The experiment was performed using a scanning confocal microscope (Leica SP5) with an infinity tube length corrected (ITLC) objective (Leica HCX PL Apo, 63x, ) or a finite tube length corrected (FTLC) objective (Leitz, PL APO, 100x, ). Figure 1 shows a schematic drawing of the sample chamber. Two cover glasses (Menzel Glaser, #1.5) were separated at one side by means of two stripes of double-stick tape to have gold nanoparticles attached to the cover glasses at various heights inside the chamber [Fig. 1]. The diameter of the gold nanoparticles was (BBInternational). The size was chosen so that they would provide a reasonable signal level in the nonaberrated case. The inner surface of the lower coverslip was chosen as zero for the depth measurements. The depth of the gold nanoparticles attached to the upper coverslip was measured as the distance traveled by the objective until the particular particle was in focus. To visualize the gold nanoparticles, we used immersion oils used with different refractive indices from Cargille (refractive index liquids set A). After deducting the average background intensity from all pixels, the maximum intensities at the positions of the nanoparticles were measured and the difference between the maximum intensity and the background noise at a given depth can be used as a measure of the confocal visualization efficiency. The gold beads were visualized by the reflection of a laser line (operated at 43% of maximum power) with the following settings: zoom 65, pinhole , and field of view .