إنجاز العدسات .. كتاب التصوير علم وفن

Second: Lens Performance lenses

After making sure that the lens is suitable for the task entrusted to it, the completion of the lens depends on the degree to which the lens designer was able to fix the defects inherent in it.

As well as on the way it is made and collected

To find out the performance of your lens, take pictures of the type you want and enlarge them.

Then analyze these pictures for:
Degree of sharpness. Degree of color correction.

3- The degree of occurrence of internal reflections and the formation of light halos. Degree of flare and halation. 4- Evenness of light distribution.

e - Degree of distortion.

And do not forget that I said (to such and such degree... because there is no complete lens). Likewise, every photographer needs special features, and what one may consider a defect may be an advantage in the eyes of another. For example, a portrait photographer, especially for women, may prefer a lens in which the defect of spherical aberration appears. Clear, and this helps him hide facial imperfections such as wrinkles, freckles, and acne, while we find that a scientific photographer wants a very sharp lens to show fine details, etc... etc.

(a) Sharpness and clarity: Sharpness

Achievement of any lens is improved by closing the pupillary aperture from 2 to 5 apertures less than the widest aperture. The more we narrow the pupil after that, the sharpness will not increase in the distance control level, but only the depth of field will increase (see beyond) until we reach very narrow apertures, we find that the sharpness may deteriorate due to the dispersion of light, and here we note several observations:

Lenses made for 35-degree cameras are usually sharper than lenses made for larger format cameras, such as studio cameras

Very fast lenses, such as F/1.2, are usually less sharp than those with smaller focal lengths, such as F/2 or F/2.8.
Lenses made for close-up photography (macro) are sharper than those made for normal distances.

The presence of a light filter in front of the lens of low quality or dirty) or not setting the distance completely) or (corruption of the relationship between the rangefinder and the lens) or (corruption of the position of the mirror in the reflex camera) or (slow shutter speed) or (movement of the camera during shooting) or (Bad zoom) All this leads to a less sharp image.

The presence of (b) the degree of color correction: the presence of a defect in color correction leads to colored halos around shapes in color images, and in (black and white) images, it leads to a slight blur. This defect is usually found in very cheap cameras

Some amateurs notice that some lenses are pictures with natural colors, and some of them lead to pictures that tend to be yellowish or blue. This difference is usually due to the different type of optical glass used or because of the material with which the lens is coated. This defect may be fixed by installing an appropriate filter, permanently.

(c) The degree of internal reflections: which lead to the occurrence of halos and haze on the film. Although the lens coating led to a significant decrease in this defect, there is no doubt that the rays are still reflected back and forth on the surfaces of the lenses, which leads to this defect, especially when photographing in the direction of light some

(D) An even degree of light distribution: All lenses send more light to the center of the negative than to the corners and sides. But this difference
with be to varying degrees. In standard lenses, this defect is not noticeable. But it starts to show up with wide-angle lenses, especially with positive color films that don't accept a high contrast in exposure. This defect leads to the edges of the image being darker than the middle. This defect is clearer in the case of photographing details and average darkness, such as a gray surface, for example, or as the sky, a cloudless surface.

Zarqa.

Using the appropriate lens from (e) the degree of distortion: the real distortion (not to be confused with the distortion of perspective, which is always a defect of the photographer (because of the lack of the right place) appears in straight lines that appear curved inwards or outwards, and this curvature increases whenever We moved away from the center of the image, and this defect appears most often in the wide-angle lens and in the variable focal length lenses
However, in the absence of straight lines and non-architectural photography, the

This defect is not noticeable, but may give a pleasant roundness to some scenes, such as
The flowers