You are right that a normal optical instrument has only one focal length. But here, you look at a combination of a lense and(!) a sensor that consist of a 2d-array of detectors (formerly known as pixels). Your focal length therefore has the unit meters/px. If your camera's pixel pattern is a perfect square, fx equals fy. If the pixels are a bit rectangular and not quadratic, fx differs from fy. This effect can be captured by using two different focal lengths.
There is even a version of the intrinsic calibration where there is a skew value that incorporates an angle between the rows and columns of your pixel array that is not 90deg.
If you need more explanation about the theory, have a look at the first section of the calibration tutorial here. It will explain why fx and fy are needed.
But basically, as @Ziyou said, it's because image are not square (I should say not spherical) but a rectangle. Therefore, you need an estimation of the focal on Height and the focal Width.
An image has two dimensions, thus have two focal length values.
Asked: 2014-08-11 13:34:54 -0600
Seen: 14,060 times
Last updated: Mar 29 '16
A question, can i compare thees values with the original focal lenght of my camera? My camera has a focal lenght =60mm , and after the calibration in open cv i have fx=38246.8 fy 38074.0 the pixel size is 0,00571
Hi,
how can I get focal length in pixels from fx and fy? I need to get disparity using z = b*f / d,
where z is the depth (in meters), b is the horizontal baseline between the cameras (in meters), f is the (common) focal length of the cameras (in pixels), and d is the disparity (in pixels). At zero disparity, the rays from each camera are parallel, and the depth is infinite. Larger values for the disparity mean shorter distances.
I am using kinect with the focal length: fx_rgb = 5.1885790117450188e+02 * ratio; fy_rgb = 5.1946961112127485e+02 * ratio;
Kinect is not a stereo sensor, therefore I dont have a "baseline".
Please help Ayesha
^^ @ayeshas , please do not make answers, if you have a question or a comment.
^^ @ayeshas , please do not make answers, if you have a question or a comment.