Photography is about using the light. It takes practice and experience to see the light on a subject in the way the camera will do. But sometimes it just comes together. I could see the light on these white orchids, coming from a large window facing the morning sun. There was a clear contrast between the direct sunlight and the rest of the flowers, offering both a nice backlit view and an interesting contrast between the orange interior and the white flower.
The 17-50 zoom lens allowed me to get close enough for a reasonable composition. The top photo was the result. A smaller aperture let me capture a sharp picture. The position of the light gave a nice texture to the flower petals contrasted with the orange centres.
Opening the lens to f/2.8 and going to the other side of the flower gave me a backlit view with the background nicely blurred. Desaturated in GIMP, and masked to let the orange through finished the shot below.

Camera Gear

This was shot with a Canon EOS 450D with the Sigma 17-50mm OS EX lens. It is not a macro shot, just as close as I could get with the Sigma.

White orhid flowers with orange centres