as otherwise no real images would be produced. ƒ2 was
99.7 mm for the convex lens (+100 mm), ƒcomb. was 180 mm
for the combination of two lenses (+100 mm/-200 mm) so that
ƒz = -223 mm represents the focal length of the concave lens.
(The combination of two lenses involves a systematic error as
the distance between the principal planes is disregarded).
3.1 The magnification is obtained from the relationship
between object size and image size