NA determines intensity. NA determines intensity. A triple-labeled MDCK epithelial cell monolayer was imaged by confocal microscopy (LSM 510; Carl Zeiss MicroImaging, Inc.). Image A was acquired using a 10×/0.25 NA objective (A-Plan), and B using a 10×/0.45 NA objective (C-Apochromat). Single optical sections were acquired using identical acquisition settings, and with pinholes of 1 Airy Unit for the red channel and adjusted to maintain constant optical slice thickness in the other channels. Image A was acquired first using the minimum laser power and gain necessary to visualize signal in each channel. The image acquired with the higher NA objective (B) was so much brighter that all channels are highly saturated. This is because intensity is proportional to the fourth power of the NA. Alison J. North J Cell Biol 2006;172:9-18 © 2006 Rockefeller University Press