Q : How is a curved screen not out of focus at the edges, unlike the flat cinemascope projection screens?

Although the edges of a curved screen are (physically) further from the wall than the center of the projection screen is, it does not impact the actual focus of the picture for this reason. When a screen is flat and it is being illuminated by a single center-point light source (aka 1-projector) the light is actually travelling a longer distance to reach the edges of the screen as opposed to the screen’s center. This uneven “throw distance” can actually create distortion that manifests itself in the form of an “hourglass-shaped” image. If a projection screen is properly curved, the light hitting the edges is actually traveling an equal distance to the light rays that hit the center of the screen. If anything, a curved screen provides enhanced clarity on the edges. This is why curved screens and geometric correction features on projectors are a must for many of today’s video aficionados.