App Smart Extra: Painting With Light Decide.com, a Web site that tracks millions of price changes on consumer electronics and appliances to predict when buyers are most likely to get a deal, said laptop prices tend to dip to seasonal lows in late June, the last two weeks of September and again in mid-December. One of the more interesting effects you can do with a smartphone is night "light painting," where you take a long exposure image of a scene with your phone, and moving lights leave striking tracks of color in the final photograph.