Disclosed is a method of metrology comprising using measurement illumination to measure a target, said measurement illumination comprising a plurality of illumination conditions. The method comprises performing a first measurement capture with a first subset of said plurality of illumination conditions, e.g., each comprising a positive weighting, to obtain a first parameter value and performing a second measurement capture with a second subset of said plurality of illumination conditions, e.g., each comprising a negative weighting, to obtain a second parameter value. An optimized parameter value is determined as a weighted combination of at least the first parameter value and the second parameter value.