  The first penetration field H_{J}(T) of Josephson vortices is measured
through the onset of microwave absorption in the locked state, in slightly
overdoped $\rm{Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta}}$ single crystals (T_{c} ~ 84
K). The magnitude of H_{J}(T) is too large to be accounted for by the first
thermodynamic critical field H_{c1}(T). We discuss the possibility of a
Bean-Livingston barrier, also supported by irreversible behavior upon flux
exit, and the role of defects, which relates H_{J}(T) to the c-axis penetration
depth $\lambda_{c}(T)$. The temperature dependence of the latter, determined by
a cavity perturbation technique and a theoretical estimate of the
defect-limited penetration field are used to deduce from H_{J}(T) the absolute
value of $\lambda_{c}(0)=(35 \pm 15) \mu m$.
