This image shows the full frame view with a 400mm lens. Because the birds are small in the field of view the auto focus system is being challenged to locate and focus on the subject. Even with this “large” field of view the photographer had a hard time tracking these fast flying birds.
This appears to be a "Banded Pennant" Dragonfly flying about 15 feet away.
The full frame is shown in this image with the insert showing the detail in an enlargement. Notice the camera didn't focus on the background.
