2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/1322Most new sub-mm/mm surveys, both deep and shallow, are being targeted at rich cluster fields. I explain why, comparing surveys that have exploited weak lensing by massive foreground clusters with those done in blank fields.6 pages, 2 colour figures. Proceedings of the XXth Moriond Astrophysics Meeting, `Cosmological Physics with Gravitational Lensing', eds J.-P. Kneib, Y. Mellier, M. Moniez, J. Tran Thanh VanAstrophysicstext