العوامل المؤثرة في قوة التحديد .. كتاب آلة التصوير

Factors affecting selection strength

Determination is affected by two main factors: the lens and the sensitive layer. We will now talk about the effect of the lens on the power of determination, provided that we hope to explain the effect of the excited layer and the processes of display until we start talking about the sensitive layer in its part.

The effect of the lens on the power of determination:

Camera lenses are equipped as above - so that they are able to resist the natural optical defects in the lenses, such as chromatic aberration, spherical aberration, etc. It has been proven in practice that whatever the lens is corrected, it is not without defects. The point of light emanating from the object will not be transmitted by the lens as a point of light in the image (Fig. 56). Rather, it is, at least from a theoretical point of view, more like an unsharp patch of light, and around it are rings of light that gradually shift the intensity of its illumination as it is farther from the center of this spot. The strength of the determination in the image depends on the extent of this spot of light, the smaller this spot, the greater the strength of the determination. There are different names for this light spot. They are sometimes called circles of optical mixed circles of confusion or disk optical or anti point.

And assuming that the body on which the distance is set is completely flat (that is, it does not have a third dimension). The diameter of these mixed optical circles depends on the factors that the light discs do not show as follows:

(a) Optical wavelength:

The diameter of the mixing circle decreases as the wavelength decreases. In other words, this means that the diameter of the mixing circle is relatively small if the light rays are blue. (or less than it in length), and large if the rays are red (or more than it in length).

(b) Diameter of the aperture:

As the diameter of the mixed circle decreases as the lens aperture widens, and it increases. Its diameter is the narrower the opening (Fig. 57). This may seem strange to many, because the prevailing belief among the vast majority of photographers is that the diameter of the mixing circle decreases as the lens aperture narrows, and therefore the sharpness of the images must increase whenever the aperture narrows. And then we see that we should provide
explanation for this discrepancy.

The appearance of natural defects in lenses decreases (as we have previously shown) if the aperture is narrow, and this increases the power of selection (in addition to increasing the depth of field), but the narrow aperture has another opposite effect that reduces the power of selection on the other hand, as it stops The diameter of the mixing circle is the angle confined between the two extreme rays that pass through the lens. In (case B, Figure 57)
We see that the angle between these two rays is narrow, which leads to an increase in the diameter of the mixed circle, while the diameter of the mixed circle decreases if the angle between these two rays is widened (the case is Figure 57). It is clear that the reason for the widening or narrowing of this angle is the diameter of the aperture, and thus we see that the widening of the aperture increases the strength of determination from another side.

In summary of the above, it can be said: The aperture of the lens is disputed by two opposing factors that affect the power of resolution. The first factor determines that the narrowness of the aperture of the diaphragm increases the power of resolution as it helps to overcome the optical defects in the lenses. As for the second factor, it determines that the power of resolution is transferred according to narrow aperture lens. And since there are two opposing forces that affect the diameter of the mixing circle, therefore, we finally find that each lens has a medium aperture (neither narrow nor wide), if used during photography, which leads to the maximum possible obtaining of the sharpness of the image, and this medium aperture is known as Optimum Aperture, and this aperture may be 4 f, 5.6 f, 8 f, or 11 f sometimes. . The greater the degree of lens correction and the greater the amount of optical defects, we found that this medium aperture tends to widen, such as 4 f, for example (1). Accordingly, we find that the ideal corrective lens is the one whose maximum image sharpness is associated with the maximum aperture wide.

Experiments were conducted by the research departments in Factories (2) to demonstrate the relationship between the power of determination and the focal number (i.e., relative aperture) in lenses that received a large degree of correction against optical defects, and the results that we show in the following table came from the aforementioned source.

(1) Harris Tuttle mentions in the following reference that the lens aperture that would lead to the maximum sharpness of the image is the one that is two stops less than the maximum aperture of the lens. Use the f/9 aperture. This was stated in the following reference:
Harris Tuttle, Identification news, John 1966, Vol. 15-16, No. 12-1, page. 9. Edwin Mutter-Leica Photography- August, 1957 ( 2 )

The power of determination can be estimated mathematically with a simple equation based on our knowledge of the two factors that determine the power of determination, namely:

1- The wavelength of light (let us denote it by the letter m).

2- The relative aperture of the lens, and let us denote it (1: f, i.e. 1 over f).

Example 1: If we assume that the light rays that pass through the lens were blue and have a wavelength of 0.0005 millimeters, i.e. (5 microns), and that the relative aperture of the lens = 2:1 (that is, the ratio between the diameter of the aperture to its focal length = 1: 2.

That is, the limiting force is equal to 1,000 lines per millimeter. (S.T. 1000 1 Mali).

Example 2: What is the power of determination if we assume that the optical wavelength remains the same as it was in the previous example with a focal number of 4 f?

That is, the determination force = 500 lines per millimeter (sq.t / 500 / 1 mm). Then we see, based on the previous two examples, that the power of determination increases with the increase in the aperture of the lens, assuming that the optical wavelength remains constant.