We have studied the influence of probe-sample interaction in a scanning near-field optical microscopy (SNOM) in the far field by using samples with a step structure. For a sample with a step height of ～λ/4, the SNOM image contrast between the two sides of the step changes periodically at different scan heights. For a step height of ～λ/2, the image contrast remains approximately the same. The probe-sample interaction determines the SNOM image contrast here. The influence of different refractive indices of the sample has been also analysed by using a simple theoretical model.