ASL, or Aspherical, refers to the hybrid aspherical elements used in Tamron lens construction.
The hybrid aspherical elements provide excellent image quality along with compactness, allowing Tamron to create lenses that incorporate impressive zoom ranges in really compact designs.
Tamron has advanced optical design with hybrid aspherical lenses, almost eliminating spherical aberration and image distortion from the high-power zoom series of lenses.
Using hybrid aspherical technology, multiple lens elements can be replaced with just one, without compromising the performance in any way. It is this technology which allows Tamron to create lenses delivering a high level of image quality no matter what the focal length or aperture.