68% (inner) and 95% (outer) confidence contours of the 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 = 70   arcmin^-2 in 5 fields of 2  ×  2   deg², obtained using the covariance 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. For the left panels, the cosmic covariance parts of the data covariance matrices have be used as given by the approximations. For the right panels, the cosmic variance parts have been modified where needed to ensure positive-semidefiniteness (see text for details).