# Camera sensor size vs pixel size and focal length [closed]

I'm writing a small software for motion tracking (from two cameras, videos) in 3D space and research. To do that, I need some information about cameras. Sensor size and pixel size relationship is one of them.

For some things, I need the sensor size, for other, pixel size, elsewhere, focal length. I need to understand if and what is the relationship between the two, and is it possible to calculate one or another.

At the moment I have a plain Canon A720 IS. The listed focal length is 5800, sensor size is 1/2,5'' (5.744 x 4.308 mm) and the pixel size i 3,1µm² (~1,76µm). How are these numbers related (mathematically)? Is there a way to calculate pixel size from the sensor/resolution/focal length/whathaveyou?

PS: After calibrating my camera with OpenCV (inaccurately, but close enough for debugging), I got a focal value of 633. This has to be multiplied by sensor size, so I end up with a value of 253,2. How is this related? This is nowhere near 5800µm, Canon-listed focal length, and 4620µm, for which I am getting the most accurate 3D positions.

• See What is “angle of view” in photography? for a visual explanation of how focal length relates to sensor size. – mattdm Jul 14 '15 at 14:10
• – mattdm Jul 14 '15 at 20:23
• @mattdm how is that related to the relationship between pixel size and sensor size? Also, I have focal length, I even have three, the problem is I don't know which one is true. – Petersaber Jul 14 '15 at 20:49
• Let me step back a bit. How do you think these values might be related? – mattdm Jul 14 '15 at 21:00
• @mattdm I don't know. That's why I asked in the first place. Pixel size... I don't know. Sensor width/height divided by the amount of pixels on the matrix. However, the matrix is bigger than the picture it takes... isn't it? calibCamera value * sensor size should give me the focal length, but I get different result from that... so that's something I also asked about. – Petersaber Jul 14 '15 at 21:09