62 The Digital Filmmaking Handbook, 4E
Figure 4.1
An Olympus CCD image sensor.
Image Quality
Film shooters have an advantage over video shooters in that image quality is heavily dependent on lenses and the film stock they stick in their camera. It’s not so simple with digital video. In addition to the usual concerns about lens quality and camera features, you also have to worry about how good a job the camera does at actually capturing, processing, and storing an image.
Two factors contribute the most to your camera’s image quality (or lack thereof ): the camera’s lens and the chips the camera uses to create an image.
Sensors
In the old days, video cameras used vacuum tubes for capturing images. Today, video cameras use special imaging sensors called CCDs (charge-coupled devices) or CMOS (complementary metal-oxide semiconductors). Just as their tube-based predecessors used either one or three tubes to capture an image, chip-based cameras use either a single sensor to capture a full-color image, or three separate sensors to capture separate red, green, and blue data, which is then assembled into a color image
An image sensor looks like a normal computer chip, but with a sort of light-sensitive “window” on the top (see Figure 4.1). The imaging window is divided into a grid, and the finer the grid, the higher the resolution of the sensor will be. The circuitry controlling the sensor can determine the amount of light striking each cell of the grid, and that data is used by the camera to build an image.
Single-chip cameras have red, green, and blue filters arranged over clusters of cells in the sensor. These filter the light coming through the lens and allow the camera to record color images. In a three-chip camera, a series of prisms split the incoming light into separate red, green, and blue components, and directs each of these components onto a separate sensor. Because the camera dedicates an entire sensor to each
color, color fidelity and image detail are much improved over single-chip cameras (see Figures 4.2 and 4.3).
The image data gathered by the sensor(s) is passed to an onboard computer that processes the data and writes it to digital storage media. How the computer processes the data can have a lot to do with how images differ from camera to camera. Some cameras tend to produce warmer images, with stronger reds and magentas, while others might produce cooler, lesssaturated images with stronger blues. One approach is not better than the other, but you may find that you have a personal preference, or that one is better suited to the tone of your project.
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Figure 4.2
In a single-chip camera, light is focused by the lens onto the sensor. Red, green, and blue filters placed over alternating cells of the sensor enable the camera to calculate color. The
resulting data is passed on to the camera’s processor. (Notice that there are far more green cells to accommodate your eyes’ high sensitivity to green.)
Figure 4.3
In a three-chip camera, light is focused by the lens onto a series of prisms that split the light into red, green, and blue. Each component is directed toward its own sensor.
Evaluating a Sensor
When evaluating a camera, first look at its color reproduction. If you’re a stickler for accuracy, you’ll want to see if the camera can create an image with colors that are true to their originals. Even if you’re not concerned with color accuracy, look for color casts or odd shifts in color.
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It’s also important to pay attention to video noise. Noise comes in two flavors: luminance noise, which appears as monochromatic speckly patterns in your image (see Color Plate 5), and chrominance noise, which shows up as colored blotches, usually red, green, or magenta (see Color Plate 6). Although you ideally want an image without noise, this isn’t always possible, especially when shooting in low light. However, when evaluating noise response, you’ll find luminance noise less annoying, since it tends to look a lot like film grain and it’s easier to remove later on in postproduction. Chrominance noise, however, is never attractive, and is extremely difficult to minimize in postproduction.
You’ll also want to check the camera’s response to different lighting situations. Unfortunately, your average electronics store is not the best place for testing a camera. However, if you can manage to point the camera out a window or into the dark recesses of a shelf or cabinet, you should be able to get an idea of the sensor’s dynamic range, or latitude, which is the range from dark to bright that the camera can record in the same shot. In addition to the dynamic range, look for color consistency, casts or shifts in color, and keep an eye out for noise.
CCD-based cameras can have a tendency to create vertical white bands when exposed to bright elements in a scene. Different cameras employ different techniques to deal with this problem, and some are better than others. When evaluating a camera, point it at a bright light (but never at the sun!) and then quickly tilt the camera down. Look for vertical banding and smearing during the camera move. Vertical banding is not a reason to reject a camera, since you can always work around it, but it is important to know if your camera has this tendency.
CMOS-based cameras are immune to the above, but can be prone to visual distortion, known as the rolling shutter effect, which happens when the shutter is slower than the physical movement of the camera. The result is a “Jell-o” effect during pans. Tops of objects might move across the screen more slowly than their bottoms.
Sensor Size
Thanks to DSLRs that shoot HD, there’s a lot of talk about image sensor size floating around these days. That’s because DSLRs are small, relatively inexpensive cameras, but they boast large image sensors that help them record a very high-quality image.
The current gold standard for image sensor size is based on 35mm still photography. With 35mm still film, the image is exposed directly onto the negative, which is approximately 35mm wide and 24mm tall (see Figure 4.4). Higher-end DSLR still cameras try to replicate the image quality of 35mm film by using similarly-sized image sensors, also known as fullframe sensors.
Micro four-thirds sensors are smaller than full-frame sensors, but they use special technology to make up for the size difference. The Panasonic AG-AF100 and the Sony NEX-VG10 are recently developed camcorders that have micro four-thirds sensors. By contrast, most HD video cameras have smaller sensors, ranging from 1/8 to 2/3 of a full-frame.
Bigger image sensors make better-looking images. They have a greater dynamic range, a lower tendency toward noise, and they are more capable of capturing a shallow depth of field.
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Figure 4.4
A full 35mm still film frame compared to 4/3-, 2/3-, and 1/2-image sensors.
Why Is Shallow Depth of Field So Important?
One of the defining trends in modern still and motion photography is the prevalence of shallow depth of field. When shooting with shallow depth of field, things in the background will be out of focus, which helps bring more attention to the subject in the foreground. To achieve shallow depth of field, you need a camera with a larger image sensor and the right lens with the right lighting conditions. We’ll talk more about how to achieve this look in Chapters 7 and 10.
Compression
The first place that your digital video gets compressed is in the camera. Every camera uses a compression algorithm, or codec, to turn the analog subject into a digital signal, and that process affects image quality greatly. In Chapter 3, “Digital Video Primer,” we discussed compression in great detail and provided a list of acquisition formats.
Most lower-budget indie filmmakers will find that 8-bit, 4:2:2 compression is acceptable for their needs. Less than that (such as HDV’s 4:2:0 color), and the image quality will be too low; more than that, and it will look fantastic but be very expensive. Be aware that some high-end cameras can record in more than one codec (see Figure 4.5).