Image deblurring is essential to high resolution imaging and is therefore widely used in astronomy, microscopy or computational photography. While shift-invariant blur is modeled by convolution and leads to fast FFT-based algorithms, shiftvariant blurring requires models both accurate and fast. When the point spread function (PSF) varies smoothly across the field, these two opposite objectives can be reached by interpolating from a grid of PSF samples.Several models for smoothly varying PSF co-exist in the literature. We advocate that one of them is both physicallygrounded and fast. Moreover, we show that the approximation can be largely improved by tuning the PSF samples and interpolation weights with respect to a given continuous model. This improvement comes without increasing the computational cost of the blurring operator.We illustrate the developed blurring model on a deconvolution application in astronomy. Regularized reconstruction with our model leads to large improvements over existing results.