I've used this technique when doing long exposure photos with star trails in them. The main exposure at something around f/8 for 10minutes (or however long) and then opened up to f/1.7 (on a 50mm) for about 1 minute. This produces a very skinny star trail we have all seen, but at the end of the trail is a fatter circle, the star.... This makes seeing the constellation easier and also adds something thats a bit more interesting to the common star trail photos.