Any reason for 15 seconds instead of 20-30? I'm just trying to learn. Also, wondering why you guys suggested ISO 1600 instead of 3200.
As others said it is a matter of star movement. I expect with the other noise you will see, 30 seconds wouldn't be that noticeable but it is still something to keep in mind.
A good starting point is to divide 500 by the equivalent focal length. So in this case 500 / 35 = 14.3, so 15 seconds is a reasonable starting point. This is just a general guideline because the direction you are facing will also make a difference.
Pointing to the north will result in longer times. As you move away from the north the times will get shorter.
It is really a matter of getting out there and trying different settings.
Take this image as an example:
The North Star between the cactus barrels is effectively just a dot. As you move farther away from the North Star the trails get longer and longer. So if you are trying to capture points the effective motion of the stars get faster the further you are away from north.
Also note that when facing due north the motion will be circular but as you face east or west they will be diagonal lines that appear mostly straight. And to the south the lines will start to arc back the other way.
Here's another example facing about due west:
This is with an ultra-wide 12 mm lens (full frame sensor). In the upper right you can see the circles starting around the north. In the center of the image the lines start to straighten out and to the left they arch back the other way.
The best thing you can do is get out there and give it a try. Do some shots locally and see if you can get a good star point on the phone's camera. If it works then the pictures will only get better as you head to darker skies.
[doublepost=1506031266][/doublepost]
Hmm.
@Laird Knox you were closer to me.... Poppills calculates the max exposure time for the Canon A650 at something like 3-5 seconds. Hmm, that wont be enough to get light in in that amount of time.
@Laird Knox thoughts? you are better at this than me...
It all depends on your tolerance and which way you are facing.
I love this shot but in a 20 x 30 inch print you can see the elongation of the stars. In this case I don't think it detracts from the image in the least.
As I mentioned in the post above 500 / focal length gives a good starting point. Then it becomes an issue of how demanding you want to be. Keep in mind that this is effective focal length, so...
FX
500 / 24 = 20.8
DX
500 / (24 * 1.5) (crop factor) = 13.9
So that same lens on a full frame sensor would be good for 20 seconds but only 10 or 15 seconds on a crop body. (Rounding to typical camera steps.)
That 3 to 4 seconds you mentioned might be because it truly isn't a 35 mm view equiv. I'm not sure how crop factors impact things on such small sensors.
[doublepost=1506031451][/doublepost]According to this page
https://www.digicamdb.com/specs/canon_powershot-a650-is/
the crop factor is 4.6 so:
500 / (35 * 4.6) = 3.1
In that case I would say the 2-3 second suggestion is spot on.