Set
13s
20mm · ƒ/2.8 · longest 13.9s
The 500 rule says
25s
Made for film and small prints. Today’s sensors show the streak sooner, so it runs long.
Viewfinder
Your setting, 13s: the star moves 3.2 px.
How it works
The sky turns once a day, so every star is always moving. On the sensor that movement is tiny, a few of its pixels, which is why it is shown here on the pixels themselves. The NPF answer keeps that movement to about 3 pixels: at 100% a slightly stretched dot, at any normal size a point. Twice as long and it is a dash. A longer lens or smaller pixels show the same movement over more pixels, so they need a shorter shutter.
How far the star moves
Pixels 6.0 µm wideA still star already spreads over about 2 pixels. About 3 pixels of movement still reads as a point in the whole photo; 6 starts to show as a dash.
- Your setting 13s 3.2 px
- The 500 rule 25s 6.1 px
- Twice yours 26s 6.3 px