Hi .I am reall y confused about the focal length of lens.i understand that it is the length from the lens to the point where parallel beams coming from infinity after passing through the lens cross one each other.My question ,knowing thew focal length wher exactly does it help?does it have any relation with the point where the lens focuses?a lens with a focal length of , lets say 30mm in comparaison to one with 100mm ,what diffrenece they have especially as far as the point of focus (the distance where the image of an object is focused).thank you very much.look forward