  We present a prescription to correct large-scale intensity variations
affecting imaging data taken with the Wide Field Imager (WFI) at the MPG/ESO
2.2 m telescope at the European Southern Observatory at La Silla in Chile. Such
smoothly varying, large-scale gradients are primarily caused by non-uniform
illumination due to stray light, which cannot be removed using standard
flatfield procedures. By comparing our observations to the well-calibrated,
homogeneous multi-colour photometry from the Sloan Digital Sky Survey we
characterise the intensity gradients across the camera by second-order
polynomials. The application of these polynomials to our data removes the
gradients and reduces the overall scatter. We also demonstrate that applying
our correction to an independent WFI dataset significantly reduces its
large-scale variations, indicating that our prescription provides a generally
valid and simple tool for calibrating WFI photometry.
