An optical system and detector capture a distribution of radiation modified by interaction with a target structure. The observed distribution is used to calculate a property of the structure (e.g. CD or overlay). A condition error (e.g. focus error) associated with the optical system is variable between observations. The actual condition error specific to each capture is recorded and used to apply a correction for a deviation of the observed distribution due to the condition error specific to the observation. The correction in one practical example is based on a unit correction previously defined with respect to a unit focus error. This unit correction can be scaled linearly, in accordance with a focus error specific to the observation.