Some question about this Sigma lens

[stl]

Saw this lens and it's specs...
http://www.sigmaphoto.com/html/pages/15_30_ex.htm

The filter size is stated as rear (gelatin filter insert type), what that?

It has a non-removable petal shaped lens hood, wonder how to protect the lens leh?

 

[Shadus]

Filter is inserted into the lens itself rather than in-front as most lens are.

Hmm....wht do you mean for your 2nd question? The petal hood itself is the protection for the lens as well as reducing the chance of flaring.

 

[stl]

how is the filter like? a piece of plastic insert into the lens? which brand is it? not those from Hoya rite...