The Power of Gray - The Photoshop: Professional Retouching and Compositing Tips, Tricks, and Techniques - page 139

Figure 2.66
2. From the choice of lens flares, for this effect I'm going to use the 105mm Prime.
In the small preview window, drag to position the flare where you would like it.
This can be challenging due to the size of the preview, but don't worry about being
entirely accurate ( Figure 2.67 ).