اختبار العدسات .. كتاب آلة التصوير

Lens testing

Before the photographer begins photographing subjects that may require exorbitant costs to be re-photographed (such as cinematography, for example) or subjects that may not be re-photographed due to their infrequent nature, or when purchasing a new or used camera, or when suspecting any defect in the lens, the photographer must The photographer has to test the lens to verify that it is free of previous optical defects, and there are many ways to perform this purpose, the simplest of which may be the setting of a lens. magazine or daily newspaper on a flat surface - completely and then photographed. The lens is considered free of defects if the negative appears sharp in all its parts and the longitudinal lines are present. The transverse is completely straight as the original. The previous test, despite its simplicity, achieves the following purposes:

(a) Ensure the central definition of the image.

(b) Ensure the sharpness of the image away from its center (this includes the sharpness of the sides of the image).

(c) Ensure that the lens is capable of transmitting parallel lines faithfully so that they appear parallel in the image as well.

It is useful for the photographer to carry out his previous tests with the use of different apertures of diaphragms, as the image may appear sharp if the aperture is narrowed, while these defects appear clearly and clearly if the aperture is wide.

Selection strength test:

Because there are optical defects that result from the rays emanating from a point far from the axis of the lens, so it becomes more accurate if we want to estimate the power of determination to show the distance between the axis of the lens and the point (located on the image) in which the power of determination was measured. It is said, for example:

The determination force at the point facing the axis of the lens = 42 lines per millimeter, while it decreases to an error per millimeter if this point is 25 degrees away from the axis.

We conclude from the foregoing that the power of determination increases in the middle of the image at the point facing the axis of the lens, and shifts as the point is farther away. about this axis.

Whereas, unless the lens is completely corrected against the coma defect, it is certain that the image of the optical point appears in the form of a light spot that is not completely round but rather expanded in one of its sides than the other, so the numerical estimate of the determination strength differs according to the direction of the lines that form the image.

And if we measured the strength of determination by means of several test charts, some of which bear longitudinal lines, others transverse (Fig. The strength of determination varies according to the direction of the lines on the test plate, despite the stability of the rest of the factors that may affect it (such as lens aperture, type of sensitive film, and light wavelength).

For this reason, it may be more appropriate to measure the power of determination by means of test plates with radial lines that are at right angles with the axis of the lens.

The degree of contrast of the lines on the test plate has an effect on the result of the lens quality test (1). Therefore, it is better to conduct this test by photographing two plates: the first with completely black lines on a white surface, and the second with gray lines based on a light gray surface.

And since the distance between the lens and the test plate has an effect on the sharpness of the lines (i.e. the strength of determination as well), it was agreed that this distance should be equal to the focal length of the lens x 26.

In view of the effect of the type of film as well, it is better for the sensitive film to be ultra fine grain film and slow sensitivity and for the sensitive gelatin layer to be very thin Thin Coated Emulsion