الضبط البؤرى أو ضبط المسافة على العدسة ١-a .. كتاب التصوير علم وفن

Chapter 2 Focusing or adjusting the distance on the lens

The purpose of focus adjustment is to adjust the distance between the lens and the film so that the silhouette of the subject to be photographed is clear or sharp on the film (in focus), and the shorter the distance between the subject and the lens, the more we need to lengthen the distance between the lens and the film, and vice versa. If the subject is very far away in infinity, practically more than 100 meters), then the distance between the lens and the negative of normal lenses) is equal to the focal length (i.e. the shortest distance of the lens that can give a sharp and clear image).

The focus adjustment device consists of two parts: a mechanical part: it may be a ring that we wind, an adjustment disk that operates on the chain, or a bellows, and the purpose of any device is to move the lens with respect to the film forward or backward.

Optical part: to help you know the exact distance of the subject. This may be a rangefinder (such as in cameras with viewfinders) or frosted glass (follow the instructions in the instruction manual that came with the camera).

In short, focus adjustment is done in one of the following ways:

A - In cheap cameras: most of them do not have a way to adjust the focus, as the lens is set at the hyperfocal distance (see after it )
Some of these cameras have signs that are set on them when photographing half a person, a person in height, a group of people, or distant scenes, or they may have distances that are set at the distance determined by the photographer according to what? for his appreciation of the dimension of the subject.

Distance adjustment icons in the rudimentary camera

2- In cameras with a viewfinder (mm): usually in the center of the viewfinder there is a circle of a different color (usually green) and in this circle a double image of the subject appears and when the lens is moved it becomes one, then the distance is set to the subject. This is a photo

set distance

Unset distance

3 - In reflex cameras of different types: the adjustment is done on frosted glass so that the image is clear, and this method is preferred by professionals, and some of these cameras have other aids for focus adjustment.
(a) There may be a circle in the middle of the frosted glass divided by a transverse, longitudinal or inclined line. The distance is not correct if the subject is divided at this line, and the distance is correct if the subject is not divided. This method is preferred if the subject has clear lines

(b) There may be a circle of fine prisms in the middle of the glass, and if the distance is not correct, we find that the image inside this circle appears crumbled and dances when the camera is moved. If the distance is correct, the fragmentation will disappear and the image will become stable. This method is preferred if the subject does not have clear lines.
(c) In some types of cameras, the frosted glass contains

the two previous methods

Frosted glass

Min leaflets

space delimiter

There are cameras that focus automatically in several ways:

(a) Using infrared radiation and post-reflection angle measurement

It hits the subject like a Canon AF camera.

(B) Using ultrasound like some cameras

Polaroid. (c) Using a snapshot method such as a Konica camera

. autofocus

After setting the distance, a small motor moves the lens to the required distance

Types of image sharpness or clarity:

In order for us to consider the image sharp or completely clear, sometimes lack of sharpness or clarity may be required, as we will see later, the image must be

sharp on three levels:

<-1 Clarity and sharpness in the distance: that is, the level on which the distance wheel is set on the lens (focusing) will appear sharp
Clarity and sharpness in depth: the narrower the pupil aperture or the higher the number (the greater the sharpness in depth and vice versa. Clarity and sharpness in movement: that is, there is no vibration or flickering with a specific direction. That is, moving objects during photography appear sharp, and this is a function of shutter speed The faster the shutter is, the more it can stabilize the movement, and the slower the shutter, the more blunt the movement appears.

Reasons for blurring or lack of sharpness of the image:

Camera shake during exposure: This is the main reason for the lack of sharpness of the image, especially with beginners. To avoid this disadvantage, you must:

(a) Do not use a shutter speed less than the reciprocal of the focal length of the lens used in millimeters when using the camera held by hand. Example: If the lens used is the standard normal lens with a focal length of 50 mm for a 35 mm camera, then the lowest speed that you can use while holding the camera in your hand is less than a second. Lower speeds can be used if the camera is firmly attached to a stand or pressed against a stable surface such as a wall, pole or piece of furniture.

This is done when you take a picture with the camera in your hand, prove yourself well by opening your feet and pressing the elbows on the body, pressing the camera on the cheek, then take a deep breath and exhale part of it, then stop breathing and press the trigger gently and slowly until the shutter releases. Do not press suddenly because this leads to shaking the camera at the moment of exposure. It also helps to rest on a solid surface such as a wall or a fence..etc.