The focal length of a lens depends upon 

Focal length depends on colour of light, because changing color changes wavelength which in turn changes the convergence point, so focus position changes and hence focal length also changes. It also depends on curvature of lens,

$\text{ i.e. }\frac{1}{\mathrm{f}}=\left(\frac{{\mathrm{M}}_2}{{\mathrm{M}}_1}-1\right)\left(\frac{1}{{\mathrm{R}}_1}-\frac{1}{{\mathrm{R}}_2}\right)$

where R₁, R₂ are radius of curvature of two sides of lens. And M₂ is refractive index of material of which lens is made of and M₁ is the refractive index of surrounding medium. And by this formula we see that focal length is also dependent on the material of lens.