From the above it follows that a camera obscura, in order to form images of distant objects, must be so constructed that the focus of the convex lens falls exactly upon the ground-glass plate; and conversely a camera obscura whose screen lies in the focus of the convex lens can give sharp images of only such objects as lie practically at an infinite distance from the lens. Precisely these optical conditions exist in the emmetropic eye; when its accommodation is wholly relaxed the retina lies exactly in the principal focus of the dioptric apparatus.
Fig. 1.
If the object-point is not at an infinite distance the image-point will not coincide with the principal focus, but its position will be determined, as has already been mentioned, by the distance of the object-point and by the focal length of the convex lens.
It is evident from Fig. 1 that rays of light which proceed from are so refracted in the lens that they will be focused at an infinite distance beyond it, or what is the same thing, and is expressed by the dotted lines, they take a direction as if they had proceeded from a point at an infinite distance in front of the lens.
The farther the object-point is removed from the lens the nearer the image-point approaches it, until finally, upon infinite removal of the object-point, the image-point coincides with the principal focus. Thus rays of light which diverge from a point more distant than the principal focus, become convergent after passing through the lens and intersect each other in an image-point whose distance is likewise greater than that of the focus.
Let , in Fig 2, be the focus of the convex lens, the luminous point, and the image-point, then an inverted, diminished image of an object lying at will be formed at , while if the rays diverge from their union takes place in , and an inverted, enlarged image of would there be formed. The distances of and are thus conjugate focal distances, each of which, just
