A few things to keep in mind when taking a portrait of a person or pet by window light:
ONE: Natural window light usually means wide open apertures and slower shutter speeds, so consider boosting your ISO if necessary, be sure your anti-shake mode is on, or use a tripod to prevent camera shake.
TWO: The closer you are to a window, the more dramatic the shadow on the opposite side of the face will be. Positioning your subject further from a window means softer, more even light.
THREE: You can actually stand in front of the window and position your subject facing the window and it won’t cast a shadow! This results in very flat lighting on the face. Four, shoot by a window even if it is overcast or there are soft sheer curtains. The light will be beautiful.