Post Magazine

August 2013

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Ensuring image quality in production A chieving perfect image quality is equal parts equipment and user. To help ensure high-quality image production, steps in camera set up, alignment and matching measurements, such as white shading, colorimetry, exposure and color adjustments are great but frequently overlooked techniques. Here we take a look at the various adjustments necessary to achieve high-image quality using a suitable camera chart and waveform monitor.The aforementioned techniques are critical to ensure a high-quality image and prevent mistakes from occurring — mistakes that are costly to fix in post production. WHITE SHADING During camera set up, white shading is an important function that is often overlooked or performed using inadequate equipment. The basic tools required to properly adjust white shading are a high-quality HD waveform monitor and a very even light source, which will provide a precise illuminated field with evenness that can be controlled to close tolerances. Every camera operator should be familiar with white shading techniques, where overall brightness must be even across the whole image. In the case of studio cameras with fixed lenses and shooting in the same controlled environment, white shading will need less checking than that of a digital filmmaker on location who is often changing lenses. These measurements should be done in a controlled environment, where a qualified operator can ensure each channel is properly calibrated. The resulting adjustments can then be saved as scene files so they can be quickly recalled on-location when changing lenses or filters. A common problem is when the center of the image appears brighter than the edges.This type of aberration is caused by differences within the camera's optical system components, such as the prism or optical block, and is most usually found within older lenses. Differences may also occur when changing filters. There is a difference between shifts in color and neutral shifts in evenness of an image. For instance, adding a Wratten neutral density filter will invariably lower the brightness and give the image a warmer look. Changing lenses can also change coloration, but this effect is typically minimal. Larger differences in edge-to-edge brightness are often more common, with a hot spot in the center of the image. CORRECT COLORIMETRY and EXPOSURE While white shading primarily deals with the luma adjustments of the signal, it is also important to ensure the color fidelity of the image. The goal is to reproduce the image on a monitor or television set as closely as possible to the original scene. To achieve this, accurate test charts that produce a reference pattern are required. The camera converts this light image via the charged coupled device (CCD) to digital voltage levels. The camera then converts the signal to the HDSDI output, which can then be measured using a waveform monitor to quantify the characteristics of the signal. Accurate evaluation requires a precision test chart that complements an electronic test pattern generator, providing test signals that facilitate camera alignment and image control. Optimizing the image largely depends on the adjustment capabilities of the individual camera. It is important to understand the differ- of the vector boxes. By increasing the gain of the vectorscope to 2.0X this will place all primary colors signals within their boxes when a camera is reproducing color accurately. Today's cameras have advanced multimatrix settings, and it is possible to line up each primary color signal to be in the vectorscope boxes, but this inadvertently reduces the overall color gamut. When aligning a camera's matrix, increasing the sensitivity of a particular color can affect the positioning of many other colors. The Combi DX-1 or ChromaDuMonde simplifies the time-intensive process of color alignment because every DSC primary has the same RGB level combination as every primary color. If patterns were made with different luminance relationships, color matrix adjustments would displace the gamut showing a false primary as a true primary further distorting the working color space. In many instances a particular color cannot be selected on its own. Instead alterations can be made to the B-Y and R-B, etc. ences in colorimetry between HD and SD when viewing the waveform monitor display. For example, the difference of the Y-channel green-magenta transitions will vary between the HD and SD signals. Users familiar with SD signals may initially consider the HD signal to be incorrectly adjusted. This is not the case. This difference in levels of each color component is normal and is due to the different colorimetry equations used between HD and SD formats. (L-R) Figure 1 shows test patterns, while Figure 2 illustrates chroma vector alignment. COLOR ADJUSTMENT The DSC ChromaDuMonde test patterns in Figure 1 (above) are designed to represent saturation levels found in real life. Consequently, when using a vectorscope with the scale set to 75 percent, the CamAlign and ChromaDuMonde color signals will fall short By Mike Waidson Principal Engineer Tektronix Beaverton, OR When these settings are increased or decreased, all of the other colors on the vectorscope will shift. Adjusting one color incorrectly will affect the entire color space; this should always be taken into consideration. Once all the adjustments have been completed, the configuration of the camera can be saved as a preset for later use. Leveraging proper techniques will help achieve high-image quality whether multiple cameras or various lenses are used. Determining the proper white space shading, correct colorimetry and exposure, and getting the color adjustments just right, will be invaluable techniques in perfecting image quality (see Figure 2, above.). Post • August 2013 47

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