Canon EF 2x II Extender : Yes or no ?

[David]

Not being in a position to buy a 100-400 L lens for longer focal length in my kit I was thinking of maybe (in the meantime) getting a Canon EF 2x II Extender.

One problem with this idea is that a Canon 70-200 L 4.0 lens [the one I am thinking of attaching this Extender to] mounted over a 2x Extender becomes a 140-400mm f/8 lens, You gain double the focal length but you effectively lose 2 stops of aperture with this thing.

F8 is not exactly fast is it and although it will ‘allow’ deeper DOF it will also slow down the shutter speed and for moving subjects (sports, birds, wildlife) that would probably be a nightmare. Also, the background better be ‘interesting’ because I would have a hard time giving it a bokeh/.blurred background effect except in Photoshop, right ?

I am thinking this Extender might be more suited to a 2.8 for both sharpness and speed IF I had a 2.8 L lens.

So, for moving long focal length images (sports especially) where you need a fast SS (and consequently wide aperture) this purchase for my current lens kit would be a waste of time ?

But, maybe for not so twitchy wildlife (kangaroos etc) and still subjects out of subject feature size in the frame (the moon etc) using the 70-200mm L 4.0 lens the Extender would be worthwhile IF the light conditions are good and/or I can up the ISO without crashing the pixels ?

I dunno, thoughts and comments welcome -I get the feeling people are going to tell me to sit on my hands and wait till I can afford the 100-400 L.