The Better Beamer is made up of two bracket arms, a fresnel lens and strap, the Better Beamer will add approximately 2-stops of light output to your flash by concentrating the flash pattern.
The Better Beamer was designed for use with telephoto lenses of 300mm in focal length and longer. Folds flat for easy storage.
The FX-4 Flash Extender is specific for the Canon 580EX but will also fit the Metz 54-4.