjust for the prism loss, which to take Andy's advice and keep things simple and open up a third of a stop - nothing to so with the lens - the lens is a lens ;)
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