图像法进行航空相机调焦过程中，图像噪声的影响会使得调焦精度降低.然而对于图像信噪比和调焦精度两者之间关系的研究主要是基于实验结果和主观判断，没有通用性的理论和确切的结论。从理论上分析并给出两者之间确切的关系表达式，并通过实验验证了调焦精度和图像信噪比之间呈正相关的关系，图像的信噪比越高，调焦精度越高。以评价函数的方差值作为衡量调焦精度的标准，理论上计算了图像的信噪比对评价函数的期望和方差的影响，提出信噪比越高的图像中噪声越少，评价函数的方差值越小，调焦越准确。设计实验记录不同信噪比下，应用同样的调焦方法，得到的图像评价函数方差的变化规律，实验结果显示当图像的信噪比由27 dB升高到39 dB时，评价函数方差由0.262降低为0.0611，相当于调焦精度提高了约30%，证明了理论分析的正确性。

In the use of auto-focusing technology based on image processing of aerial camera, the precision decreases due to the influence of image′s noise. Currently the relationship between signal to noise ratio (SNR) of image and auto-focus precision is mainly based on the experimental results and subjective judgments, but there is no universal theory and exact conclusion on them. The relationship and an exact mathematical expression are analyzed and given between the SNR and auto-focus precision, experiments show that the SNR of image has a positive correlation to auto-focus precision, the higher SNR of the image, the higher precision of the auto-focus precision. Through the standard of the expectation and variance of focus measure, the influence of SNR on precision is analyzed, and results show that the influence of noise can be weakened by calculating the focus measure more times. Experiments are designed and the changes of SNR with variance of focus measures are reconded by applying the same focus method in different conditions. Results show that when the SNR of image rises from 27 dB to 39 dB, the variance of focus measure function decreases from 0.262 to 0.0611, meaning that auto-focusing accuracy is improved by about 30%. The result proves the correctness of the theoretically analysis.