Very cool, but let me ask this: Is this combo getting you better results than just cropping? I'm not saying it is not, but that is the thing about a teleconverter, or combination of converters is it has to outperform just cropping the image.
If you shoot the moon with just the 2.0X and then crop, is there less less detail?
After reading the original post I gave it a try just for fun. And yes the results were really quite good and better than cropping.
When you think about it, it seems almost self-evident that shooting at the higher magnification would provide better results than cropping in to an image taken at smaller magnification - ignoring for the moment any IQ issues induced soley by higher magnification such as camera shake or shallow DOF. With those other variables under control, the higher magnification has to produce an image of more exquisite detail.
For any given detail in the image - a crater for example - doubling the magnification will increase the number of pixels used to image that detail by a factor of 4.
Let's take a hypothetical. Assume I shoot an image of the moon with a 400mm lens and one of the craters in that image takes up a matrix of pixels on the sensor measuring 50X50 pixels, for a total of 2500 pixels. Now I want to increase the apparent size of that crater by cropping in to the image to give an angle of view equal to what I would have had if I'd shot with an 800mm lens. Even though I have doubled size of that portion of the image in each dimension, I still haven't increased the number of sensor pixels contributing to that part of the image so there is no additional detail to be garnered.
However, if I had taken the image with an 800mm lens to begin with, that portion of the image would be twice as large on the sensor than the previous shot, measuring 100X100 pixels. Because of the squaring effect, there are now 100X100, or 10000 pixels contributing detail for that same portion of the image. The uncropped image will be similar in angle of view to the previous cropped image, but will have a much higher pixel density and thus much higher resolving power vis-a-vis the image details.