Gravitational Lensing by Galaxy Groups in the Hubble Deep Field
  We investigate strong lensing of galaxies in the Hubble Deep Field by foreground groups and clusters of galaxies with masses from $10^{13}$ to $10^{15} \MSun$. Over this mass range, groups with the profile of Navarro, Frenk, & White (1995) are less efficient than singular isothermal spheres at producing multiple images of galaxies, by factors of $5 \times 10^{- 2}$ to $10^{- 3}$. This difference highlights the sensitivity of the lensing cross section to the central density profile. Nonetheless, with either profile we find that the expected number of galaxies lensed by groups in the Hubble Deep Field is at most $\lesssim 1$, consistent with the lack of clearly identified group lens systems. 
David C. Fox and Ue-Li Pen