There is a short window of time when the sky is dark enough for the stars to show, but not so dark as to lose the deep blue color. I also wanted to avoid too much streaking of the stars due to the rotation of the earth, so this limited the length of time the shutter would be open.

I stopped down a little from wide open to avoid “coma” at the edges of the frame. Coma is an optical distortion at the edges of the frame of virtually all wide angle lenses when used wide open. I also needed just enough depth of field so both the stars and the tree appear sharp. If I stopped down too much, the shutter speed would be too long and star trails would be the result.

Canon 20D. 17-40mm lens at 17mm. Aperture: f/5. Shutter speed: 30 seconds.