It's good that it worked for you but I doubt it's just a filter though. Sharp image is a combination of several variables: correct focusing point, steady hands (if not using camera support), as well as the right exposure/aperture setting for the desired DOF. Even a minor deviation from a correct focusing point would momentarily re-focus the lens to where you're aiming it instead of where you want it. When shooting a moving target from afar, long tele-lenses are an obvious choice. As more light is needed, we tend to open the aperture as much as possible; thus, reducing the DOF. When focusing on bird's eye with a long tele, the feathers often get out of focus, especially if a bird is big and/or facing the lens.
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