How to find depth of gray image?

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Hello , I am using calibrated raspberry pi camera. I want to find scaling factor from two image(from monocular camera) taking at t and t+1 time. I have 2 sets of image points(x,y), from this i am calculating rotation and translation using essential matrix(it's 2D-2D transformation).

But now i want to find 3D points(or z- depth of image) from image point, how can i find depth of image? (For 3D point i have camera matrix, distortion coefficient, transformation between /base_footprint to /base_link and /base_link to /camera is there any requirment for this?) or how can i find scaling factor from 2D points ? Thank you in advance!