  The lower critical magnetic field, $H_{c1}$, of superconductors is measured
by using ensembles of NV-centers-in-diamond optical magnetometry. The technique
is minimally invasive, and has sub-gauss field sensitivity and sub-$\mu$m
spatial resolution, which allow for accurate detection of the vector field at
which the vortices start penetrating the sample from the corners. Aided by the
revised calculations of the effective demagnetization factors of actual cuboid
- shaped samples, $H_{c1}$ and the London penetration depth, $\lambda$, derived
from $H_{c1}$ can be obtained. We apply this method to three well-studied
superconductors: optimally doped Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$,
stoichiometric CaKFe$_4$As$_4$, and high-$T_c$ cuprate
YBa$_2$Cu$_3$O$_{7-\delta}$. Our results are well compared with the values of
$\lambda$ obtained using other techniques, thus adding another non-destructive
and sensitive method to measure these important parameters of superconductors.
