Chromatic aberration in confocal images. Eleven optical sections, spaced at 0.2-µm intervals, of a single, 0.2-µm diameter bead labeled with four different fluorophores. The dotted cross in each section marks the x–y location of the center of the bead in the green image at best focus. Notice that the other three wavelength bands are shifted in x, y, and z with respect to the green. The yellow and blue images also reveal some spherical aberration. A composite color image for the sixth optical section is shown at the top. (Red) Excitation (ex), 633 nm; emission (em), >650 nm. (Yellow) ex, 568 nm; em, 585–615 nm. (Green) ex, 488 nm; em, 505 nm–600 nm. (Blue) ex, 364 nm; em, 385–470 nm with a 63× (NA = 1.2), water-immersion lens.