What is the duration of a single projected film frame?

[denti alligator]

I've been AWOL from the forum and am returning for help that I somehow know I will get here faster than anywhere else.

The problem is a technical one, and maybe I'm over- or mis-thinking this one, but here it goes: if it takes 1 second for 24 frames to pass by the opening of the projector where light passes through the frame and onto the screen, how long is each of the 24 frames actually AT THAT OPENING to be projected? That is, doesn't it also take time for the shutter to hide the movement of one frame to the next? And how much time does that take? I'm bad at math, so bear with me here. I realize the frame doesn't stop or linger at the opening, it just passes by, hence the need for the shutter, to hide all those moments when the frame is NOT perfectly aligned at the opening (what's the proper word for this, anyway?). But wouldn't that mean that each frame actually only appears for a fraction of a fraction of 1/24th of a second, while the rest of that 1/24th of a second is black?

And if this is so, what happens on a DVD? Does that mean that we somehow see each frame for a longer period of time? But that doesn't make sense, does it, since the effect is the same?

Why is this driving me nuts?

[denti alligator]

This wikipedia article clears a lot up.

So it appears the exposure time is adjustable, though a 180-degree shutter is the norm, which means, to answer my question, each frame is projected for 1/48th of a second (but according to the article, this varies). It also appears that the shutter "pulls" the frame into place, so that the film does not (?) exactly move smoothly past the aperture...? Can someone familiar with this technology explain this, please. And at what point is the shutter angle that determines the exposure time decided? When filming? How does one know what shutter angle to use at projection if the film shutter angle is unknown?

And this still doesn't answer what happens on a DVD. Obviously the "black out" of the shutter is not transferred to digital. So does that really mean we are seeing each frame for about double the time? If so, why does that not affect the appearance of the film?

[deadlovers]

So when the frame is pulled down, register pins pop out and into the film sprockets, securing the frame in place for that 1/48 (at 24fps). Then, once the shutter covers the gate again, the frame is released and another frame is pulled down and locked in place, and so forth. It's all pretty mechanically amazing.

The shutter is rotating, round, and adjustable (usually). 180 degrees is the standard shutter angle--so you've got a half-circle-mirror, spinning, throwing light up to the viewfinder (your eye) half of the time, and through the gate and to the film plane the other half of the time. If you reduce the angle to, say, 90 degrees (3/4 of the shutter is mirror, 1/4 is empty), you'll be getting half the exposure time of 180, creating that hyper-crisp feel that is sometimes used in action scenes. It's all controllable at the time of shooting. It's like a photo camera, quicker shutter speed offers a crisper image, but it also requires more light.

As for projection, the visual results of a chosen shutter angle are permanently baked in at the time of exposure. Now, I'm not too well versed in film projection, but I understand that modern projector shutters run at speeds of 72hz so as to hide the perceivable flicker of a traditional 24hz projector. The projector still runs at 24fps, but flickers more than the once in-between frames (in the middle of frames, too, I'd imagine), and requires a lot more light.

As for the translation between 24fps projection and digital 24p: I suppose you're technically seeing more of each frame for the duration of their allotted 1/24 second, but the difference is not perceivable by the human eye/brain. I don't rightly know how long the pulldown process takes, so a projector could plausibly have a much larger shutter angle (mostly empty), only creating flicker for a fraction of the time associated with the actual filmmaking. I'm sure there are projectionists, who frequent this forum, that could provide a better explanation.

Please, correct me if I'm wrong on any counts.

[zedz]

Kluge (I think it was, anyway) has a a very poetic answer for this, and he talks about every film you watch being half imagery and half darkness, with the darkness being an absolutely essential part of the filmic experience.

[denti alligator]

Zedz, do you remember where he said that? I've heard something similar, but from another source (can't remember now).

It still don't quite understand how this works, since the moment at the frame is perfectly aligned at the aperture would seem to be so minute. Am I understanding it correctly that the frame is held at that moment for the entire time the shutter is open (i.e. it doesn't actually move), before then getting pulled away while the shutter covers over the aperture?

[zedz]

Yes, the film is not dragged through the projector at a constant rate, but advanced one frame at a time by the claw, held in place, flashed up on the screen, blacked out, and advanced another frame. All of this happening 24 times a second.

[AK]

An interesting topic (in which I can't be of any assistance, I'm afraid), and a wonderful quote from Kluge! If anyone could track that down for the rest of us I'd be thrilled!

[zedz]

Denti: Fortunately, I excerpted the relevant dialogue in my final annotation of the box set in the Kluge thread:

I'll give Kluge the last word, in the form of a dialogue between Nobel prize-winner Professor Kandel and his unnamed interlocutor, from "The cinema in the head of the spectator." This piece expands on an intriguing idea featured in The Blind Director:

- For a forty-eighth of a second it is dark and for a forty-eighth of a second there is an image. This is an interesting kind of movement for the brain.
- What does the brain 'see' of this? Does it see the black between the images, the transport phase? Does it react independently to the moment in the transport phase when it is dark in the cinema for a forty-eighth of a second, i.e. with signs that it creates itself and only understands itself?
- Similarly.
- Like in a dream?
- Or under the influence of drugs. It 'sees' the black continuously, whereas the same brain sees the 'image' as continuous, even if it is also 'flickering.' A polyphonic impression.
- Unconscious?
- Non-conscious. I know, of course, that I am sitting in a cinema. Or rather doubly conscious. I see TWO FILMS, one made by the brain itself out of darkness, and one in light and colour, as reported by the eyes, also with a collective impression already created by our ancestors, as triggered by the content of the photographic images.
- The stimulant that causes the brain to dream is the rapid exchange?
- Which, however, in the case of a two hour film produces a whole hour of darkness (the brain works autonomously) and a whole hour of images (the brain responds to stimulation).
- And that is better than reality?
- Much better.

Edit: this exchange is exerpted in one of the DVD booklets, but I believe the original essay can be found in Cinema Stories.

[denti alligator]

Yes, the film is not dragged through the projector at a constant rate, but advanced one frame at a time by the claw, held in place, flashed up on the screen, blacked out, and advanced another frame. All of this happening 24 times a second.

Thanks. Makes perfect sense now.

[denti alligator]

Thanks for the quotation from Kluge, too.

So does this mean that with a DVD we get *more* image, the single frame appears to our eye for twice the length of time?

[zedz]

I think that's the case, which is why the absence of flicker is considered such a big deal by many experimental filmmakers - we really do receive and process the images in a different way with the different media. I certainly feel a difference in the way I receive film and video images, even if it's hard to articulate. For one thing, it's much rarer for me to 'drift off' while watching projected film. Partly that's down to the viewing context, but this also applies when I've watched 16mm projected at home.

[Alan Smithee]

Couldn't you just jack up the frame rate in post and fill every other frame with a black frame?

[denti alligator]

Couldn't you just jack up the frame rate in post and fill every other frame with a black frame?

I wonder what the effect of this would be. Anyone want to try it out with a short sequence? (I'm not technically capable.)

[Adam X]

And this still doesn't answer what happens on a DVD. Obviously the "black out" of the shutter is not transferred to digital. So does that really mean we are seeing each frame for about double the time? If so, why does that not affect the appearance of the film?

I suppose this could account for the perceived increase in sharpness/clarity well-authored blu-ray's afford, compared to film, being that your eyes get to see each frame literally twice as long.

... we really do receive and process the images in a different way with the different media. I certainly feel a difference in the way I receive film and video images, even if it's hard to articulate. For one thing, it's much rarer for me to 'drift off' while watching projected film. Partly that's down to the viewing context, but this also applies when I've watched 16mm projected at home.

I definitely feel the same way, and really miss regularly seeing films projected on 35mm vs video. I remember reading/hearing somewhere that the brain does indeed perceive the two formats differently, and not just for the difference in the way the image reaches the screen (light thrown through film vs an image projected onto a screen). I wish I could quote the source*, but it was stated something along the lines of the brain perceiving projected film "actively", while watching TV (and no doubt digital projection) is perceived "passively" - meaning that the brain is in different states while viewing the two different formats. It's not surprising it's more common to fall asleep in front of a TV with this being the case, but it brings up interesting notions of how we actually perceive any given film, depending on the format used.


* = sadly an internet search only resulted on pages about projection or active vs passive 3D.

[martin]

I found a topic/discussion elsewhere: In Digital Projection, are we missing the "black space"?

If I'm understanding this correctly, the "black space" (approximately 50% of the film's duration, but might vary), is missing in digital projection and in digital medias like DVD and BD. But apparently some digital projectors are able to make "dark frame insertions" (page 2 in the link).

Whether the "dark frames" are important for the viewing experience is not a trivial discussion at all. There seems to be general agreement that the appearance of dark frames in analogue film projection isn't noticeable when the flicker rate is above a certain rate. But the dark frames could be important on a subconscious level (like Kluge suggests). And while a digital projection without these black frames could seem similar to an analogue projection on a conscious level, it could be a different experience on the unconscious level (?).

[TMDaines]

Could people tell the difference between film and video if both "prints" are immaculate and projected perfectly in a double-blind test? The biggest reason I notice the difference is the shitty projection of film and damage on prints. I'm sure I'm part of the minority here but I don't think I'd care too much if film died out.

[htshell]

I can always tell the difference between film and video. It's not that hard to tell either because reels last about 18 minutes so you're going to see the cue mark in the top right and then the reel change. Even before that though you can tell from the density, the type of dirt or damage or other marks and so on.

[zeroism]

From a post on the page that martin linked:

FYI, film projectors usually use either a double or triple bladed shutter, so you essentially get 48 or 72 "frames" shown per second, but as the film is advanced only every 24th of a second, each frame is shown either 2 or 3 times - this reduces visible flickering, without changing the motion chracteristics of the format.

I was under the impression that 48hz and 72hz modes on HDTVs were designed to simulate this behavior, at least to whatever degree that's possible with a digital display. I use the 48hz mode on my Panasonic Viera with 24p content, which produces an occasionally noticeable flicker, which, while not to my eyes nearly as obvious on all content as many comments on home theater forums and review sites would suggest, does look and "feel" more like projected film to me than anything at 60hz. Does the flicker I'm seeing indicate something comparable to "black frames" in the manner of a film projector, or is it fundamentally different?