Modulation of the frictional force of a fingertip sliding over a surface-haptic device can produce compelling sensations of texture and relief. The virtual sensation is particularly apparent and feel as fixed in space if the stimulus is rigorously correlated with the displacement of the finger. While frictional textures tactually resemble their real counterparts, some exploratory conditions under which the sharpness of the texture declines exist. We postulate that this decline in sharpness is caused by the perceptual limitation of the attempt to interpret the variation in friction as an out-of-plane sinusoidal topography. To investigate these questions, we measured the detection thresholds of sinusoidal friction-modulated gratings for a wide range of spatial periods explored at two different speeds. We compared the results with the detection thresholds, reported in the literature, of real gratings and vibrotactile stimuli. We found that the detection of spatial friction-modulated textures does not follow the same trend as that of real textures but is more similar to the vibrotactile rendering, which is strongly influenced by the exploratory speed. This study provides a better understanding of the perception of friction-modulated textures and provides insight into how to design impactful stimuli on surface-haptic devices.