تصحیح أخطاء العدسة للحصول على صورة أحسن .. الانحراف اللوني .. فن التصوير الفوتوغرافي

Correction of lens errors to get a better picture.. chromatic aberration..

The possibility of obtaining an accurate image with a simple lens, as we described in the previous experiment, is theoretical. In fact, a simple lens cannot produce a perfect image, rather it gives a poor, blurred image. The reason for this is that the simple lens has many defects called aberrations. These deviations are:

1- Chromatic Aberration
2 - Spherical Aberration
3- Curvature of field
4- Distortion
5- Aberration (Coma)
6- Astigmatism

In order to study the art of photography, one must know the reason for the occurrence of these aberrations and how to correct them using modern composite lenses.

1- Chromatic aberration:

If we direct a beam of white light towards a prism, it emerges from it in the form of successive rainbow colors, called the prismatic spectrum. The explanation for this is that white light is a mixture of various types of rays that are refracted by prism glass to varying degrees. Figure (3)
Notice in this figure also that the red light is refracted to a lesser extent, while the blue-lilac light is refracted to a greater degree. We call this disintegration of the white color "scattering".

Therefore, when a beam of white light passes through a regular convex lens (Fig. 4), the blue rays are refracted at an angle greater than (the angle of refraction of the green rays, and greater than the angle of refraction of the red rays). .

At the (R) level, there is a clear and precise image of red intertwined with overlapping images of green and blue. And at level (G) there is a clear and accurate image of the green color intertwined with overlayed images of red, blue and lilac colors, and in fact we cannot see these images separately with their different colors, but we see a white overlaid image that is the product of the intertwining of all these images.

This lens defect. It is called "chromatic aberration" and it is caused by refraction and scattering of light. These two phenomena occur together, but each in a different amount. Therefore, it is possible to combine two lenses: a positive lens with a large refraction coefficient (i.e. refracts light strongly), and a small scattering coefficient (light scattering in a small amount), and a negative lens with a small refraction coefficient, and a small scattering coefficient, in a way The effect of scattering one of them cancels the effect of scattering the other (because the direction of scattering in both lenses is in the opposite direction). This lens, consisting of two lenses, can focus all colors on one focal plane. This lens is called a chromatic aberration corrector lens. It can be said that all color-corrected photographic lenses are corrected in this way.

In fact, it is not possible to correct all the colors of the spectrum with a lens composed of two lenses only. Despite this, the correction resulting from this lens gives sufficient results for different uses.
There are compound lenses made up of more than two lenses that can correct chromatic aberration for the entire visible color spectrum. They are called (complete achromatic lenses) (Apochromats).