Formulae below relate to lenses, and make use of the following international symbols:
f = focal length
u = distance of object from lens (measured from front nodal point *)
v = distance of image from lens (measured from rear nodal point *)
d = total distance between object and image, ignoring nodal separation
0 = object height
1 = image height m = magnification
(*Warning: these distances are difficult to measure accurately with telephoto and inverted telephoto lenses, because the nodal points may be well in front or behind the centre of the lens. See page 71).
You can calculate focal length, or image and object distances, or magnification, when other measurements are known. The key equation is:
From which ...