The optical layout · Magnification: how much you really need

23.1 The optical layout

The path of the light crosses four assemblies. The objective, in the bell, gathers the light and forms a first image of the target, inverted. The erector assembly turns it right side up and, in variable-power scopes, scales it: it is the heart of the zoom, and it is also the assembly the turrets displace in order to move the line of sight. The eyepiece, with its diopter adjustment, presents the final image to the eye. And the reticle sits in one of the two available focal planes: ahead of the erector assembly (first focal plane, FFP) or behind it (second focal plane, SFP). It is an architectural choice whose practical consequences are treated in Chapter 26. Figure 23.1 gives the layout, with the two possible focal planes marked on the ray path.