Streak Tube Imaging Lidar (STIL) is today one of the leading technologies for airborne and underwater laser-based imaging systems. There is a vertical blurring effect unique to the STIL sensor which has a simple explanation in the process of the STIL image formation. Although we believe the effect is familiar to many workers in the field, to our knowledge it has not appeared in the literature, and in particular we believe the simple mathematical description required to characterize the effect has not appeared. One reason may be that the effect can mimic or be dominated by transmitter vertical blurring effects such as finite transmitter pulse width, pulse spreading, or other receiver blurring effects such as finite CCD element size and electron-path dispersion within the tube. In addition, the magnitude of the effect depends on the amount of scattering and is small for clear water or short propagation lengths. However, it is not difficult to demonstrate that the effect can be large for a realistic hypothetical system under realistic optical conditions, and, just as importantly, that relatively minor design changes having little effect on other image formation processes can significantly mitigate the STIL effect for the same system.