| 摘要 |
To detect a deviation of an object focal plane of a printing lens from a reference focal plane that is perpendicular to an optical axis of the lens, a focusing chart with a test pattern thereon is held inclined to the reference focal plane with a center of the test pattern aligned with the reference focal plane, and test prints are made from the focusing chart through the printing lens while setting the printing lens at different printing magnifications. The test pattern consists of a plurality of lines extending in parallel to a transverse direction to the inclined direction of the focusing chart, and being spaced equally from each other in the inclined direction. The deviation of the object focal plane of the printing lens at each printing magnification is detected on the test print as a deviation of the lowest density area from the center of the test pattern, and is measured as a relative height from the reference focal plane by use of a scale that is provided adjacent the test pattern along the inclined direction of the focusing chart.
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