I finally did some resolution tests with the Sony FDR-AX100. This camcorder records 4K to internal memory cards, and I was really wondering if 4K was going to be that good, or would it be so compressed that there was virtually no difference to the HD video. The camera also has an XAVC recording option (and you can buy a cheaper version of the camera, the HDR-CX900, that just has the higher data rate HD format recording) so I wanted to see how that compared to both 4K and HD AVCHD recording.
I actually wondered whether it would be better to get the HDR-CX900 instead, as it would be cheaper and frankly, all I was looking for was better HD, rather than 4K.
So for this test, I took the 4K footage, and the HD footage, and output it to HD in ProRes 4:2:2 from Final Cut Pro X. And if you're looking for really good HD resolution, than the FDR-AX100 in 4K mode is pretty amazing as a source. The FDR-AX100 4K footage blows away the best AVCHD camera I currently have (the NEX-EA50.) And while the FDR-AX100's XAVC footage is better than AVCHD, it's still clearly between the AVCHD and 4K.
I threw in another AVCHD camera, the Canon G10, just because I had it.
Clearly there are HD cameras that will do better than the NEX-EA50, though we're talking even more expensive (like the Canon C300). Even with cameras like the C100 or NEX-FS700, you'd probably have to use an external recorder to get your HD image close to the results of scaling down the 4K footage.