Focal visual optical systems magnification (magnifying glass, oculars, microscopes...)

These optical systems are used to allow an observer to obtain, a long distance image from an finite distance object, visible with eye without any accommodation.

If we admit that we assimilate such a system to a thin lens L, figure 50 represents an object FB of dimension y in the focal object image of L. The image B' of B is at the infinity in the direction θ' = y/f = -y/f '.

The Gc magnification said « commercial » of these systems is the ratio between θ' and θ'0, angle under which we directly see the same object with the eye at a distance of 250mm, thus:

A x5 magnifying glass has a 50mm focal distance. An x20 ocular has a 12,5 mm focal distance.