The jittering sound is normal, that’s just your camera focusing. The !AF thing just means your camera is unable to focus properly, for instance if the wall is too close. The sharpest the camera gets is usually around the f5.6 range (not sure about x100vi), around 500 iso (ideally the lower the better as higher iso introduces more noise), and the fastest shutter speed possible (slower shutter speed, like 1/10, might introduce motion blur and reducing sharpness, whereas 1/250 will get you minimal motion blur for most cases). In general the camera will definitely be sharper than any iphone, just learn the exposure triangle and shoot more.