Gravitational Lensing Deflection angle

Does anyone know where I could find (online) a derivation of the deflection angle in gravitational lensing:

alpha = 4GM/Rc^2

Thank you
Hi Jrod
Deflection angle by E=hf (Planck) and so: m=hf/c^2? Restmass of photons=0 but dynamical could be different? What could we ever learn more from Einstein than this? In my lecture about the nonexistance of stellar black holes I come to half of your value, but about factor two-faillures in astrodynamics a whole book could be written. (I myself have two recommended books about astrodynamics witch differ factor two in central pressure in the Sun!). Cosmical lensing is what you want to discuss about isn't it.
greatings Jm

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