# Optical instrument camera

Parts of camera

A simple camera consists of a convex lens, diaphragm, shutter, and film.

Convex lenses function to form the real and inverted image on the film. Unlike the lens of the eye that has a focal length that can change, the camera lens has a focal length that cannot change. The camera lens is a convex lens, not a concave lens because the image produced by the concave lens is always virtual. Instead, the image produced by a convex lens is real when the object distance is greater than the focal length. A real image is an image that exists because this image can be recorded on film. On the contrary, the virtual image is a false image so that the image cannot be recorded on film. The position of the real and inverted image produced by the convex lens coincides with the position of the film.

Diaphragm or f-stop. The diaphragm functions to control the amount of light that affects the film. The size of the opening diameter depends on the size of the lens and is expressed by the f-number or f-stop. F-Stop is defined as the ratio of the focal length of the lens (f) to the diameter of the opening (D): f-stop = f / D. If the lens has a focal length of 100 mm and the opening diameter is 20 mm then the lens has a f-stop of 100 mm / 20 mm = 5. In this case, the lens is set at f / 5. If the lens has a focal length of 100 mm and the opening diameter is 25 mm, the lens has an f-stop of 100 mm / 25 mm = 4. In this case, the lens is set at f / 4. Based on this calculation, it is concluded that the higher the diameter of the opening, the smaller the number of f-stops and the lighter about the film. The smallest f-stop number (the most significant opening diameter) is the speed of the lens.

Shutter speed. The length of the open shutter time interval affects the amount of light that hit the film because when the shutter opens, the film also opens and is subjected to light. Shutter speed is also called “lighting time”. The shutter speed is related to the shutter time interval open. Therefore, the shutter speed is expressed in seconds. The shutter speed is between several seconds to a second. The smaller the time interval for the shutter to open, the faster the shutter speed, the less light will hit the film. Fast shutter speed is needed when the amount of light is small. Fast shutter speed is also required to eliminate image blur due to camera movement.

Film. The film functions to record the real image formed by the camera lens. A few years ago the film was still used on the camera, but now the film is no longer used on the camera. At present, the image formed by the camera lens are recorded electronically. Cameras that record images electronically are called digital camera. Cameras that record image using a film are called analog camera.

Focusing and formation of the image

The quality of the shooting, in addition to depending on the size of the diaphragm opening and shutter speed, is also affected by focusing. Before reviewing the focusing of the camera lens first understand the following explanation. On the topic of convex lenses it is explained that if the object distance (s = do) is very far and is considered infinite, the real image produced by a convex lens coincides with the focal point of the convex lens. So when the object distance is infinite, the image distance (s’ = di) is equal to the focal length (f). On the topic of the image formation of the convex lens, it is learned that if the object distance gets smaller, the image distance gets more prominent and the image size is also getting bigger.

Focusing the camera lens is different from focusing the eye lens. Focusing on the eye lens is done by changing the curvature of the eye lens (changing the focal length of the eye lens) until the image falls right on the retina so that the most precise image is produced. If the camera uses a single lens (not a zoom lens that has many lenses), the focal length of the lens cannot be changed. Focusing the camera lens is done by changing the image distance (di) until the most precise image is produced.

If the object distance (s = do) is very far or infinite, the image distance (di) is the same as the focal length (f). Compare Figure 1.

If the object distance is smaller than infinite, the image distance (s’ = di) is greater than the focal length (f). The smaller the object distance (s = do), the higher the image distance (di) and the higher the image size. Based on this explanation it is concluded that if the distance between the object and the lens is closer, the distance between the lens and the film must be enlarged. If the distance between objects with the lens farther away, the distance between the lens and the film is reduced. Compare Figure 2.

Focusing on the camera lens also needs to pay attention to the focal length of the camera lens used. The larger the focal length of the camera lens, the higher the image distance (s’ = di). Lenses that have a higher focal length also produce the larger image. If we want to get a precise figure while the distance of an object is distant, use a lens that has a large focal length. Lenses that have a large focal length are usually flatter or less curved.

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