Lens hoods are primarily designed to avoid
unwanted stray light from entering the lens by extending and shading the end of
the lens. In addition, since the end of the lens is extended, you also get the
added benefit of some extra protection from accidental impact.
Dedicated lens hoods are designed to match
the specific focal length of the lens it was designed for.
JJC LH-65B replaces Canon Lens Hood ET-65B
CANON EF 70-300mm f/4-5.6 IS USM
CANON EF 70-300mm f/4.5-5.6 DO IS USM