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Focal Length vs Field Of View
Since cameras are involved, often customers are constantly concerned with focal length. They write them down, they try to decide if a value from SynthEyes is correct or not.
Important : A focal length value is useless 99% of the time — unless you also know the sensor(plate) width of only those pixels used to form the image (typically in millimeters, to the hundredth of a millimeter). Unfortunately, this value is rarely available at all, let alone at a sufficient degree of accuracy. Even when a manufacturer provides a size, what does it represent? Sensor chips typically have dummy cells, and the camera mode (HD, “4K”, “QHD”) often means that a different subset of sensors is used to make the picture you use. It takes a careful calibration of the camera and lens to get an accurate value. Sometimes an estimate can be better than nothing; read on.
SynthEyes uses the field of view value (FOV) internally, which does not depend on sensor (plate) size. More precisely, it uses the horizontal angle of view. It provides a focal length only for illustrative purposes. Set the sensor (back plate) width using the Shot Settings dialog. Do not obsess over the exact values for focal length, because finding the exact sensor (back plate) width is like trying to find the 25” on an old 25” television set. It’s not going to happen. Lens manufacturers will tell you that the rated value is only intended to be a guideline, something like +/-5%, which is a long way from what is required for pixel accuracy with thousands of pixels (0.025% at 4K).
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