FIGURE 4.

The “turning point” between donut and spot‐like images can be defined in various ways: (a) equality between the maximum intensity of longitudinal and radial components, (b) stationary point of the image center, or (c) equality of total energy contributions of longitudinal and radial components. Plotting the turning point for a range of objective NAs results in a “turning line”; the color of these lines matches the colors of the corresponding turning point boxes in Figure 3. Higher objective NAs result in equality of longitudinal and radial field components at smaller tube lens NAs. This is caused by the higher field strength at the periphery of the pupil when higher NA objective lenses are used. Note that in (b) negative values are saturated to better visualize the turning line