1. A method for estimating a PSF kernel for at least a portion of an image, the method comprising the step of: estimating a PSF kernel for at a least a portion of the image using a PSF cost function having (i) a first fidelity term that includes a derivative in a first direction, (ii) a first weight for the first fidelity term, (iii) a second fidelity term that includes a derivative in a second direction that is different from the first direction, and (iv) a second weight for the second fidelity term, the second weight being different from the first weight.