利用Ronchi光栅的Talbot效应和Moiré条纹测量了长焦透镜焦距，并根据焦距与莫尔条纹斜率之间的定量关系可求得透镜焦距.为获得条纹斜率，用CCD采集莫尔条纹图像后，利用数学形态学滤波方法进行图像滤波平滑等预处理，有效地增强了图像的对比度.通过二值化、细化获得单像素宽连通的条纹，通过对条纹进行标记, 为条纹斜率的直线拟合计算提供了可靠的数据，采用这些数据计算的焦距的误差为0.10%.

Based on Talbot effect and Moiré fringes of Ronchi gratings,focal length measurement of the long focal length is carried out. The focal length of measured lens can be figured out accurately according to its relation to the slope coefficient of Moiré fringes. To get the slope of Moiré fringes, the image of Moiré fringes is mainly filtered or smoothed by mathematics morphological method to enhance the contrast of fringes effectively. Then the filtered image is processed by thinning to single pixel width. The fringes are labeled to fitting lines to calculate the slope coefficient accurately,and the error of focal length is 0.10%.