I am a big fan of Indy Mogul (indymogul.com) but I was a little disappointed with one of their recent episodes of Q and Erik, their weekly filmmaking question and answer show. In response to a reader question about achieving a film look, Erik claimed that frame rate was one of the two big factors differentiating the film look and the video look. He suggested looking for a camera that shoots 24P, or 24 progressive frames per second. This is a commonly held misconception about video and I’d like to try and clear this up a little.

Film cameras in the US run at 24 frames per second. Television and video runs at 30 frames per second. So, what is the difference? I’ll explain.

One of the ugly downsides to the video image is what is often referred to as “the video edge.” The edges of objects seem perhaps too crisp or hard, especially around bright areas. The video edge has two main sources, one is loosely related to frame rate and the other is not.

1.  The charge coupled device (CCD) in the camera, the chip that registers the image, is divided into distinct pixels that are isolated from one another. If light hits one but not the other, the edge between them is very distinct. Film does not have this problem.  The light receptive molecules on a piece of film are scattered randomly and they overlap. Also, light can pass through the film emulsion and bounce of the back of the film to expose nearby areas of the film. This is called halation and is the reason for the “glow” around bright objects in a film frame. Different film stocks will halate differently. Pro videographers will often use a filter such as a Tiffen Pro Mist filter to soften video’s hard edge.
2. The common misconception, (promoted by video manufacturers who cannot solve the first problem) is that the video edge comes from a lack of motion blur. It makes sense that more motion blur could soften the video edge. The fewer frames per second, the longer the exposure of each frame, and thus the more motion blur. That is the party line. Is it true? Not exactly.

A film camera has a shutter shaped like a disk with a pie piece cut out of it. It rotates at 24 times per second to give 24 frames per second. During the time when the pie piece cut out is over the frame, the film is being exposed. The time when the film is not exposed is used to pull the next frame into place. Generally, shutters use a 180 degree cut so the actual exposure time for a given frame is 1/48 second. An ordinary consumer video camera recording 30fps will actually record 60 interlaced fps. Because there is no film to pull through the gate, the video camera doesn’t need to use a shutter and thus can expose each interlaced frame for the full 1/60 of a second.

The above is a naive generalization of the issue. In reality, video cameras do have an adjustable shutter speed to cut down on light and film cameras also have adjustable shutter angles. There is a distinct difference in the look of a short shutter, but it’s not the thing that separates film and video. For example, do you think Saving Private Ryan looked like it was shot on video? No, but it did have that sharp crispness to the image because of a small shutter angle.Good video cameras today can shoot 30P, or 30 progressive frames per second. The exposure can be set to 1/30 of a second which is even slower than a film camera do, but it doesn’t eliminate the video edge. 30P is a better choice than 24P for most videos. But here is the real killer…

The 24P camera will stretch your 24 frames into 30 frames by interlacing the progressively shot frames, thus counteracting the fact that it shot progressive frames in the first place. Interlacing probably contributes more to the bad video look than frame rate so why would you add interlacing just to lower the frame rate?

If you bought a 24P camera, don’t feel bad. It probably shoots great video at 30P.

My Conclusion: 24P is a gimmick. Shoot 30P unless (and only unless) you plan on getting a film print made, in which case, shoot 24P advanced (Google this if you want to know how it differs from vanilla 24P).

This entry was posted on Thursday, December 20th, 2007 at 3:23 pm and is filed under Film Look. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.