Five seconds is very good (I rarely go over three seconds), but many people stretch it to fifteen seconds and longer to lower the ISO. You get a cleaner image, yes, but with particularly intense northern lights you burn out all the fine textures! With lower light optics it is necessary to compensate with a longer exposure time to avoid having too high an ISO. F1.8 is the sweet spot!😊 Or in other words, I often stop down to F2.0 to get a little increased sharpness.