How about a 1000mm lens, that weighs less than a 50mm f/1.4?
Here it is ... because you asked ... well, kinda. I heard that: "What?"
Some say, "That lens is better left in the case."
I say, "You may be right." LOL
If you really need some reach without toting a monster lens ... well, you probably should use the better lens and hire someone to tote it for ya. But, if you are supremely cheap ... and just have to get something to take that Bull Elk, standing out there waiting to take your hunting arrow ... this may be just the thing.
This is really a 500mm f/8 Reflex lens with its 2x extension tube on the back of it. It's completely manual focus with a fixed aperture. When you add the extension tube, the aperture goes from f/8 to f/16. I know what you're thinking ... f/16? Yeah ... so dark you can almost take sun shots and still get noise.
I break it out when I'm bored and want a challenge. The A700's Super SteadyShot does help it some what, but the focus on this is hardly what one might call "crisp." My experience in going to ISO-6400 only seems to take the shot from bad to worse. Anyway, thought I'd share that.