تقوس الميدان .. كتاب آلة التصوير

Third: the curvature of the field:

The curvature of the field is one of the defects that are easy to see in the images formed by the simple lens. It results that the rays reflected from objects far from the optical axis of the lens do not gather on its flat focal plane, but rather gather at the distance of the focal length and on an imaginary curved line, and the center of the curvature is the optical center of the lens. The gathering of the image on this curved surface contradicts the nature of films or sensitive glass panels, which are always flat.

The basis on which the method of overcoming chromatic aberration was built
"colour segregation":

1 - A glass prism made of flint glass, and we can see that the white rays of light penetrated when it penetrated the prism. The degree of segregation was great for the second case.

2- Crown glass prism. And when the light rays pass through it, they also decompose according to the general rules, but the degree of their scattering is less than in the first case (because of the type of glass).

3- By combining two prisms made of the above two types of glass, and by placing one of them in an inverted position with respect to the other, we find that one of them has rays with the same degree of separation as the other, and the result is that the rays come out white.

If we bear in mind that the lens does not differ in its characteristics from being representative of the properties of the prism, and that prism (B) represents the converging lens, while prism (A) represents the dispersing lens, we finally found that combining two lenses with different properties and the calculation is correct, is the means that guarantees us Overcoming the chromatic aberration suffered by the single lens.

This defect is seen clearly when displaying positive, transparent images in some types of projectors, where we may see the image sharp in the middle without the sides, and if the focusing distance is re-adjusted, the image appears sharp in the sides without the middle. Hence, the best solution then is to mediate in adjusting the lens between the two previous cases, so the whole image appears medium in sharpness.

In order to overcome this defect in photographic lenses, the following is followed:

(a) In cheap primitive cameras, the surface on which the image is formed is curved so that the optical center of the lens becomes the center of this curvature (as long as the user is a film and not a sensitive plate), while making the location of the film a distance slightly less than the focal length, thus mediating its location between the distance The middle of the image gathers on it, and the one on which the image of the bodies located at the edges gathers (Fig. 44). In fact, this method is nothing more than a cover-up for the error instead of fixing it.

(b) As for the lenses used in precision cameras, they are corrected when manufactured by adding a dispersed (concave) lens that lengthens the distance on which rays that are not parallel to the axis of the lens gather, and thus reduces the curvature of the field.