The lens mounts are the same on all current Nikon cameras. Maybe you're confusing lens zoom ranges? The biggest "real" difference as measured by the "Man I Just took a picture with the D7000 and it puts my D3100 to shame" metric. Is the sensor in the D7000. Head and shoulders above anything in the 3000 and 5000 series. Its reads light WAY better, meters better, better saturation and color rendering.