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Eumig 881 PMA, time exposure feature, hacking it (plus reciprocity failure)


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#1 Michael Lehnert

Michael Lehnert
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Posted 19 August 2016 - 01:06 PM

I get alot of Personal Messages to my profile here from people who don't even post their questions in the forum (registered users, zero posters), and instead asking me directly for help or support, or putting questions to me. While I am very happy to field answers, it's kinda sad that internet forums have become so "old-fashioned" that people don't see the value in asking their questions in public to get better hive mind anwers than most direct PMs.

 

So, from now on, if a question is concerning a good and rare camera, I'll feel compelled to post the correspondence, anonymously, here in the forum, simply to make it available to the public. Someone may have had the same question, someone may web search it in the future, or learn or be inspired by the issue at hand for their own work.

 

Yesterday, I got a question in regarding the Eumig 881 PMA, it's time exposure feature (similar to the one found on the Bauer A 512 top production camera), hacking it for daytime with NDs, and a related question on reciprocity failure.

 

Here's the correspondence. I hope someone who has more shooting experience with the Eumig 881 PMA can chime in, as it's been years since I had one in hands and shot with it.

 

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Hi ____,


 
it's been a great many years since I have used an Eumig 881 PMA with Eumig Makro-Viennon 1:1,8 / 7-56mm, and I may not remember the functionality of the intervalometer (variotimer) and time exposure feature fully, but cross-checking with the original manual, let me answer your three questions as best as I can:
 
1. Is f1.8 the diaf aperture set automatically when you do long itme exposure? I would like to set it around f11. Is that possible while doing long exposure?

 

First: The exposure index of the camera is set automatically using the Super 8 cartridge notch codes, reading either ISO 25/40 or 100/160. You can't alter or override the exposure meter's reading with dedicated EI dials or the filter key for the Wratten 85, as is possible on other Super 8 cameras where the systems are interlinked.
 
When you activate the time exposure feature, the camera will calculate and control its own "correct exposure time" of a film frame based on the film's exposure index and the camera's exposure range of 1/12 sec to 1 min, and yes, the diaphragm automatically opens up at f/1,8 in that setting.
 
I have never tried this out on the Eumig, but the diaphragm is accessible by the manual aperture settings, and you can close it and lock it. In fact, the manual tells you so when shooting with ISO 100/160 film stock to close it manually down to f/2.8 and lock it at that. So I suppose you should be able to close it down to f/11 and lock it. You can check yourself as you should see a change in the exposure through the film gate: angle a mirror at 45 degrees to the gate in the cartridge compartment, with the compartment door open.
 
You would need to calculate the film speed, your chosen aperture opening and the neutral density of the filter you want to use for the specific shooting situation I can only guess you are aiming for.
 
 
2. Is it possible to combine the time exposure, with the time lapse of the film? Doing time lapse with long time exposure of each frame.
 
I can kinda see what you are trying to achieve, but technically, you can't shoot the time exposure feature (LT mode) – which in itself will lead to time lapse in its original purpose scenario – in conjunction with the intervalometer (VT mode) of the camera, in a daytime situation. You can't set LT and VT at the same time, and the filming speed selector know of the intervalometer won't affect the shutter release in VT mode.
 
Your proposed ND hack for LT should get you the maximum out of this experiment. But in the end, you have to test-reel it! Grab a cartridge and just shoot it to see how the camera reacts to the daytime ND'd time exposure that you I think you are after. Cinematography is about physics and chemistry, but it's still an art practice.
 
 
3. And last. I would like to know if you know that in long time exposure the open or closing stops works in the same way when you shot in normal time exposure. There is something called "fail of reciprocy law" that i m not sure i understand.
 
If by "open or closing stops", you don't mean opened-up or closed-down f-stops but the mechanical shutter release, the "opening and closing" of the shutter, then yes. The opening angle of the shutter is 190°, and mechanical transportation is not changed. In LT, it's just governed by "sufficient cell-measured" exposure time rather than filming speed when shooting normally at, say, 18 or 24 fps. That's why you have the film-sequence timer knob, telling you when you have 10 sec of film at a projection speed of 18 fps exposed "in the can" or rather in the cartridge chamber.
 
Reciprocity is mathematically defined in a formula that says that aperture and shutter speed are inversely proportional. As you decrease the aperture, you will need to increase the shutter speed by the same corresponding setting to retain the same exposure. You get the same exposure at various combinations of f-stops and shutter speeds, which manifest to you in the lovely optomechanical interplay between f-stop setting ring, exposure index setting control, and filming speed setting selector (one mark down there, one mark up here, to put it primitively).
 
Reciprocity failure is when the light situation is such that this interplay breaks down and you need to add extra exposure time to the usual formulaic rule of one f-stop, say, half'd, to double exposure time. This failure happens at the film stock level, as the silver halide grains on the emulsion do not evenly react to photonic exposure, and so the density of the exposed frame is lower than the exposure index of it technically suggests. Film stock data sheets have density curves where you can check on the characteristic behaviour and cut-off points of the film.

 

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