When I first started taking photos years ago, all I wanted to do was to zoom in as much as I could. I longed to have a big telephoto lens that I could take amazing images with. It was probably my love of birds and sports that drove this obsession. Of course the dream never became a reality. Partly because of the cost involved, not only in getting such a lens, but in those days the cost of printing photos was a small fortune. Not to mention the fact that I didn't have a clue what I was doing.
Today it is totally the opposite. Granted from time to time I will attached the 70-200mm or something with an even longer focal length to the camera in order to compress a foreground, take a panorama or isolate a subject. Having lenses like this in your bag is an important part of being a landscape photographer. However the majority of the time my go to lens is my Canon 16-35mm f2.8 L iii. I just love the versatility that this lens gives me and how by using it correctly it opens up a unique perspective. Tilt the lens forward and the background in an image appears to get bigger and closer to you. The opposite it true is you tilt the lens backwards. Understanding how this works can greatly improve your compositions. From an Astro perspective then, the wider you shoot the longer an image you can take. This is due to the rotation of the earth. Following the 500 rules means that you can't go wrong.
To explain it simply. If you divided the focal length that you are shooting at into 500 it will give you a number. This is how long your exposure can be before you get trailing in the stars.
Here is a quick example. If you were shooting at 20mm, then the following maths would apply.
500 divided by 20 = 25 <------ your maximum exposure would be 25 seconds.
I have recently purchases a super wide angle lenses. It has a focal range of 12-24mm. At 12mm it is insanely wide. Again this has opened up a whole new dimension to my photography. But a word of warning. Shooting at an extreme wide angle is not easy and takes a lot of practise. I have been using this lens for 3 months now and I am finally getting to grips with it. You have to concentrate on getting the lines in your image correct, with such a wide angle its easy to get the legs of your tripod or other objects into your image, not having the images correctly composed can make things look very unnatural. Get it right though and the resulting compositions can be amazing.