sparedog, 5x is a digital teleconverter of 3x, 24 mm becomes a 72mm equivalent
we call this 5x because we get that with the 5x mode zoom of the cam, but the image we obtain is 3x.
but no more than 3x: 5x mode is not linked to video mode.
the good, the image is on the center of the lens. distortions. less than full frame image
optical quality better than full frame image but image quality less than a raw photo image: it's a 100% crop of a raw photo image, but better than full frame raw video image, because this one is "like" a crop of the sensor (same number of points selected) but getting points sparsely over all the surface of the sensor = moire and aliasing.
The best cams for 5x mode: the cams with the less sensor density of points: full frames, ... 6D perhaps the best