We have focal F400,F420,F550,1000 and so on.According to the use of measuring ... instruments and testing to use. Collimator instruments.F550 focal length is 550mm,Parallel to the beam optics,we ... have focal f400,f420,f550,1000 and so on.According to the use measuring instruments ... measure the rail line and other large plane. Adjustable collimator F550