= Einstein radius
{c}
{title2=$R_E=D_l\theta_E$}
For a point lens of <mass> $m$ at observer distance $D_l$ and source distance $D_s$ in a nearby Euclidean geometry, the <Einstein radius> in the lens plane is $R_E=\sqrt{4GmD_l(D_s-D_l)/(c^2D_s)}$. Its angular version is $\theta_E=R_E/D_l$. This is the ring radius for perfect alignment and the characteristic transverse scale of a <gravitational microlensing> event. A crossing timescale is $t_E=R_E/v_\perp$, proportional to the square root of lens mass at fixed geometry and speed.
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