The primary goal of a lens hood is to stop light coming into the lens at extreme angles and being reflected inside the lens. It is most necessary when you want to take pictures with the camera pointing close to a strong light source, particularly the sun.
Lens hood should match the angle of view of the lens. It should exclude as much light as possible without being visible in the picture.
Small lens hoods for a consumer grade camcorders can also be regarded as fashion statement, especially rectangular lens. A camcorder with attached rectangular lens looks more "professional" and cool, though it is unlikely that a small piece of plastic or rubber will turn a father videotaping his son's first steps into a movie director.
Round And Rectangular Hoods
Since pictures are rectangular, you can exclude more excessive light using a rectangular hood, but in practical terms the difference between this and a circular hood appears to be minimal. One disadvantage of the rectangular design is that it needs to be precisely aligned.
A round hood can be simply screwed in using standard filter thread. A generic rectangular hood usually has a mounting ring and some sort of locking mechanizm, like a screw that locks the hood on the ring in selected position.
Lens hoods also have two other very practical uses. They help to protect the lens from accidental impacts and also help to keep your fingers away from the front of the lens. It is very easy to put your fingers on the front element, leaving dirty finger marks on the surface that will seriously degrade the picture.
The hood shown on the pictures came with a 37mm aluminum mounting ring. After some tinkering I decided to take this ring off and to mount the hood directly onto a UV filter. I decided to keep a UV filter permanently attached to the camcorder to protect the lens. True, the Elura 100 has lens cover, but the cover is not automatic. Therefore, the filter protects the lens, and the hood protects the filter. Even with the hood attached the Elura 100 fits perfectly into a small Lowepro bag.