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Depending on how accurate you need the focal you can get a good quick estimate if you know either of the following:

• CCD/CMOS sensor physical size
• Horizontal field of view (FOV)

If you know the sensor's physical width in mm, then the focal in pixels is:

focal_pixel = (focal_mm / sensor_width_mm) * image_width_in_pixels

And if you know the horizontal field of view, say in degrees,

focal_pixel = (image_width_in_pixels * 0.5) / tan(FOV * 0.5 * PI/180)