how to get depth from disparity map?

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hello

I'm beginer. Now, I am using stereo camera.(python) Using stereoBM_create(), I made a disparity map. Disparity boundary is (-16 ~ 496). As I known, disparity means that it is gaps between two pair of image. Therefore, it is not depth right? I don't want actual depth. But, that disparities means not the same values.

(ex) image 1 : real max depth 80m / of disparity 10

image 2 : real max depth 40m / of disparity 10

these are not same, right??)

As a result, how to get depth from disparity map??

+) Z = B*f / disparity . In this formula, f (focal length) is static value?? I am using MS lifecam-hd3000. But I don't know focal legnth..