العدسات الشيئية .. اختيار واستخدام المواد .. كتاب التصوير الضوئي

Objective lenses .. Selection and use of materials .. Book of photography

Selection and use of materials
Objective lenses

- The deflection lens:
The central deflection lens is a large angle lens invented to cover a larger size than the film size, often 24 x 36 mm. The lens can be moved parallel to the optical axis to allow the observation of a perspective that is comparable to the perspective obtained by the deflection of the single-track machine (see page 220). It is used especially for photographing tall buildings without tilting the machine, and this allows obtaining parallel vertical lines on the image while they are close together if we use another lens. The deflection movement can be controlled in four directions up and down. To the left or to the right.

- Zoom lens:
Changing the lens to change the focal distance is not recommended and for intermediate focal lengths between two lenses is not available. Therefore, an interchangeable lens called a zoom replaces several fixed focal length lenses and allows, at the maximum limit of the shortest and longest focal lengths given by the zoom, to select the desired framing with the highest degree of accuracy. But its drawbacks are its weight and sometimes its price. As for the quality of the image, we can say that - for modern models - it is equal to the quality of the images provided by the popular fixed focal length lenses.

- Mirror lenses:
Knowing that the telephoto lenses have a simple formula, it is difficult to make them coherent. A solution has been found for this, which is to fold the light beam on itself by its successive reflection in two times. Therefore, the mirror lens with a focal length of 1000-500 mm is three times shorter than a normal telephoto lens. Since there is only one obstruction wall for this type of lens, it is necessary - for correct photography - to influence either the duration of taking the image or the gray - neutral filters.

- Focal distance:
Photographic lenses belong to three families: normal focal, wide-angle and long focal: this name is completely independent of the size of the film used.

A normal lens has a focal distance approximately equal to the diameter of the size. For example, the diameter of a 6 x 6 image is approximately 80 mm, so the normal focal distance for this size is close to 80 mm. All the lower focal distances for the same size - are suitable for wide-angle lenses and the upper focal distances are suitable for long focal points.

For a 24 x 36 mm lens, a focal length of 80 mm is a long focal length lens because for a small lens the normal focal length is 43 mm (in fact, the normal focal length for this size is between 50 and 35 mm). For a 4 x 5 (10 x 12.5 cm) lens, the diameter is 150 mm, which is suitable for a normal focal length. Therefore, 80 mm is a major angle, while 57 mm would be a minimum. For a long focal length (for example) 200 or 240 mm.

- Field angle:
There is a second factor to consider in the relationships between the normal focal length, the major angle and the long focal length, which is the field angle that the lens sees (see page 42). A true macro lens, whatever the size used, sees a field angle of more than 70°. A normal focal lens has a field angle of 45° 50°. The field angle of a long focal lens is usually less than 35°.

- Macro lenses:
The maximum aperture of a macro lens is usually lower than that of a normal lens. This lens tends to exaggerate the perspective of objects, especially at the edges of the image, and therefore its use is precise for portraits unless the intention is to distort the subjects for some effect. The use of a macro lens requires the instrument to be close to the subject. Thus, a large depth of field cannot be avoided, especially at a small size, due to the short focal distance of the lens. Of course, the macro lens is necessary when you cannot step back to frame - with a normal focal lens - the entire subject. But it is also used for landscapes and allows for a wide panorama, giving special importance to the masses located on the first level.

- Fisheye lens: It is a very large angle that gives an image whose central section has the appropriate size and perspective for the focal distance, but its lines gradually curve as they move towards the edges of the image. The image taken with a fisheye lens resembles the image seen in a mirror in the form of a smooth ball. All the lines are curved and twisted inward, giving the image a strange appearance.

- Long focal length lenses:
On the contrary, the long focal length lens allows the subject to be photographed from a distance to obtain a large area without entering the subject.

It is widely used in the field of portrait photography, sports photography and hunting photography. The limited field angle requires a distant point of view that brings a kind of flatness in the perspective and the depth of field is relatively limited.

- Telephoto lens (tele or projective):
It is a special type of long focal length lens whose optical structure is such that it gives a long focal length but with a small draw (lens-film distance).

- Depth of field:
The drawing to the right shows how the depth of field varies according to the focal distance. The depth of field (for all lens focal lengths) was calculated by adjusting at a distance of 30 meters and an aperture of 5.6 / F. Note that the depth of field extends as the focal distance shortens.

- Standard lens:
The standard or standard lens - the main lens in most machines - is a compromise between the wide-angle lens and the long focal length lens. It suits most subjects, but that doesn't mean that these subjects don't sometimes require more realistic treatment. A normal lens requires precise framing that doesn't contain any subjects outside the subject being treated. Sometimes it is useful to have a standard lens with a longer focal length, for example an 80mm lens on a 24x36 machine (instead of a 50-55mm lens), which allows for framing to be more compact.

- Minimum expression distance:
The longer the focal length, the further away the minimum focal length. This is not a big mistake since the magnification depends on the focal length.

With a long focal length, you usually stay far away from the subject. But with a wide angle lens, you have to be close to it.

- Depth of field:
The depth of field also varies depending on the focal length. This depth is smaller the longer the focal length. The adjustment is more precise and difficult with a long focal length lens. This is a bad result if you want an image with all surfaces clearly visible. It is good if, on the contrary, you are looking for a calibration that you can select on one surface of the image.

Lenses, field angle and minimum distances for calibrating the distance.

Changing lenses
6mm fisheye
18mm wide angle
35mm wide angle
50mm normal focal length
85mm long focal length
200mm long focal length
800mm long focal length

The images above show you how, while maintaining the same point of view, you can adjust the magnification of the image by using different lenses.