Microscope objectives

Back focal plane

The back focal plane (BFP) is an important design and build parameter that may affect, for example, the kind of tube lens that is used with the objective to form a finite image. It is also common to image the BFP onto other optics like scanning mirrors or adaptive optics etc, and so knowing the size of the BFP may be important. Fortunately, we know from the infinity correction, that the BFP diameter (\(D_{BFP}\)) scales with the focal length and numerical aperture of the objective according to: \[ D_{BFP} = 2 f NA \tag{1}\] However it may also be useful to know the location of the BFP for mechanical and alignment considerations, for example how far the BFP is from the objective flange. Not all manufacturers provide this information, so it may be down to the builder to find out for themselves. In practice the BFP is often located inside the objective, i.e. a focusing laser beam into the rear of the objective will produce an output of minimal divergence when the focus is some distance past the flange. Sometimes the only way to find out is to do this test and measure!