It is demonstrated that ML estimating error can converge to CRB quickly with increasing synthetic aperture length and Signal to Noise Ratio (SNR), but a threshold effect exists.
研究表明，随着合成孔径长度和信噪比的增加，ML估计误差可以很快地收敛于CRB，但存在阈值效应。
If you know the focal ratio but not the focal length, you can calculate it: focal length is the aperture (in mm) times the focal ratio.
如果你不知道焦距，只知道焦比(focal ratio)，你可以通过这样计算的得到焦距：孔径(单位是mm)乘以焦比就是焦距。
For example, the focal length of an 8" (203.2mm) aperture with a focal ratio of f/10 would be 203.2 x 10 = 2032mm.
比如，孔径为8英寸(203.2mm)，焦比为f/10的透镜，其焦距为203.2 x 10 = 2032mm。
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