العدسة والحدقة - ٣ - طريقة عمل آلة التصوير .. كتاب التصوير الضوئي

Lens and pupil - 3 - How the camera works .. Book of photography

Lens and pupil

Most things reflect part of the light they receive and absorb the other part, while some materials such as glass, for example, transmit light rays. All things are seen by the human eye or by the photographic device according to the appearance that the light gives them and illuminates them. Light spreads in a straight line, which explains the formation of images by stenopic or pinhole lens. A simple hole in cardboard forms an image that can be received on a screen. This image is inverted, meaning that the head is from the bottom. Since the rays coming from the upper part of the subject pass through the hole to form an image at the bottom of the screen and vice versa. We can obtain (acceptable) images by stenopic, but since the depth of field is unlimited (due to the very small diameter of the stenopic), the images as a whole are not completely clear. In addition, taking the image certainly requires a long time, which limits and reduces the subjects that can be photographed in this way.

A simple lens that reflects the focal length of the lens and replaces the stenopic lens improves the quality of the image thanks to its ability to focus the light rays emanating from a point in the subject at a point in the image. The lens is the photographer's eye and the basic element of the photographic instrument. A simple glass lens, or in other words any lens, acts like two prisms attached at their bases that transform light as they refract it. The angle of refraction is related to the angle of incidence or reflection of a light ray and the curvature of the crystalline lens surface. The role of the lens is to increase the capabilities of the photographic instrument and to improve the quality of the images. To handle all subjects freely and invest all their capabilities, you must acquire a set of lenses with different focal lengths. There are countless lenses, but if you are not specialized in a specific field of photography, there are three lenses that cannot be dispensed with:

- A regular or standard lens - usually delivered with the machine - has a focal length close to the diameter of the used size. For example: 50 mm for 24 x 36 mm, the field angle is close to 45° and is approximately equal to the eye angle.

- A lens with a long focal length: gives greater magnification possibilities for the image with a narrower field angle. Its focal length is usually more than 85 mm for a 24 x 36 mm machine. Its major advantage is that it does not distort the perspective for close subjects (half-length image). This observation is specific to half-length images and photography with a 24 x 36 mm machine, which gives a large three-dimensional image that suits a very narrow field angle.

How to force light to travel in a straight line

Light rays reflected from an object diverge in every direction and do not form an image. A piece of cardboard with a hole in the middle (Stenopian) limits and directs the rays reflected directly from the object to form an inverted image.

Image calibration.

A prism turns the light to its base. The light rays emanating from each point in the object converge and meet, forming a point in the image (lower left image). This point of convergence is called the main focus. A clear image is formed with an unlimited number of points.

Focal surface

The surface perpendicular to the optical axis and passing through the main focus (upper right image) is called a focal surface.

Focal distance

It separates the optical center of the lens from the main focus of the image when it is set to infinity (image to the right). A lens with a short focal distance is very close, so the image is formed very close to the lens.

Field Angle

The field angle determines the boundaries of the subject included in the image size. It is related to the focal distance and the film size used in the photographic device.

Image Enlargement

Changing the size of the image without changing the location of the device or the subject. A lens with a different focal length is used. If it is longer, the focal length increases the image, and if it is shorter, it reduces its image. A lens with a focal length twice the length of another lens gives an image twice as large (images to the left). We must not forget that each lens must be used with the film size for which it was created.

Photographic Objects

These objects consist of at least two crystalline lenses (often more) close together or apart. Their abundance has led to the manufacture of a large number of models that give good images from which errors are removed, thus improving the overall quality of the image. With a suitable larger aperture and a larger angle of the exposed field.