To get night shots with star points, rather than smeared stars, you need to choose a shutter speed that won’t show star movement.
The longer the focal length, the less time it takes for star movement to show.
Traditionally, for film cameras, the Rule of 500 was applied. This rule basically says that for 35mm film, divide 500 by the focal length of the lens. This gave you the longest shutter speed you could use before seeing star streaks.
But with the advent of high-resolution, pixel-dense sensors, this rule needed re-thinking; star streaks are more apparent the higher the resolution.
For some time, the Rule of 400 was used. (For cropped sensors, it was 400 divided by the focal length times the crop factor: 400/(focal length x crop factor)
But with today’s pixel-dense sensors, even the rule of 400 was resulting in star movement. Now we have the NPF Rule.
Basically, the NPF rule takes into account the size and resolution of your sensor, the focal length, quality factor, the f-stop being used, the minimum declination from the celestial equator of the stars captured in the frame and probably more!
I know, that’s a ton of stuff to consider.
(Google “NPF rule for sharp stars” if you want more details and to see the math).
For example, using the rule of 500 with a 14mm lens on a full frame camera, the longest shutter speed would be about 36 seconds.
With the rule of 400: about 28 seconds.
However, personal experience showed me that I was getting star movement even at 20 seconds.
The NPF rule tells me that when using my 14mm lens at f/2.8 on my Nikon D810, the longest shutter speed should be about 16 seconds. And if I’m aiming to create large prints, that that time should be dropped to about 8 seconds.
What’s interesting is that if I increase the f-stop number to f/4, I could increase the shutter speed to about 19 seconds (about 10 seconds for large prints). Of course, doing that means I’ll also need to increase the ISO by one stop. Always the trade-offs.
So how do you find out these shutter speeds for your own combination of equipment? The best answer is the PhotoPills app, available for iOS and Android. http://photopills.com
This remarkable app is really the Swiss Army knife of photo apps and will tell you a ton of stuff. Highly recommended. Be sure to watch their online how-to tutorials. (I have no affiliation with PhotoPills)
The PhotoPills folks have more info here: https://www.photopills.com/calculators/spotstars