Abbreviation: IF = "Internal Focus System" (Tokina)

The abbreviation IF means "Internal Focus System" and is seen on Tokina products.

Explanation from Tokina

The two most used methods of focusing a lens are either the complete straight forward movement of lens elements (used mainly with single focal length lenses) or the rotation of the entire lens barrel group (used mainly with zoom lenses). The internal focusing system used by Tokina move each element group within the lens, but does not change the overall length of the lens. This is especially useful with telephoto designs.

The internal focusing system has a number of advantages including;

1. Faster focusing
2. Improved handling due to fewer movements near the center of gravity
3. More compact lens designs
4. Superior use of filters because the barrel with the filter thread does not rotate.

IFS (Internal Focus System)
By movement of the forward elements within the stationary main barrel: AT-X 235 AF PRO, AT-X 287 AF PRO SV, AT-X 280 AF PRO, AT-X 828 AF PRO

See examples of effected products

Tokina AT-X Pro 16-50mm f/2,8 DX

See links