Shadowing in the Recording

The quality of the tube is excellent. The threads are smooth and fit very well. But one major disadvantage of tube P7700 58 mm (for the Nikon Coolpix P 7700), he shadowed at zoom levels 25 and 38 mm, the image in the corners off very strong. With 25 mm, it is not at all to use because only a clear round surface remains visible in the center of the picture, everything else is black.
Only from - preset zoom level of 50 mm (in the camera can be preset) - the tube even in the corners is clear and pure.