I am setting up cameras for an experiment filming animals. In a experimental protocol paper that I'm following, it states that:
Based on a pinhole camera model, the bound x_min on the pixel span of the animal in the image can only be guaranteed if the observation distance between animals and each camera is at most:
where X is the length of the animal, f is the focal length of the camera and p is the physical width of a pixel. For our studies, in the interest of observing the flight paths of the animals over a large distance, we chose to allow a small image size.
My question is what is the physical width of a pixel, and how do I obtain that number? In the paper, it was stated that their equipment was:
three thermal infrared cameras (FLIR SC8000, FLIR Systems, Inc., Wilsonville, OR, USA) with variable-focus 25 mm lenses and a pixel width of 18 μm, providing a 40.5 deg field of view. The 14 bit grayscale−1 video has a frame size of 1024×1024 pixels and frame rate of 131.5 Hz.
It wasn't explained how the pixel width was obtained. The number seemed strange to me as well. If the physical pixel width is 18 μm, then the physical length captured by the whole image would be only 1024 * 18μm = 18mm, while their subject are birds flying in the sky in the scale of several tens of meters across. I'm probably understanding the concept incorrectly.