The sunstar was not created with a star filter (which can soften the overall image). It is a natural by-product of using a wide angle lens at a small
aperture. Whenever you have the sun in a deep blue sky, you can create a sunstar by using a wide angle lens, stopping down to f/16 or f/22, and meter the blue sky next to the sun for your base exposure. It
is good to bracket around the base exposure by 1 or 2 stops by changing shutter speeds. If you don't understand bracketing, read the article on exposure. The pictures above were taken using a 15mm full-frame fisheye lens while laying down on the ground, and shooting up
through the poppies at the sun.