Once the lighting and cameras are properly set up, it is recommended to calibrate the balance of their optical parameters. This is especially important in a telepresence context since there is no post-production for live video. Depending on the lighting level of the stage, the ideal exposure depends on the following parameters:
¶ Aperture
This is the setting that adjusts the diameter of the diaphragm opening. It reduces the illumination of the camera sensor, increases depth of field, reduces geometric and chromatic distortions, and increases the influence of light diffraction.
A large depth of field is achieved with a small aperture diameter, while a smaller depth of field is achieved with a larger aperture diameter. A reduced depth of field is ideal for isolating a close subject from a more distant background, or for chroma keying, using a green screen for example. An extended depth of field allows you to keep focus on the entire capture area, both background and foreground. This is ideal for live broadcasts or recordings covering a wide area.
¶ Sensitivity (ISO)
This is the measure of the sensor’s sensitivity to light. Good exposure depends on the amount of light received, to obtain an image that is neither too bright nor too dark, presenting midtones. The sensitivity of digital capture devices can be controlled by adjusting the gain of analog or even digital amplification. High sensitivity makes it easier to capture in low light. However, too much amplification can also introduce noise into the image, making it appear more grainy. Most cameras offer the possibility to add a noise reduction filter to reduce this grainy effect. But, this filtering sometimes comes at the cost of losing details, contrast, and fine textures.
¶ Exposure time
This is the interval during which the camera shutter lets light pass during a shot, i.e., the duration of sensor exposure. A longer exposure time allows more light to be captured over time. Conversely, any movement captured this way will appear blurry; a very short exposure time allows moving objects to appear more clearly, but lets in less light, which can be problematic in some low-light situations. Usually, a shutter speed of 1/50s combined with a frame rate of 24 frames per second (fps) is perceived as the configuration offering the most natural result.
These parameters affect each other reciprocally: increasing the exposure time requires closing the diaphragm so that the amount of light reaching the sensitive surface remains the same. Generally, the less illuminated the scene, the more complex it will be to achieve quality exposure.
Other important parameters, color balance, contrast adjustment, and noise reduction can also positively affect the quality of the visual rendering, upstream of its local or remote transmission:
¶ Color balance
Modifies the rendering of colors, including black and white, in the image. It is recommended to adjust this balance from the camera, especially in a telepresence context where there is no post-production. In a low-light situation, it may be useful to use a color checker, a gray card, and a white balance filter to make these adjustments:
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Automatic white balance can be performed directly from the camera, by zooming in on a white image, which will allow it to adjust automatically;
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Fine adjustment of the amplification of Red and Blue tones eliminates any undesirable coloration;
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Adjustment of Hue and Saturation, as needed.
¶ Contrast
This is what adds a sense of dimension to images. Good contrast means the image includes the full spectrum of color tones.
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By setting the camera to display the image in black and white, it is easier to adjust the difference between light and dark tones.
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The adjustment depends on the atmosphere you want to create, whether brighter or darker;
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In the case of chroma keying involving a green screen, contrast and hue must be adjusted for optimal results.
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Ideally, use ambient lighting—with bright projectors—rather than relying only on the luminance parameter, to optimize image clarity;
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A more defined (sharp) image can be useful in some cases, but a softer image can also appear more natural.
¶ Noise reduction
Image noise is generally defined as pixel aberrations that do not faithfully represent the colors or exposure of the image. Noise is usually due to too low a brightness level, causing exaggerated amplification of luminance. Noise can be very distracting for the viewer and should therefore be minimized to obtain image quality suitable for broadcasting.
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2D noise reduction technology is ideal for capturing moving objects or subjects;
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3D noise reduction technology is ideal for capturing static and stable visual fields;
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By combining 2D and 3D noise reduction technologies, it is possible to efficiently optimize both static and moving imagery.
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Hot (or stuck) pixels appear in the image as bright colored dots. They occur more often with slow shutter speeds and high exposure (ISO). Many cameras have an option to eliminate them.