Compound Lens Focal Length Formula

Equation for calculate compound lens focal length is,

$$\frac{1}{f}=(\frac{1}{f_{\mathrm{<mn>1</mn>}}}+\frac{1}{f_{\mathrm{<mn>2</mn>}}})-\frac{d}{f_{\mathrm{<mn>1</mn>}}·f_{\mathrm{<mn>2</mn>}}}$$

f₁ = Focal length of one of the lenses (m)

f₂ = Focal length of the other one of the lenses (m)

d = Distance between the lenses (in air) (m)

f = Compound lens' focal length (m)