Optical Considerations: Chromatic Aberration 1
In terms of image quality, values like sharpness you want a higher number. With chromatic aberration (CA) you want a lower value. The lower the chromatic aberration the better the image result.
The comparison image here shows there is clear colour bleeding and chromatic aberration in the image taken with a flat port on the left, in comparison to the water-corrected lens on the right.
The image on the left is of a flat port and the image on the right is of a water-corrected optic. As you move from center to edge on a flat port, the issue become progressively worse. In the water-corrected lens you can see that the issue is reduced and evened out across the image.
Flat port lenses have a uniform increase in CA from center of lens to the edges.
The edges have a very high CA > 10
Water corrected optics dramatically decrease CA. The edges are approximately CA = 2