Leica's New Lens Holder Doubles as a Stabilizer
From the standpoint of someone that always wants to have an extra lens on hand, it seems like a solid idea. The lens holder attaches to the tripod mounting so it's going everywhere your camera goes. Of course, that means that if you drop your camera, both lenses go with it.
The lens holder does have a solid base, which means you can use it as a sort of mini-monopod without having to worry about scratching up your lens. Still, something feels weird about using an expensive lens to hold up an expensive camera.
Leica
