焦面：光线经过镜头后形成清晰影象的平面。
Focal plane: The plane where light entering a lens forms a sharp image.
苗床整平后，用铁锹拍平，浇上10%腐熟稀人粪尿，再住床面亡铺一层过筛的焦泥灰或黄泥土，厚度约5厘米即可。
Seedbed leveling, Yongtieqiao shoot-ping, doused with 10% who maturity dilute waste, then living death bed screening of a shop Coke Nihui or yellow soil to a thickness of about 5 cm can be.
线长变化后，焦线宽度、光强分布均匀性、靶面位置仍然不变。
After the line length varied, the width, intensity distribution along the line length, and the target position are still unchanged.
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