In this note, we explore the precision of the optical imaging method for measuring the free surface position of a micrometer fluid shape. For this purpose, images of a liquid film deposited on a rod were acquired and processed. The resulting contour was compared with the corresponding solution to the Young-Laplace equation. The average deviation was about 30 nm, 25 times smaller than the pixel size, reflecting the validity of optical imaging for most applications in microfluidics. (C) 2009 Elsevier Inc. All rights reserved.