You mentioned it was taken in low light which might mean a slow shutter speed was used with a wide aperture. Test again with an aperture in the lenses mid-range like f8. Mid-range values are usually sharper than wide open or completely stopped down. It might also be possible the image is not sharp due to slight camera movement during the exposure. To rule out camera movement use a tripod or place the camera on a stable surface then make the exposure with the self timer. Using the self timer will eliminate the possibility of jiggling the camera a bit when the shutter button is pressed. Its also possible wind might have caused some movement in the branches. Lots of reasons an image might not be sharp with no fault to the equipment. As mentioned knowing the exif data would help.
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