I have two Canon 1.4X extenders and while you can't connect them directly to each other because of the way they are designed, adding a short 12mm extension tube between the extenders makes it possible. What I didn't know was how it would work.
The goal was to make a lens combination that would be even better for shooting insects and small birds without having to be right on top of them. The 300mm lens and one extender creates a lens with a focal length of 420mm. Adding another 1.4X extender would create a lens with a focal length of 588mm. And, if I attach it to my crop sensor EOS 50D, I've got an apparent focal length of 940mm.
Here's how the set up looks on the 50D. The lens, and body caps are all from this combination of lenses, not even including the Promaster extension tube. I left this photo full size so you can zoom in to see the details.
The extension tube, in addition to allowing the extenders to be stacked, shortens the minimum focusing distance to about 3 feet. But, it also eliminates focusing at infinity. The farthest this setup will focus is about 115 feet. But remember, the goal here is to photograph insects and small birds - not the moon.
While autofocus still functions with this setup it isn't accurate as the extension tube moves the plane of focus forward. It's possible, however, to get close with autofocusing and fine tune by turning the focusing ring on the lens. As I use back button focusing, this isn't a problem. People who keep the focus activation on the shutter button would have to switch the lens to manual focus.
Image stabilization still works fine and is a big help. All of these sample photos I took hand held. In general, I think image quality is pretty good. At least close up. I expect that most use will be between 5 or 6 feet and 30 or so feet.
The three images below show the farthest and closest focusing distances and a distance of about 12 feet, respectively. All images are taken at what the camera shows to be f/8, but which, in reality, is f/11. Only one extender is taken into consideration by the camera. The image taken from 12 feet, however, is shot wide open, which would be equivalent to f/8, while the camera showed f/5.6. All are at ISO 800.
|about 115 feet|
|about 3 feet|
|about 12 feet|
And here are two images that show the combination at a range of about 6 feet.
My next project, which I have tried out but not documented, is to combine the two extenders with two extension tubes and the 40mm pancake lens to make an 80mm macro lens.