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Bolex lenses & depth of field


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#1 sonickel

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Posted 20 November 2005 - 01:47 AM

Hello everyone,

I've just bought a Rx 5 Bolex, with the following lenses:

16mm RX Switar f1.8
25mm RX Switar f1.4
75mm Yvar f2.8 (non-RX)
16-100 RX Vario Switar POE f1.9

I've found this online calculator, which can do depth of field for 16mm lenses in general:

Depth of field calculator

However, I've read that the RX lenses are made differently to most lenses. They compensate for the 25% of light lost through the prism, by marking the fstops differently (hence f2 on a RX lens isn't really f2 at all).

It's all very confusing. Do I need to find special depth of field tables/ calculators for these RX lenses, or any other Bolex lens? Or will the online DOF calculator be accurate?

Thanks in advance B)
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#2 Boris Belay

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Posted 20 November 2005 - 12:53 PM

Hello everyone,

I've just bought a Rx 5 Bolex, with the following lenses:

16mm RX Switar f1.8
25mm RX Switar f1.4
75mm Yvar f2.8 (non-RX)
16-100 RX Vario Switar POE f1.9

I've found this online calculator, which can do depth of field for 16mm lenses in general:

Depth of field calculator

However, I've read that the RX lenses are made differently to most lenses. They compensate for the 25% of light lost through the prism, by marking the fstops differently (hence f2 on a RX lens isn't really f2 at all).

It's all very confusing. Do I need to find special depth of field tables/ calculators for these RX lenses, or any other Bolex lens? Or will the online DOF calculator be accurate?

Thanks in advance B)


Hi, First off, welcome to the wonderful world of 16mm. film (and Bolex cameras).
RX lenses are the greatest source of confusion outside of the world of politics, it seems... And certainly Bolex's greatest thorn in the side ! You've been carelessly advised, but you're not the first or last one to be confused.
When designing their Reflex H16/H8 model, the Bolex engineers opted for a fixed prism (most other reflex cameras have a mechanism that moves the prism mirror away from the light path during the time of exposure). The advantages were a very sturdy prism assembly (since it doesn't move) and no flickering in the viewfinder while shooting (permanent light flow). But this (fixed) Bolex prism introduced two problems : a light loss of about 25% indeed, and optical aberrations.
The light loss IS NOT corrected-for by the lenses. A f:1,4 on a RX lens lets the same amount of light in as a f:1,4 on an equivalent non-RX lens.
Which means that you should be careful to take the light loss into consideration in your light-measurement process. This can be done in several ways : the Bolex lightmeter for H Reflex cameras is already calibrated for this light-loss, so it's straightforward. (Two things to be cautions about, though : 1) this is the small Gossen-made lightmeter meant to mount on the accesory shoe mount above the camera turret, NOT the American Bolex Company hand-held lightmeter, which predates H16 Reflex models ; 2) these lightmeters are old by now, so should be checked carefully against another known-good lightmeter -- readings will be different, obviously (+25% light), but if the ratio remains the same in high, medium and low lights, your meter may be good enough). The other way is to use the adjusted shutter speeds published in every Bolex Reflex User manuals (make sure to get the right one for your camera) when measuring light with a regular lightmeter (a better solution since you can use more recent models). The User Manuals give two sets of shutter opening angles/exposure speeds : one 'real one', mathematically correct, and the adjusted one that corrects for the light loss and lets you use any regular light-meter reading.

RX lenses are corrected for the second effect of the fixed prism : the deformations introduced in the flow of the light by this piece of glass (it's flat, but it still acts a little bit like a lens element). For the techninal specifics of this correction, check out this site (and rummage through the other, very informative pages on Bolex history and more) :
http://www.city-net.com/~fodder/bolex/

So, your Depth of field should be the same whether on RX or non-RX lenses. I will compare your linked calculator to the Bolex D-o-F tables I have and confirm that.
Those are very good lenses you got, by the way -- and a good kit indeed !
-Boris
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#3 sonickel

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Posted 20 November 2005 - 05:36 PM

Thanks very much for your advice, Boris.

So: when shooting with a Bolex, read with light meter (using adjusted shutter speeds), calculate fstop & depth of field using regular DOF tables, and shoot - and it should turn out well exposed and sharp as a tack?

That clears things up immensely.

* can't wait to start shooting *
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#4 daniel mahlknecht

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Posted 22 November 2005 - 11:33 AM

hi,
just in case you haven't noticed yet, the kern bolex lenses have a built in depth of field calculator, the little orange dots appearing in the small holes when you change focus or the iris.

daniel
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#5 Chris Keth

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Posted 23 November 2005 - 12:33 AM

Thanks very much for your advice, Boris.

So: when shooting with a Bolex, read with light meter (using adjusted shutter speeds), calculate fstop & depth of field using regular DOF tables, and shoot - and it should turn out well exposed and sharp as a tack?

That clears things up immensely.

* can't wait to start shooting *



Yes it should. I've metered for a bolex running at 24fps using a shutter speed in my meter of 1/90th and it turned out quite beautiful.
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#6 gregorscheer

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Posted 13 January 2006 - 08:54 PM

Yes it should. I've metered for a bolex running at 24fps using a shutter speed in my meter of 1/90th and it turned out quite beautiful.


The right setting would have been closer to 1/70
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