Empirical Calibration of Hubble Space Telescope WFPC2 F606W and F814W   Photometry
  Using the ground-based V and I photometry of a sample of stars from the Groth Strip, we obtain the first empirical calibration of the F606W and F814W Hubble Space Telescope WFPC2 filters for 0.5<V-I<4.5. We present results in the form of corrections that need to be applied to the two synthetic calibrations currently in use. Both calibrations are found to require corrections to zero points and color-terms. 
Samir Salim and Andrew Gould