| 摘要 |
A focal length measuring device comprises a light source unit for generating a collimated light, a diffraction grating for deflecting the collimated light by a deflection angle theta, and an image-pickup element for measuring a position of a focused spot light after the deflected light passes the lens to be tested. The diffracting grating is disposed near a front focal plane of the lens to be tested. The image-pickup element is disposed near a rear focal plane of the lens to be tested. A focal length is calculated according to a relationship such as h=f tan theta under condition that h indicates an image height as a distance from the optical axis. By doing this, it is possible to measure the focal length accurately and easily while restricting an influence of a depth of focus and an aberration.
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