Yes, it is clear enough for bird watching. 

Step 1: Being careful not to touch the lens, cover the right objective lens (or close your right eye) and turn the central focusing dial until the image seen by the left eye is clear. Step 2: Cover the left lens (or close the left eye) and this time instead of turning the central focusing wheel, turn the diopter adjustment ring (located near the right eyepiece and displayed on the small image in the upper right) until the image is clear. Step 3: Finally look through the binoculars with both eyes until the image is clear and sharp. 

You can focus clear about 6 ft as far as I know 

I don't see why not, remember that the filter goes on the lens, not on the camera. 

Hi, thank you for your appreciation ,this filter just for camera lens , so it couldn't be used on Sandmarc lens, thanks