Fig. 3.

(a) Aberration and depth of field control demonstrated with 4 $\mu$m polystyrene beads and a variable iris placed at the back focal plane of objective 2. Full frame (a) and region of interest (c) taken with objective 2 NA $=0.75$. Full frame (b) and region of interest (d) taken with objective 2 ${\mathrm{NA}}_{\mathit{\text{eff}}}=0.33$. An increase in particle number, likely due to depth of field increase, and improved particle visibility, from aberration control, are both observed by decreasing the effective numerical aperture of collection. The vertical line of intensity superficial to the sample is formed by specular reflection from the coverslip.