Can someone explain

[aladyforty]

Looking at some tamron lenses I noticed the word "Aspherical" used. Is this the equivalent of ultrasonic on a canon lens?

[granthagen]

"Aspherical" is a lens design that helps to maintain a more uniform sharpness and color rendition over the entire coverage area. Aspherical elements are harder to produce than non-aspherical elements, so their incorporation jacks up the price of the lens.

Grant

[aladyforty]

Thanks:-)

[Bob Nichol]

http://en.wikipedia.org/wiki/Aspherical_lens

An aspheric lens or asphere is a lens whose surfaces have a profile that is neither a portion of a sphere nor of a circular cylinder. In photography, a lens assembly that includes an aspheric element is often called an aspherical lens.