It must have been soon after the invention of the videotape recorder that the first predictions of the demise of film arose. The latest perceived threat to celluloid (well, not really celluloid) is 1080P/24 fps video. Without question 24P marks video's closest approach yet to being a substitute for film in television and movie production. It has 2K resolution - not as high as 35mm film but pretty high. It has a frame rate of 24 per second, as does almost all film shot for theatrical projection or television programming, which makes it "move" like film in the theater and on the TV screen.
Although there are those who disparage the judder generated by 3/2 pulldown when 24 Hz film is converted to 60 Hz video, it is a part of the "film look" coveted by those who tell dramatic stories on television. We now have a 24P camera (720/24P) that can operate off-speed (undercrank and overcrank).
For those productions heavy on special effects, 24P video's image stability and lack of grain, along with the fact that it is acquired in electronic form, makes it work better for blue-screen or green-screen effects than film. It requires no developing or film-to-video transfer. Artistically, it offers the freedom to shoot either movie-style with a single camera or television-style with multiple cameras and a switcher.
The use of 24P video is on the rise among both movie and television producers for the above reasons and others. It is finding its place as a useful tool for television and movie production, and its use in these areas will without doubt continue to increase. It is well-recognized that despite its similarities 24P video is different from film - particularly 35mm film - in some respects. Leaving aside considerations of differing colorimetries, gammas and dynamic ranges, let's take an in-depth look at one fundamental technical difference between film and 24P video, one that has more to do with optics than electronics.
RAYS OF LIGHT
First, we need to review some background. A camera lens works by collecting the light that comes through it and focusing it on the film or the sensor. Much of the light entering a lens can be considered to be a group of parallel rays. These rays are bent or refracted by the lens, and this serves to converge them on a point called the focal point. The distance from the lens to the focal point is called the focal length. When you were younger, you might have enjoyed setting paper on fire by focusing the rays of the sun on it with a magnifying glass. When the glass is held at the distance corresponding to its focal length from the paper, the sun's rays are concentrated into a tiny dot that contains enough energy to heat the paper to its ignition point.
It should be pointed out that not all the rays entering a lens are parallel, and the de-focusing that is generated by nonparallel rays is called spherical aberration. Further, not all the rays entering the lens are of the same frequency, and the color-scattering caused by the resulting differential refraction is called chromatic aberration. A characteristic of good lenses is that these aberrations are in various ways minimized.
The focal length of a video or film camera lens is a key parameter. Another key parameter is the aperture size, which is the useful diameter of the lens. The relationship between these two parameters is expressed as the focal ratio. The focal ratio of a lens equals the focal length divided by the aperture. This relationship between a focal length and aperture is known to photographers as an f-stop: As the focal length of a given lens is constant, the f-stop effectively indicates the degree of iris opening or aperture.
To see what this means in movie and television shooting, first consider a 35mm still camera, something familiar to many of us. A "normal" lens for such a camera has a 50mm focal length. The image projected by this lens on the 35mm film frame creates a scene on the film in which things look to be about the same distance from the viewer as they look to the eye. If we substitute a lens with a focal length of 100mm, this is a telephoto lens and the images on film are magnified, appearing to be closer to the viewer than they appear to the eye.
This magnification also results in a narrower angular field of view: Less of the scene will now be captured in the picture on film. Because the aperture (diameter) of the lens is about the same as the 50mm lens, increasing the focal length results in a larger focal ratio - whereas the f-stops of the 50mm lens might go down to f/1.2, with the telephoto they may only go down to f/3.5, for example.
Conversely, if we put a lens with a focal length of 35mm on the camera, the effect will be to "push" the images farther away than they appear to the eye, with an attendant increase in angular field of view, and this is a wide-angle lens. The focal length of the wide-angle lens results in a smaller focal ratio than the 50mm lens, and it will be capable of smaller f-stop numbers than its 50mm cousin.
DEPTH OF FIELD
The final parameter we must consider is depth of field. When a camera is focused on an object, that object will be the sharpest thing in the picture. But points closer or farther away than the focused object will still look acceptably focused. The range of distance in front of and behind the focused object that are in acceptable focus is called the depth of field. Depth of field varies inversely with the lens aperture, the distance to the focused subject and the focal length of the lens. As a result, the more distant the focused subject is from the lens, the smaller the aperture, and/or the shorter the focal length, the greater the depth of field.
Finally, the linear size of the image projected by the lens on the focal plane - the spot where the film or sensor is located - is equal to the photographed scene's angular size (angular field of view) multiplied by the focal length. If we wish to project a smaller image on the focal plane while holding the angular size of the photographed image constant (giving us the same apparent distance from the viewer and the same magnification of the photographed image), the focal length must decrease, which as we have seen will cause the depth of field to increase.
Now let's get back to movie and video cameras. One of the photographic effects most frequently used in shooting 35mm film for dramatic television and movies is selective focus. Often, the foreground - on which the director wishes our attention trained - is held in focus, while the background is intentionally defocused. Our attention is shifted from one subject to another within the frame by defocusing one subject and refocusing on another. This type of photographic effect is a staple of dramatic storytelling on film. It is relatively easy to achieve on 35mm film, because the focal lengths of the lenses used to project an image on the relatively large 35mm film frame are long enough to result in a relatively shallow depth of field, even when stopping down the lens aperture to a reasonable degree.
The image width at the focal plane when shooting 35mm is 0.864 inch using Academy camera aperture (which has an aspect ratio of 1.33 and is the full frame exposed by the camera) and 0.945 with Super 35 aperture. When we reduce the image size to fit on a 2/3-inch CCD sensor (which does not have 2/3 inch of useful area), the image width at the focal plane is reduced to about 0.346 inch. Our lenses are therefore going to have a markedly shorter focal length and a significantly increased depth of field. We will now have a much more difficult time defocusing the background while keeping the foreground in focus. This may be mitigated to some degree by opening the lens iris all the way, maximizing aperture, but the lens can have only so much aperture before the image becomes defocused at its edges, an effect called field curvature.
The net result is that selective focus is much harder to do when shooting video than when shooting 35mm film. Super 16 film has somewhat the same problem, but the image at the focal plane is still larger than that for a 2/3-inch CCD imager at 0.493 inch, so the effect is not as great. One-inch CCD sensors would help, but they are not available to the 24P shooter. Lack of selective focus is often worked around in practice by the application of artful lighting - obscuring the background by reducing the amount of light falling on it.
We have seen that 24P video is a tool that movie and television producers can and do use to tell their stories. We have also seen that while it has similarities to film, the two are different. As with any artistic medium, the photographer must understand the medium and its characteristics in order to effectively employ it.
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