Joint posterior distributions of the mean matter density Ω_m and the matter power spectrum normalization σ₈ of a flat ΛCDM universe inferred from a cosmic shear survey with n_g = 25   arcmin^-2 (assuming flat priors for Ω_m and σ₈, and all other cosmological parameters known). Shown are the 68% (inner) and 95% (outer) confidence contours for a small survey in 5 fields of 2  ×  2   deg² (left panels), a survey in 6 fields of 4  ×  4   deg² (middle panels), and a large survey in a 120  ×  120   deg² field (right panels) obtained when using the covariance matrix measured from our lensing simulations (solid lines), the normal approximation (top, dashed lines), the log-normal approximation (top, dotted lines), the empirical corrections to the normal approximation by Semboloni et al. (2007, bottom, and the corrections by Sato et al. (2011b, bottom.