Tamron Low Dispersion Glass

Tamron lenses use low dispersion glass for greater sharpness. 

Reduced chromatic aberration is achieved when low dispersion glass elements are used in a lens. Chromatic aberration is the tendency of light of different colours to come to different points of focus at the image plane. It can reduce the sharpness of an image if bad, and cause fringes of colour along boundaries that separate dark and light parts of an image.

Glass with a low dispersion index has less of a tendency to defract a ray of light into the rainbow of colours. This allows the lens to, in effect, compensate for chromatic aberration at the center of the field.

This can be a particular problem at long focal lengths, and towards the edges of the field of view at short focal lengths.