Every angle, proportion, and facet in a cut diamond contributes to its optical efficiency, so variations in any or all of them have a definite effect on the beauty of the diamond.
Pavilion too Shallow
If the pavilion is too shallow, light will strike the first pavilion interface within the critical angle, and most light will escape through the pavilion (bottom) where it will not be seen.
This will result in a diamond with considerably less brilliance. It may also show a gray ring and appear dark and lifeless. (referred to in the trade as a fish-eye)
If the pavilion is too deep, light will strike the first interface outside the critical angle and be totally reflected to the other side of the pavilion where it will strike within the critical angle and escape through the side of the pavilion (bottom).
This will result in a diamond with considerably less brilliance and an obvious dark center.
Antwerp Ideal Proportions
In an Antwerp Ideal diamond most of the light entering through the crown, will be totally reflected within the pavilion and then will exit through the crown at an effective angle, resulting in a maximum combination of brilliance and fire.
Most experts agree that a pavilion angle close to 40.5° creates the best optical efficiency and beauty.