Metrology method and associated metrology and lithographic apparatuses

A method of metrology comprising using measurement illumination to measure a target, said measurement illumination comprising a plurality of illumination conditions;

• said method comprising:
• performing a first measurement capture with a first subset of said plurality of illumination conditions, to obtain a first parameter value;
• performing a second measurement capture with a second subset of said plurality of illumination conditions, to obtain a second parameter value;
• and
• determining an optimized parameter value as a weighted combination of at least the first parameter value and the second parameter value,
• wherein said first subset of said plurality of illumination conditions each comprise a positive weighting and said second subset of said plurality of illumination conditions each comprise a negative weighting.