The resonant frequency is a function of material and geometric properties of the beam and is independent of the deflection.
For a cantilever beam of uniform cross-section,
[tex]f=\frac{1}{2\pi} \sqrt {\frac{8EI}{\rho Al^4}}[/tex]
I: moment of inertia (depends on cross section)
E: Young's modulus
[itex]\rho[/itex]: density
A: cross-section area
[itex]l[/itex]: length