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Metering for a Bolex H8RX4 with a Sekonic L-398A


Erren Franklin

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Hello!

I know there are various forums where I could post this question, but I know from experience that there are some very knowledgeable folks here.

I've been shooting 8mm (regular 8 ) for many years now but have mostly relied on cameras with automatic exposure or TTL light meters, like the Leicina 8v or the Canon Cine Zoom 512. I've recently accomplished a long-term goal of mine- getting a CLA'd Bolex H8RX4 with the three H8RX lenses (5.5mm, 12.5mm, and 36mm). To really shoot properly, I need to up my metering game. I've had a Sekonic L-398A for 15 years. First question - does the photo sensor ever wear out? If so, would it have worn out if I kept in the dark most of the last 15 years? Second - I've found guides on calibrating it, which I could follow, but I'm wondering if there is a reason to and a way to calibrate that is geared toward the cine meter/cinematography? 

Lastly and most importantly, if I was to take a simple light meter reading with the Sekonic (set ISO, meter and set footcandles, find aperture setting according to cine framerate) and adjust the lens to the aperture reading on the Sekonic, would it be accurate for the Bolex? Or does the Bolex shutter angle and shutter speed complicate this setting? If so, how would I adjust it and what would I need to know to be able to do so dependably, no matter the light reading?

Thanks so much for your help!

 

 

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Okay, let's begin with your BOLEX, the shutter angle on the Reflex models is typically 133 degrees, so, use 1/65th second for 24fps or about 1/43rd second for 18fps [this is where having the original owner's manual comes in helpful].  Also the viewfinder prism diverts about 25% of the light to the viewfinder, thus anywhere from about 1/4 Stop to 1/3 Stop light loss to the film [this is relative to the condition of the prism as well.  With a small light meter probe it is possible to remove the pressure plate from the gate and actually meter the light coming in via leaving the shutter open....of course...not practical for most, but possible for evaluating the camera without having to waste film].   The shutter angle is much small than on non-reflex versions and older BOLEX cameras due to the mechanics of the variable shutter involved.  Exposure of course, is relative, relative to the film type since reversal film requires perhaps some slight under-exposure for saturation density....and negative films can benefit from and tolerate over-exposure to maintain detail.  To truly benefit from using your camera, you could shoot a very short length of film test on any stock, remove that and have it processed or do it yourself.  You don't need to shoot more than a foot or two.

   Now to the Light Meter questions.  

[1].   Yes, light meter sensors can wear out due to age, regardless if stored in the dark. However, as long as there's some decent life left via strong response to light, they can be calibrated [adjusted] to compensate for their aging.   You could and should download any one of the various free apps for your cellphone that are Light Meters.  Compare the reading off of an 18% Gray Card....or something neutral if you don't have one, or a white cardstock, and see how they rate.  It would be nice to have a known accurate light meter to reference to here......and using either a good 35mm SLR or even a DSLR or cellphone and reading the Data on a given shot to see what the exposure is.....and comparing those would be helpful.  Anyhow....once you do a few exposure reading tests.....if the meter needs some adjustment, there is usually a fine adjustment on it.  Set that to render it correctly to the Zero setting....then compare again. 

[2].  The Cine frame rate setting on most hand held light meters are based on a standard 180 degree shutter opening [usually] at whatever frames per second speed you're using.  HOWEVER, with the BOLEX Reflex, we know the shutter angle is NOT 180, it's 133 degrees so less light is getting in at the same frame rate.  AND...the viewfinder prism robs some light....so you have to factor these into your hand held Light Meter.  So I figure you would need to increase exposure by about 1/2 Stop for the variation.  It might be closer to 1/3 Stop if using Reversal Film, but for Negative, definitely a tad more.  This exposure variation increase would be the same for either 18fps [why shoot film up faster in 8mm...not necessary] or 24fps, since it's due to the difference between the Standard Cine Exposure Rating on the Light Meter versus the actual Exposure Factor on the camera due to the shutter angle and reflex prism.

[3].   For lower light levels, you can run the BOLEX at 12fps to get more exposure power, albeit the slower FPS rate means speeded up action on anything that is moving....but great for more static shots.   So, once you have determined your Light Meter works fine and is calibrated, and you understand that at the Cine Rate Set, you will have to adjust exposure to allow about 1/2 Stop more exposure, you're all set.  Other than making a practical test on the film types you want to shoot, which is easy with roll film....[since you can cut off the test part and still have practically a full roll of film remaining], you're ready to film! 

[4].  Remember, exposure is relative....relative to the subject.  A light meter is only a tool to get you in the ballpark range.....thus a person standing in front of a white or light colored wall, much lighter than an 18% Gray Card, would require metering the person up close first....and/or factoring in that the Light Meter will give you an exposure reading that will be under exposed.  Alternatively the reverse if the subject is standing in front of a very dark wall etc.  Cinematographers professionally will use either or both, an 18% Gray Card for metering each shot, or will take an Incident Light reading....which is the amount of light actually falling on the subject [here you would use the Incident Light Meter Dome on your Light Meter].....which works fine since they usually shoot Negative Film Stock. 

Before you go off and shoot lots of film in that wonderful camera, do yourself a favor and do those short film tests.....then analyze your results and make some determinations for what works best. This is time well spent.  Good luck and best of success!

  

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Thank you so much, Martin!

I'm going to follow your advice and follow these steps, and I will report back.

One question though- the short film test shoot- are you recommending that I do so separately from or prior to doing metering tests? In order to test the camera?

Thank you,

Erren

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Here‘s copy of the H8 Reflex manual:

http://www.vintagecameras.fr/images/MonSite/BOLEX/H8_Reflex/_Doc/Bolex_H8REX_Manual_en.pdf
 

Exposure times are listed on the first page. The „adapted“ times are taking the prism light loss into account. So at 18fps the exposure time is 1/50 sec, but you meter for 1/60 sec to allow for the prism light loss. Other times are shown for different speeds and variable shutter settings.

Some online guides suggest that RX lenses have aperture scales that already compensate for the prism, but this is incorrect. They are marked just like any other lens.

I would check the calibration of your meter first, then shoot a test. 

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Always do a film test PRIOR to ever shooting anything important or even just playing around factoring in what film and processing cost these days.  A short test of foot or two can be self processed...even as Negative B&W chemistry for an exposure and focus check.  Longer length would be necessary to check for even running of course.   AND...thanks for the IB Manual Link Dom,.... always default to the Factory Owner's Manual.   My 18fps shutter speed was probably more for 16fps....but anyhow, always best to double check via referring to the manual, and doing a film test.  For a more decent film test, you could shoot several feet and get an idea of how smooth the camera is running etc.  I don't think there's much difference if any exposure and shutter wise between the link to the BOLEX H8 Reflex and the H8RX4, other than the addition of the 1:1 drive shaft and flat base etc.  I have to stress the importance of testing........when I was 15 and got my first BOLEX H-8, I did a dry test and figured all was well.  Well, I ran that 100ft roll through it and got it processed (yields 200ft of course in Double 8mm) and there was shutter ghosting, film slippage etc.....I was not happy!  Had I done a small test, I would've caught that early on.  Film and processing was cheaper back then, and also I could've processed a short test...but didn't have 100ft 16mm film width processing capability back then yet.  Anyhow, it was sorted out before the next roll!

So sure, check the meter, meter your test shot and do a film test.  You'll be so glad you did BEFORE you commit to running full roll of film, of any length [those BOLEX H-8 cameras are capable of running the 25ft, 50ft and 100ft spools....though 50ft ones have been long discontinued].

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Thank so much for the tips to both of you. I will definitely run a test and let you know how it goes.

Also, one question. I had the flange depth on an H8RX modified to match that of non H8RX C-mount lenses, so I could  mount other C-mounts on it. I wanted the option to use other C-mount zoom lenses. My question is about whether or not my aperture setting on whatever C-mount lens I use would be different than an aperture setting on the H8RX lens. Is there going to be a difference?

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No, an F-Stop is a mathematical determination [based on the diameter of the lens opening, and relative to each lens.....it's commonly used for exposure.  Only a T-Stop [transmission stop type lens] that has been calibrated/checked will give you the exact light transmission.  However, for all practical purposes you will be fine since that is used to determine exposure for most lenses......of course, factoring in any other light loss via filters, prisms in cameras, light diverted for built in light meters etc.  The F-Stop generalization is used for determining depth-of-field and depth-of-focus, and hyperfocal distance.  That's why, on reflex type cameras, any other light loss has to be factored into the exposure determination.  

Just make sure to be careful with other C-mount lenses to make sure they do NOT protrude into your prism!  So your modification is to maintain proper focus range with other C-mount lenses, that's great.

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28 minutes ago, Erren Franklin said:

Also, one question. I had the flange depth on an H8RX modified to match that of non H8RX C-mount lenses, so I could  mount other C-mounts on it. I wanted the option to use other C-mount zoom lenses. My question is about whether or not my aperture setting on whatever C-mount lens I use would be different than an aperture setting on the H8RX lens. Is there going to be a difference?

 

No, there’s no difference. As Martin mentioned, f stops are a mathematical measurement and are the same no matter the lens or the format. And as I mentioned earlier, RX lenses are just like non-RX lenses in terms of their apertures. You still need to factor in the prism light loss as per the manual, no matter what lens you use. 

All lenses lose a little light internally, maybe a quarter of a stop for primes and a third to a half stop for zooms, so it doesn’t hurt to over-expose a little more to compensate for that, especially with reversal which has less exposure latitude. Many cine lenses are marked in T stops for this reason, but generally not C mounts. T stops were introduced in professional cinematography where maintaining exact exposure values between lenses was important, but I guess it was deemed too complicated or not a big enough problem for equipment aimed more at amateur users.

So with that modified camera you can no longer use the H8RX lenses, but you can use H16 RX lenses, as well as normal C mount lenses. For whatever reason, Bolex Paillard decided to make the H8RX a C mount camera but shortened the flange depth by about 2mm, so H8RX lenses are a strange hybrid that only work on H8RX cameras. Modifying the flange depth to the normal C mount standard is an interesting idea, and let’s you use some of the excellent C mount zooms designed for Super 8 as well as all the 16mm format lenses. Just be very careful of the rear protrusion hitting the prism, as Martin mentioned.

The H8RX prism is half as thick as a H16RX prism, so the optical aberrations introduced are not as strong. For this reason, I think you’ll find both H16 RX lenses (which will over-correct the aberrations) and normal C mounts (which will under-correct them) will both work OK on your modified camera. 

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