We have determined how the stereo acuity and visual acuity with Helmet Mounted Displays (HMDs) depend on the HMDs spatial resolution. We measured stereo acuity and visual acuity on 6 subjects for three types of HMD, with display resolutions ranging from 0.18 to 0.50 pixel/arcmin. The HMDs provide stereo acuity ranging from 1.1 to 2.7 arcmin-1, not far below our subjects unaided stereo acuity. The ratio of stereo acuity to display resolution is 6.3 pixel-1 and independent of HMD type. The HMDs are therefore well suited for tasks that require stereo vision, even though the HMD visual acuities are rather low; the HMD with the highest resolution provides a visual acuity (0.55 arcmin-1) which is still four times lower than the unaided visual acuity. We attribute these excellent HMD stereo acuity scores to the target used which is much more representative of real world scenes than the typical static fine line target often used in the scientific literature.