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Art imitates art - the scene slate/clap stick

Doug Kerr

Well-known member
Sometimes art imitates art, and sometimes badly. Here's an example from the fascinating world of cinematography.

Early in the emergence of the motion picture industry, it was realized that there needed to be a good way to identify the different "takes" originally on a single roll of film as they moved moved separately through the post-production process.

A very practical solution was identified. The information (production, scene, frame, etc.) was written on a small wood-framed slate, such as used by students. This was then exposed to the camera at the beginning of each take.

The onset of film-based motion picture sound brought a new (and actually unrelated) need. The sound was recorded on a separate strip of film ("double-system"). When the film was being edited, it was necessary to properly synchronize the picture and sound films in the editing machine (perhaps the famous "Moviola" machine).

The two film strips passed over separate sprockets, normally coupled together, so once synchronization was attained (for a given take), it would be maintained as the film was shuttled back and forth, whether at motor speeds or by hand. The two sprockets could be decoupled so that the proper alignment of the two strips could be set at the beginning of a session. But based on what?

The solution was simple and elegant. Two strips of wood were hinged together. At the beginning of the take, a craftsman would hold them in front of the camera, with the sound recorder running, and clap them together. The sharp click was recorded.

On the sound film strip, the click could be easily seen visually (this was of course optical sound), and could also be heard if the film strips were rocked back and forth through the editor. The exact moment that the clap stick closed could also be seen on the picture film (well, maybe, which will be the centerpiece of this story). The editor could then synchronize these two events, lock the sprockets together, and get to work.

Soon it was realized that this clap stick could be mounted on the top of the scene slate (early ones just being a single piece of wood mounted on a hinge so it would close against the slate frame top rail). Thus the two activities could be accomplished at the same time.

Before long, this had become refined into the arrangement seen in stylized form at the top of this illustration:

scene_slate_01.gif

Why the diagonal stripes? They served two purposes.

Firstly, in determining visually on the picture film exactly when the "clap stick" has closed, we basically rely on seeing if there was "any daylight" between the stick and the frame. Sometimes the background behind would be light and sometimes dark. By having both light and dark areas on the stick and the frame, we could deal with either of those.

But why diagonal black and white areas?

It turns out that by making the light and dark areas diagonal. we additionally bring to bear on the matter of determining exactly when the stick and the frame met an interesting human visual capability. It is called vernier acuity. This refers to the eye's powerful ability to perceive when a line has an offset in it, even a very slight one, even if there is a gap between the two parts of the line.

In the bottom panel of the illustration we see the clap stick not yet quite closed. In the region shown in the circular detail, yes, we can see that the stick isn't quite closed because there is a gap, but our perception is enhanced by the fact that, via vernier acuity, we recognize an offset in the black-white boundary. (The dotted line helps us see that offset; it doesn't exist when the stick is fully closed.)

Let me interrupt the technical story to talk about something organizational. In the Hollywood motion picture context, union considerations make it quire critical who does what. There was at first some uncertainty about who would handle the scene slate/clap stick. It was finally concluded that this should be the same person responsible for loading the film into the camera. Thus we see on movie credits today the craft called "Clapper-Loader".

Now, before long we had movies about making movies. In those we had to have scene slates/claps sticks as props. Real ones weren't suitable - they were generally very untidy looking. So prop makers made nice presentable ones. And evidently, the first guys to do this didn't have a real one handy to look at, because they made them this way:

scene_slate_02.gif

Now, in a case of art badly imitating art, many actual scene slates sold today are like that - especially those sold to amateur moviemakers.

A lot of those guys got to be real moviemakers, and guess what - the makers of real professional scene slates started getting requests for ones that work this "wrong" way. And in fact, today many models of "professional" scene slates can be ordered either way.

Does this "backward" design matter. Yes.

Again, we see in the lower panel this scene slate just before the clap stick fully closes. Again we examine in larger scale the circled area. Of course, we can tell that it is now closed in the basic way - from the gap. But note here we don't get any help from vernier acuity. In fact, the eye really seems to makes us see the upper and lower boundaries as if they extended and met at a point (as shown with the dotted lines). So in a way, that aspect looks the same whether the clap stick is closed or a little open.

Now, is this a big loss in functionality? No. But its a shame to lose it through lack of understanding of the principles.

Heard that before, have ya?

Best regards,

Doug
 
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