It is well-known that the accommodative response of the visual system to Snellen letters and other targets of wide spatial bandwidth becomes less accurate as the luminance is lowered from photopic to scotopic levels, a constant state of "night myopia" eventually being reached. The present study seeks to understand this behavior in terms of the responses to the single-frequency, sinusoidal grating, Fourier components of such targets, for a constant, 80% object modulation. The focal range over which sinusoidal gratings can be detected is first investigated as a function of grating frequency and luminance; this range diminishes as the luminance is lowered. These data are then related to measurements of the accommodative response to similar gratings in the mesopic luminance range (10(-2.5) to 10 cd/m2). The response is less accurate for gratings of low spatial frequency. It appears that, provided that any grating can be detected, changes in luminance generally have only a relatively small effect on the associated accommodative response. However, low spatial frequency gratings can be detected at much lower luminance levels than higher spatial frequency gratings. The effects observed with targets of wide spatial bandwidth may thus be largely ascribed to the inability of the visual system to use higher spatial frequency information as the luminance is lowered.