Arrays of micromirrors are used to correct residual wavefront aberrations in optical systems. Each micromirror typically has either one or three actuators so it can locally correct either the piston error in the wavefront, or the piston error plus the tip and tilt errors. An array of 1000 piston-only micromirrors is compared with an array of 333 piston-tip-tilt micromirrors with the same number of actuators. The piston-tip-tilt micromirror arrays are better in all comparisons. They reduce defocus and astigmatism by 300×versus 30× for piston-only (Zernike terms Z4, Z5, and Z6). The wavefront error due to third-order spherical (Z9) is reduced by 33×versus 6×, and the fifth-order spherical (Z16) is reduced by 6×versus 2×. The micromirror array pattern is also studied. Hexagonal arrays reduce the aberrations roughly 10% more than do square arrays.