| 摘要 |
PROBLEM TO BE SOLVED: To provide a multibean scanner which never has an out-of-focus scanning position even if the scanner has a curvature of field aberration in a multibeam scanner which has plural light emission points. SOLUTION: The longitudinal power of an optical system is so set that the value of a range de-ΔS of overlaying of depth de of focus at photosensitive body surface positions corresponding to light emission points ch1 and ch2 of the multibeam scanner is positive, preferably, the value obtained by subtracting a field curvature aberration quantity Cf from the overlaying range de-ΔS is positive, where de is the depth of focus corresponding to the two light emission points ch1 and ch2 andΔS is the interval quantity between the light emission points ch1 and ch2 in the direction of the depth of focus.
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