8mm Forum: Link speed and light ?

This is topic Link speed and light ? in forum General Yak at 8mm Forum.

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Posted by Dominique De Bast (Member # 3798) on May 12, 2017, 04:30 PM:

I have a technical question. As I reported earlier, I had my Elmo ST 180 three blade shutters replaced by a two/three one. I projected some sound films at 24 fps and had a silent film with a soundtrack with them. Since I couldn't remember if it had to be projected at 18 or at 24 fps, I decided to check at 18 fps before setting the blade shutter back to the three blades position. It appears that 18 fps was the correct speed but since the flickering was almost not noticeable (unlike what happens when you test without film in the projector), I decided to project the whole film without changing anything. My question is not about flickering but about the light. I had the feeling that the picture was very bright. Of course, the projector has a 1.0 lens and a two blade shutter but it was the case also at 24 fps and I thought it was brighter than the other films I projected. Of course, it could be the film itself but then I think the flickering would have been very noticeable as I knoq it has an influence on that. So my question is : is there more light on the screen at 18 fps than 24 fps ? I ask that because I know that when you shoot with a camera the speed matters. Since the film stay is exposed longer at 18 fps than at 24 fps, the aperture varies. Is it the same with a projector ? And then is the difference noticeable or was is just my imagination (or the film) ?

Posted by Bill Brandenstein (Member # 892) on May 19, 2017, 04:16 PM:

In a word, I'd say no. Sure, the shutter blocks the light more times at 24fps, but at 18, the time of the blockage is longer. So since the proportions are the same, the light on the screen should be the same regardless of FPS speed.

You thought maybe this was due to print density, and that would be my guess, too.

Posted by Dominique De Bast (Member # 3798) on May 19, 2017, 04:33 PM:

Thank you for that explanation.

Posted by Andrew Woodcock (Member # 3260) on May 19, 2017, 06:15 PM:

I am surprised to hear you can tolerate any film projected with a two blade shutter at 18fps Dominique.

Nothing i have here looks anything like presentable on two blade shutter mode @ 18fps!
Fantastic benefits from 2/3 shutter @24fps, but awful on 18fps.

Thank God for such inventions as a variable shutter!!

Posted by Dominique De Bast (Member # 3798) on May 19, 2017, 08:11 PM:

I was surprised, too, Andrew. The flickering was only (a little bit) noticeable on some scenes. As I wrote before, it is, however, obvious when you look at the screen without film in the projector. It seems that (like for 3D anaglyph projections) perception varies from one person to another so I may just be lucky [Smile]

Posted by Brian Fretwell (Member # 4302) on May 20, 2017, 05:17 PM:

Yes perception of flicker is directly related to screen brightness. I believe it is due to how the eye works in varying light conditions, the low light receptors are active for longer so even out the light levels.
Also it was said that the old "News Cinemas" in UK railway stations had small screens and bright projection so people could see coming straight in from the street (these showed newsreels and shorts for people waiting for trains) and the flicker at 24 fps with twin blade shutters was annoying to some.

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