Schlieren Photo

Schlieren Photo of Heat Rising from a Candle Flame
photo by Pete Reinhart

Half-second exposure on Tri-X film at f/4 through 200 mm telephoto lens.  The schlieren technique uses paired parabolic mirrors to create a region of parallel light rays so that subtle changes in the refractive index of the air in the region between the mirrors will create large contrasts in the resulting image.