BF and ADF STEM images of 20-nm-diameter gold particles situated at the top and bottom surfaces of a 2 μm-thick plastic section. The BF and ADF images were recorded simultaneously with STEM detectors of 13 mrad outer semi-angle and 40 mrad inner semi-angle, respectively. ADF STEM images are displayed with reversed contrast. (a) BF and (b) ADF STEM images of gold particles located at the top of the section. In the absence of beam spreading, the gold particles appear relatively sharp in both imaging modes. (c) BF and (d) ADF STEM images of gold particles located at the bottom of the section. Beam spreading blurs the particles significantly more in high-angle ADF STEM. Scale bar is 20 nm.