[jQuery] I have to fire "each" and "$(this)" when doing something?

[aw]

$('somediv.someclass').css("opacity",".5");
is not working and I have to use :
$('somediv.someclass').each(function(){$(this).css("opacity",".5")});
But sometimes I can just go ahead without having to use "each".
What's the matter, could anyone give me a hand?