Originally posted by soycory But in reality wouldn't the 17-70 f4 be considered variable because f4 is different at 17 than it is at 70 right? I understand that the f number is constant even though the aperture is changing. Am I overthinking something
Nope, just misunderstanding. The f-number is the aperture. Constant aperture means that at the same shutter speed and ISO, f4 (for example) will give you the same exposure (brightness). Focal lengths is irrelevant. So this is why it is called constant aperture, because you can use the same minimum aperture for all focal lengths. Variable aperture means you can choose f3.5 at one end, but only down to f5.6 at the other end, for example.
Aside from that, high end zoom lenses are usually constant aperture, like f4 or f2.8. But cheaper, consumer zooms are variable (guess they are cheaper to make, but they are also much more limited, because one end of the zoom is darker, slower).