In 2002, this column rhetorically asked if film was dead once again. Since the videotape recorder appeared on the scene in the 1950s, there have been repeated predictions that film would be replaced by video. Indeed, the manufacturers of video equipment have made a number of attempts to do just that. Video successfully supplanted film in newsgathering long ago, but in spite of the video recording industry's best efforts, film prevailed as the capture method for theatrical movies and primetime television. That tide appears to be turning.
There is no disputing it's an increasingly digital world. I recently encountered a photographer from The New York Times who was not carrying a Graflex press camera with a big bellows on the front and a flashbulb the size of a 60 W household light bulb screwed into it.
She was in fact carrying a Canon EOS 1Ds Mark II digital single-lens reflex camera, which is, it turns out, standard issue at the newspaper.
HD VERSUS FILM
When HD video made its appearance, some thought it would swiftly replace film. But there were several reasons why this did not happen. One of the most popular arguing points about film and HD video concerns resolution, but the resolution argument is not as relevant as some would have us believe. There are several other ways that film and video differ. In earlier days, HD video recording was done only at 30 fps interlaced, or 60 fps progressively scanned. Interlace artifacts are significantly undesirable, to be sure, but the 30 and 60 Hz frame rates made video unacceptable in most creative storytelling situations.
Most of the film shooting for feature film and television is done at 24 fps. This means that when this material is converted to 60 Hz-based video, 2:3 pulldown must be introduced, and 2:3 pulldown imparts temporal artifacts. It is, however, the way a lot of television programming is broadcast.
When it became possible to shoot 1920 x 1080 x 24 fps video, this constituted a breakthrough for the dramatic television storyteller. The resulting video has the temporal "look" that 2:3 pulldown imparts. Regarding spatial resolution, 1920 x 1080 is generally deemed to be an adequate mastering resolution for television broadcast material. We have all also recently heard about some notable theatrical movies produced and directed by notable producers and directors that incorporate 24p video material. Some of the advantages it offers include instant dailies and a high degree of compatibility with high-resolution computer generated imaging.
A significant hurdle in much television and cinema acquisition is caused by the size of the video imager, compared to the image size of 35 mm film. This is important not simply because the larger the imager, the larger the pixels can be, and therefore the higher the signal-to-noise ratio that can be realized. It is also important because image size affects the focal length of the lens, and, thereby, the depth of field that is seen in the captured images.
LENS PARAMETERS
Two key parameters of a lens are focal length and aperture. Focal length is the distance from the lens to the focal plane of the imager or film, and aperture is the usable diameter of the lens opening. The relationship between focal length and aperture, the focal ratio, is the focal length of a lens divided by the aperture diameter.
A simple example would be when the focal length of a lens is 50 millimeters, and the aperture or diameter of the lens is also 50 millimeters, which would correspond to a focal ratio of 1.0. While such lenses do indeed exist for 35 mm single-lens reflex still cameras, they are rare, as this is an unusually large aperture for such a lens.
As the aperture diameter increases, the cross-sectional area of the lens becomes geometrically larger, and it becomes increasingly difficult to maintain image quality over the entire lens area, particularly at the outer edge of the lens. Thus, as the aperture diameter increases, the lens becomes increasingly difficult and expensive to manufacture. Typically, a focal ratio around 1.8 or 2.0 is the lowest found in practice. For a lens with a 50 millimeter focal length, a focal ratio of 2.0 corresponds to an aperture diameter of 25 millimeters. If 25 millimeters is the maximum aperture of the lens, then adjusting the iris so that 10 percent of the aperture diameter is used, or an aperture diameter of 2.5 millimeters, this produces a focal ratio of 50/2.5 millimeters or f/20.
Irises are not continuously adjustable, but are adjustable to specific aperture diameters that we know as f-stops.
In the example just cited, the nearest f-stops would be f/19, corresponding to an aperture diameter of about 2.63 millimeters, and f/22, corresponding to an aperture opening of about 2.27 millimeters.
When a lens is focused on a subject, the subject is the most sharply focused component of the picture. But objects that are both closer to and farther away from the camera than the subject will also be in acceptable focus. The range of distance in front and behind the focused subject within which objects 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, respectively.
Another way of saying this is that the more distant the focused object is from the lens, and/or the smaller the lens aperture, and/or the shorter the focal length, the greater the depth-of-field. The linear size of the image that is projected by the lens onto the focal plane--the spot where the film or sensor is located--is equal to the photographed scene's angular size (the angular field of view of the lens), 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, thereby giving us the same apparent distance from the viewer and the same photographic magnification, the focal length of the lens must decrease.
Until recently, all available 24p video cameras used sensors with dimensions that are smaller than those of a 35 mm film frame. Even if a standard 35 millimeter lens is used, the smaller image dimensions cause this lens to have a shorter effective focal length than would be the case if 35 mm film were used. This in turn results in greater depth-of-field. Greater depth-of-field is desirable for some situations, such as landscape photography, but it is usually not desirable in dramatic cinematography.
USING SELECTIVE FOCUS
One of the time-honored mechanisms in cinematic storytelling is the use of selective focus. The foreground, or the area on which the director wishes our attention trained, is held in focus, while the background, or less relevant portions of the image, are intentionally defocused. The attention of the viewer is shifted from one subject to another within the frame by defocusing one subject and refocusing on another. Selective focus is much easier to achieve using 35 mm film than electronic imagers, because the focal length of the lens in this case is relatively long, giving a shallow depth-of-field even when the iris is stopped down.
Possible ways to mitigate this problem include bringing the subject closer to the lens, and opening the iris as much as possible. The most effective solution is to make the imager larger. As 24p shooting is being increasingly used, it is not surprising that 24p cameras with full 35 millimeter imagers have appeared on the market.
When this column asked, three years ago, if film was dead again, some television shooting was being done in 24p video, but little was for dramatic programming. That worm has turned, and it is not uncommon to see dramatic footage shot on video today. Film stock continues to improve, and the "film look" is still highly desirable, but the convenience of video shooting and post production is alluring, and in many respects the gap between the "film look" and the "video look" is narrowing. We now know a lot more about the MTF, gamma, and dynamic range characteristics that make film look the way it looks, and HD camera manufacturers are engineering into video cameras the parameters that make them simulate film. Some of these parameters are particular modulation transfer function and gamma characteristics. We may expect the depth-of-field problem to be overcome, too, when video cameras with 35 millimeter-frame-sized sensors become available.
We can almost glimpse the day when film shooting for television and feature films will become a boutique activity. Old habits die hard, but 24p video shooting is becoming increasingly common.
