Looking east from my front porch I see the stars rising in the evening. The atmospheric reflectivity recedes into a dull glow from tens of thousands of city street lights.  I have always been attracted to taking pictures in the dark. Stars are tricky, as they move. Long exposures create a smear of stars. When they arc in concentric circles through the photo, these have a pleasing effect, as they seem to show an order in the universe. But I prefer the reality of sharp points. Thus I must ask myself, what is the maximum exposure time I can take before the stars begin to move? How many pixels per second does the Earth rotate?

My calculations are as follows: 

  • A 24mm lens projecting onto a frame 36mm x 24mm has a horizontal field of view of 74˚.
  • The pixel array is 2900 x 4300.
  • Each pixel will thus be covering 61 arc/seconds of space.
  • The earth rotates 360˚ in 86,164 seconds. 
  • 360˚ is 1,296,000 arc seconds. 
  • A pixels worth of rotation will take 4 seconds. 

And so in order to have a tack sharp photo with my relatively wide angle lens, I cannot expose the sensor for more then 4 seconds. That is comforting to me.