The Smithsonian Hectospec Lensing Survey (SHELS) is a window on the star formation history over the last 4 Gyr. SHELS is a spectroscopically complete survey for R _tot < 20.3 over 4 °. We use the 10k spectra to select a sample of pure star-forming galaxies based on their Hα emission line. We use the spectroscopy to determine extinction corrections for individual galaxies and to remove active galaxies in order to reduce systematic uncertainties. We use the large volume of SHELS with the depth of a narrowband survey for Hα galaxies at z ~ 0.24 to make a combined determination of the Hα luminosity function at z ~ 0.24. The large area covered by SHELS yields a survey volume big enough to determine the bright end of the Hα luminosity function from redshift 0.100 to 0.377 for an assumed fixed faint-end slope α = –1.20. The bright end evolves: the characteristic luminosity L* increases by 0.84 dex over this redshift range. Similarly, the star formation density increases by 0.11 dex. The fraction of galaxies with a close neighbor increases by a factor of 2-5 for L _Hα L* in each of the redshift bins. We conclude that triggered star formation is an important influence for star-forming galaxies with Hα emission.