Raw data must undergo processing to neutralize instrumental artifacts introduced in the observing process such as an offset voltage for each pixel introduced during readout and nonuniformity due to pixel to pixel sensitivity differences across a charge-coupled device (CCD). Additionally, instrumental and atmospheric effects contribute to the point spread function (PSF), which describes how an observed intrinsically point-like object is smeared. The PSF has anisotropic and isotropic components, both of which are problematic for the shape measurements unless corrected. Typically, ground-based and space-based data undergo distinct reduction procedures due to the differences in instruments and observing conditions. Accurate shape measurements are paramount for weak lensing because of the subtle nature of the effect. Angular diameter distances to the lenses and background galaxies are also important to convert lensing observables to physically meaningful quantities; these distances are often estimated using photometric redshifts when spectroscopic redshifts are unavailable.