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Confusion with the RX Lenses and the system


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#1 Andrew Glenn Miller

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Posted 21 December 2010 - 11:28 PM

I just upgraded from the H16 to the rex 5 system. I've been doing lots of reading on the reflex system, but now I have seem to read to much and a bit confused on the whole system, I hear different things from different people as well sometimes.

So to clarify.. so I don't hurt myself during my test roll.

Shooting with the RX lenses, are the designed so I don't have to open up 2/3 to compensate for the light loss for the viewfinder? So do I read from the 1/65 (133 degrees angle) shutter speed reading with these lenses instead of 1/80 (108 angle) (to compensate)?

Also in single frame exposures, the shutter speed is 1/30 technically (288 angle), but read from 1/40 (216 angle) to compensate right?

Then I have the variable shutter to expose for, I know the 1/2 and 1 represent how much it stops down, and I know it messes with the shutter angle in order to stop down to these exposures, but what are the exact shutter angles of these two positions ( 1/2 and 1)? I could just measure from the 1/80th degree reading and compensate for the stop down, but II'm just curious to know the actual shutter during these stop downs.

Please refer to me to external links if that helps, I just can't seem to know which ones to trust myself because some say different from another, don't know where else to go.. Thank you!
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#2 Chris Millar

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Posted 22 December 2010 - 12:31 AM

I just upgraded from the H16 to the rex 5 system. I've been doing lots of reading on the reflex system, but now I have seem to read to much and a bit confused on the whole system, I hear different things from different people as well sometimes.

uh huh - and some of these people really should know better

Shooting with the RX lenses, are the designed so I don't have to open up 2/3 to compensate for the light loss for the viewfinder?

NO!

A THOUSAND TIMES...

NO!

caps not directed at you, but at the eventual person who'll turn up here and say they are designed that way

So do I read from the 1/65 (133 degrees angle) shutter speed reading with these lenses

Thats the easiest way I think but then you're forgetting the prism light loss, which just to make things easy is about 1/3 of a stop - or you could just not bother with it (seriously) - forget converting shutter times into 'adjusted' shutter times, it gets confusing and in my opinion is part cause of all the misinformation.

Also in single frame exposures, the shutter speed is 1/30 technically (288 angle), but read from 1/40 (216 angle) to compensate right?

That will work, but I prefer knowing the actual exposure time then opening a 1/3 (same as above) - I don't know why you're converting these speeds into relative shutter angles, you're just going to confuse the issue - the speed the shutter rotates determines the exposure in single frame mode, the shutter angle doesn't change

Then I have the variable shutter to expose for, I know the 1/2 and 1 represent how much it stops down, and I know it messes with the shutter angle in order to stop down to these exposures, but what are the exact shutter angles of these two positions ( 1/2 and 1)? I could just measure from the 1/80th degree reading and compensate for the stop down, but II'm just curious to know the actual shutter during these stop downs.

'messes' with it ?? Variable shutter is shutter angle adjustment

1 stop is easy - its half of 133deg = 66.5deg

The half a stop difference will need log base2 calculators to figure out ... somewhere between 133 and 66.5 but I don't think (133+66.5)/2 - but close to it

Please refer to me to external links if that helps, I just can't seem to know which ones to trust myself because some say different from another, don't know where else to go.. Thank you!

http://www.apecity.com/manuals/pdf/bolex_lenses_for_bolex_16mm_cameras.pdf

;)
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#3 Andrew Glenn Miller

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Posted 22 December 2010 - 01:20 AM

Way to break down my post :)

But thanks for the info, it cleared up a lot of concerns I had with the system, I wasn't sure whether it was 2/3 or 1/3 either. I am going to shoot a B&W test roll this morning. Thanks a lot, this is a great forum!
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#4 Chris Millar

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Posted 22 December 2010 - 02:24 AM

Its not even 1/3 rd :lol: but some fraction of a stop arbitrarily close to that :lol:

Best way to be sure is to get a spot meter take a reading of something, then do the same but through a bolex prism ...

Phew, I'm glad I got in here before the rest!

Actually there are some Bolex people here much more learned, who have it all correct as well - some have different systems to have it all work out (that equate to the same) - some have systems that use faulty reasoning and facts, but somehow manage to get the right numbers anyway (two wrongs make a right) - some have it all wrong but get good footage anyway (which just goes to show, we're only talking fractions of a stop here with all the latitude and range of neg stock).

Basically, don't stress over it, you wont stuff up your roll because of this - you could instead forget to put in the filter holder or block off the finder when your eye is away from it - things that get much less relative forum banter but really can cause issues.
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#5 kevin jackman

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Posted 22 December 2010 - 03:09 AM

just to confirm, Im assuming you have moved from a non-reflex to a reflex camera. one main point to know with rx lenses is that they are designed to work optically with the reflex cameras. using non bolex rx labeled lenses means the lens was not designed to take the reflex prism in mind so the image quality will suffer.
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#6 Chris Elardo

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Posted 26 December 2010 - 10:54 PM

Keep in mind that when calculating shutter angle, the 1/2 setting on the lever means 1/2 of an f-stop, NOT 1/2 the shutter angle. People often make the mistake that one should equal the other, but this is not the case. When you set the variable shutter on the 1/2 mark, this IS NOT at 65 degrees. Remember that you have a circular object rotating in front of a rectangle, so geometrically, a perfect halving of the shutter angle is completely inaccurate. This leads to a lot of confusion. Using the Bolex exposure table, and the following calc: Shutter Opening = (f.p.s. x 360) / (Exposure Time), you can see that the unadjusted exposure time of 1/90th of a second with the variable shutter on 1/2 shooting at 24 fps does not equal 65 degrees. 24 x 360/90 = 96 degrees. Conversely, Exposure Time = (f.p.s. x 360)/(Shutter Opening) will give you your unadjusted exposure time. Then you only need to open your lens one-third (roughly) of a stop to compensate for the light loss through the reflex viewfinder. The Bolex table shows the corrected exposure to be 1/112th of a second. Without boring you with more math, you can calculate that the viewfinder steals a little less than 24.5% of your light.

Edited by Chris Elardo, 26 December 2010 - 10:56 PM.

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#7 Chris Millar

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Posted 27 December 2010 - 12:29 AM

Keep in mind that when calculating shutter angle, the 1/2 setting on the lever means 1/2 of an f-stop, NOT 1/2 the shutter angle. People often make the mistake that one should equal the other, but this is not the case. When you set the variable shutter on the 1/2 mark, this IS NOT at 65 degrees. Remember that you have a circular object rotating in front of a rectangle, so geometrically, a perfect halving of the shutter angle is completely inaccurate. This leads to a lot of confusion. Using the Bolex exposure table, and the following calc: Shutter Opening = (f.p.s. x 360) / (Exposure Time), you can see that the unadjusted exposure time of 1/90th of a second with the variable shutter on 1/2 shooting at 24 fps does not equal 65 degrees. 24 x 360/90 = 96 degrees. Conversely, Exposure Time = (f.p.s. x 360)/(Shutter Opening) will give you your unadjusted exposure time. Then you only need to open your lens one-third (roughly) of a stop to compensate for the light loss through the reflex viewfinder. The Bolex table shows the corrected exposure to be 1/112th of a second. Without boring you with more math, you can calculate that the viewfinder steals a little less than 24.5% of your light.

I'm too tired to figure out if what you're saying is correct or not - but I can see some of your math is at least not typed in correctly

You have a formula of "Shutter Opening = (f.p.s. x 360) / (Exposure Time)"

... but you then plug in the numbers as if it were:

Shutter Opening = f.p.s. x 360 x Exposure Time (also take note of the fact that the brackets are redundant in both cases)

24 x 360/90 is equivalent to 24 x 360 x 1/90 which as you know still equals 96

Not sure how that affects your logic - either its a typo and you're right, it isn't a typo and you're wrong, you're both wrong, or I'm both wrong :blink:

I recall doing this math ages ago and some numbers sound familiar - but I also recall putting a spot meter up to the finder and getting a real number, which was either 13% or 18% (memory fail sorry, but the second number was one of the 'round ones') - I also recall pulling an adjustable shutter apart with a protractor and finding that the bolex numbers were WRONG - at least from one of those sites purporting to be a haven of Bolex scripture. I don't have all my notes here so I can be sure of it, which is a pity as I love telling people on the net that they're wrong to trust what other people on the net tell them is right or wrong :rolleyes:

The way they leave shutter angles ambiguous like that on the side of the cameras is once big oversight huh - but yeh, you're right about it referring to stops - its kind of a give away in that it moves from the first increment to 1/2 then 1 - if it were shutter angle it would go from 1 to 1/2 to 1/4 ...

Also note the OP:

I know the 1/2 and 1 represent how much it stops down


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#8 Chris Elardo

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Posted 27 December 2010 - 11:36 AM

I'm too tired to figure out if what you're saying is correct or not - but I can see some of your math is at least not typed in correctly

You have a formula of "Shutter Opening = (f.p.s. x 360) / (Exposure Time)"

... but you then plug in the numbers as if it were:

Shutter Opening = f.p.s. x 360 x Exposure Time (also take note of the fact that the brackets are redundant in both cases)

24 x 360/90 is equivalent to 24 x 360 x 1/90 which as you know still equals 96

Not sure how that affects your logic - either its a typo and you're right, it isn't a typo and you're wrong, you're both wrong, or I'm both wrong :blink:

I recall doing this math ages ago and some numbers sound familiar - but I also recall putting a spot meter up to the finder and getting a real number, which was either 13% or 18% (memory fail sorry, but the second number was one of the 'round ones') - I also recall pulling an adjustable shutter apart with a protractor and finding that the bolex numbers were WRONG - at least from one of those sites purporting to be a haven of Bolex scripture. I don't have all my notes here so I can be sure of it, which is a pity as I love telling people on the net that they're wrong to trust what other people on the net tell them is right or wrong :rolleyes:

The way they leave shutter angles ambiguous like that on the side of the cameras is once big oversight huh - but yeh, you're right about it referring to stops - its kind of a give away in that it moves from the first increment to 1/2 then 1 - if it were shutter angle it would go from 1 to 1/2 to 1/4 ...

Also note the OP:



Forgive the error in typing the formula- I mean 'divided by' exposure time, and yes, it equals 96. This is the simplest way to at least get a reasonably convincing answer and I'm definitely not tearing apart my Rex 5 to measure the shutter with a protractor.
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#9 Chris Elardo

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Posted 27 December 2010 - 11:45 AM

The wonderful folks in Switzerland really put their foot in it when they chose to mark it in stops. Just leave the damn thing wide open and use a ND filter! I'm not wrong- trust me....


Edited by Chris Elardo, 27 December 2010 - 11:46 AM.

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#10 Chris Millar

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Posted 27 December 2010 - 04:43 PM

But you could do automated fades with an accessory... also smaller shutter angle effects - and use ND on top of it...

And as I mentioned if you really think about it the order the numbers go in is a give away - I imagine it was probably mentioned in the original manuals which all seem to have been lost when it comes to eBay (the new Bolex shop)
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#11 Chris Elardo

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Posted 27 December 2010 - 06:16 PM

Yes, very true.

I know the Bolex folks were trying to keep it simple so the average filmmaker could just turn the f-stop ring on the lens and go, but when you're playing with different filming speeds, filters, etc., it's good to know how to calculate things so you can advance your own skills. Unfortunately, Bolex doesn't provide a lot of tables or information on the deeper technical aspects of exposure. Luckily I do have an original manual for my Rex 5, but I've seen a lot of confusion over the f-stop/shutter angle issue.

I have a good table that someone put together, but they made the mistake of labeling the 1/2 stop exposure column as "65 degrees" leading the reader to believe that half a stop also equals half the shutter angle from a full-open 130 degrees. But when you calculate any of the exposure times using the formula, nothing adds up to the stated 65 degree shutter angle. This person should be publicly flogged because it just promotes more confusion-
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#12 Andrew Glenn Miller

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Posted 27 December 2010 - 11:15 PM

I'm finding it very difficult to fully understand the Bolex system than any other system I have had my hands on, maybe it's because of all the false information out there. I'm sure I'm probably making it harder than it should be. I just know I'm not advance enough to tell for myself.

I've been suggested the shutter speeds to expose for but I'd prefer to strive for perfection when it comes to exposure.
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#13 Chris Elardo

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Posted 28 December 2010 - 12:31 AM

Here is something I have:

variable shutter open variable shutter at 1/2 variable shutter at 1
130°
film speed real adapted real adapted real adapted
12 fps 1/33 1/40 1/45 1/55 1/75 1/94
16 fps 1/45 1/55 1/60 1/75 1/100 1/125
18 fps 1/50 1/60 1/70 1/87 1/110 1/137
24 fps 1/65 1/80 1/90 1/112 1/150 1/163
32 fps 1/90 1/110 1/120 1/150 1/200 1/225
48 fps 1/130 1/160 1/180 1/225 1/300 1/375
64 fps 1/180 1/220 1/240 1/300 1/400 1/500
single frame 1/30 1/40

Exposure Time = (f.p.s. x 360)/(Shutter Opening)

Example: Exposure time at 24 fps with a 130° shutter.
ET= 24 x 360 / 130 = 66, or 1/66 second

Shutter Opening = (f.p.s. x 360) / (Exposure Time)

I hope this helps!
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#14 Chris Elardo

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Posted 28 December 2010 - 12:42 AM

That table did not come out like I hoped. Here is the link where it's from:

http://www.city-net....ex/shutter.html

Ignore the 65 and 32-35 labels for shutter angle! They are wrong, but the exposure times are correct.
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#15 Chris Millar

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Posted 28 December 2010 - 12:43 AM

Keep it simple - don't use the adjustable shutter (use ND or stop it down)

As has been said:

fps x (360/135) = actual exposure time you'd put in a meter - of course its actually that fraction of a second, but its more meaningful than the reciprocal as it the actual number you'd put in a stills light meter with no shutter angle adjustment (the reciprocal is the actual time in seconds which is usually not of interest, unless you're doing weird sync stuff, sciencey things, maybe timelapse...)

then take the f-stop reading it gives you and open it a third of a stop

Done ;)


Forget all that crap about adjusted exposure times - just know that the prism takes a 1/3 of a stop of your light away, and thats all there is to it - and even if you were to forget about the prism, your footage will be fine - you're probably going to have around or more than 1/3rd a stop in error in taking the measurement anyway...
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#16 Chris Millar

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Posted 28 December 2010 - 12:52 AM

chris,

I don't have my notes to confirm - but I'm pretty sure that's not just one of the confusing sites but actually one of the wrong sites...

I'm not going to lose any sleep over this but there was a time I was pretty passionate about it - actually opening my cameras and taking measurements - now that this thread is infected with the very confusing crap that started the mess in the first place all I can say is that, sure I'm just another guy on the net telling people what I think about Bolex cameras, and so in taking that futility into account - for what its worth:

I'm right :rolleyes:

Do what I say, you wont go wrong...

... but even better yet - open up your camera yourself, take some measurements and know for yourself - its the only way you're going to be convinced for sure
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#17 Chris Millar

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Posted 28 December 2010 - 01:01 AM

The website even contradicts itself directly ...

12fps real 1/33 adapted 1/40
single frame real 1/30 adapted 1/40

some how in single frame mode the prism takes more light ???

shutter open at 130 deg:
32 fps real 1/90 adapted 1/110
shutter open at 65deg:
24 fps real 1/90 adapted 1/112

how did halving the shutter angle change what the prism does to a real exposure of 1/90 ???


etc...

But again, the errors are small - hence, once all the other possible things that can go wrong on a shoot are added up - it's nothing (but more confusion)
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#18 Andrew Glenn Miller

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Posted 28 December 2010 - 01:19 AM

Thanks a lot for all the info, I am building up footage of the escarpment I live near in Ontario and hope to make a visual piece out of it in the near future. It's a really beautiful trail all year around, Starting from Niagara, pushing right into the Georgian Bay, I hope to shoot it all on the bolex.

Thanks to Christmas, I should stock up on the cheap student price film while I can before I graduate (for every 400ft vision2 stock I buy, I get another stock for free! Plus a 30% student discount!) Yummy!
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#19 Chris Millar

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Posted 28 December 2010 - 02:01 AM

You'll be fine - have fun dude
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#20 Chris Elardo

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Posted 28 December 2010 - 12:02 PM

That sounds like a cool project, definitely! Gee, I wish I could get a discount like that- :huh:

I do agree with everything you said Chris- take your meter reading and open up a third of a stop. Amen.
I have the Bolex/Gossen meter and the meter in my POE lens, so I usually don't have to goof too much with any of it. That sight was the only one that had decent exposure times that I knew of, but it has goofy errors you have to sidestep. As I said before: Public flogging!
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