In this paper we address the problem of self-calibrating a differentiable generic camera from two rotational flows defined on an open set of the image. Such a camera model can be used for any central smooth imaging system, and thus any given method for the generic model can be applied to many different vision systems. We give a theoretical closed-form solution to the problem, proving that the ambiguity in the obtained solution is metric (up to an orthogonal linear transformation). Based in the theoretical results, we contribute with an algorithm to achieve metric self-calibration of any central generic camera using two optical flows observed in (part of) the image, which correspond to two infinitesimal rotations of the camera.