Depth calclation in stereo cameras

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mathematics

Hi

I'm multiplying reprojection matrix Q by vector [x,y,disparity,1] to get 3d coordinates of a point [x/w, y/w, z/w]. It all looks great but distance z is growing exponentialy while in reality it grows linear.
What is wrong? I understand that I should loose accuracy of my measurment further my tracked object gets from camera but this makes no sense to me.

I did ask on opencv forum