Rays entering an optical system through a curved window change directions on passage. Therefore, rays coming from an object point appear to come from a small object area. For low ratios of window thickness to its radius of curvature, the direction changes are analyzed and presented in formulas and graphs. The average change in directions for parallel rays entering a cylindrical window normal to the cylinder axis is zero, while object distances parallel to the cylinder axis are not affected by the window. The angle subtended from a window point to two closely spaced object points is increased on its passage through the window.
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