In a general camera, the image pickup surface of the image sensor is placed at a position where the "feedout amount" is 0 so that a subject at infinity can be photographed. If you shoot a subject at a finite distance in this state, the image will be out of focus because the image will be separated by the amount of extension, but with a optical lens that can adjust the focus, the image will be imaged by this amount by moving the focus ring (distance ring). You can move the position of to match the image plane.
A general optical lens has a "focus mechanism" that moves the entire lens or a part of the lens group back and forth by moving the focus ring, but because the lens moves, the main points of the lens such as the principal point, focus, and pupil position Will move. Therefore, some lens manufacturers indicate the amount of movement of the lens for focusing as the "mechanical extension amount".
The mechanism that rotates the focus ring to move it back and forth is called a "helicoid". There are two types of helicoids: a "rotating helicoid" in which the lens itself rotates with the ring and a "straight helicoid" that does not rotate.
Focus range
The optical distance at which you can focus is called the "focus range".
With a general optical lens, the range is from "closest distance" to "infinity", but with some close-up shooting lenses, the focus can be adjusted only in a very small range.
Closest distance, close distance, MOD (minimum object distance)
When the optical lens is used in the normal way, the closest distance is called the "closest distance" or simply the "closest distance". The term “MOD” that comes from English is also often used.
The "closest distance" of a taking lens is determined by the mechanical restrictions of the lens barrel (especially the helicoid part), which requires a movement amount equal to the "extension amount" required for shooting. In general, you can shoot at a closer distance by inserting a "close-up ring" between the lens and the camera.
Aperture range
The range of "F number" that can be used with the shooting lens is called the "aperture range".
The value with the smallest F-number is the "maximum aperture ratio" or "open aperture", which indicates the brightest aperture value. On the other hand, a large value is the F number on the "small aperture" side, but there are some that close the aperture completely.
Focus lock mechanism
The mechanism that fixes the focus so that it does not move is called the "focus lock mechanism".
The mainstream type is to fix the focus ring (distance ring) with a "lock lever" (knob screw) or a "lock screw" (small screw).
Iris lock mechanism (focus lock mechanism)
The mechanism that fixes the F number with an iris (iris diaphragm) type lens so that it does not move is called the "iris lock mechanism".
The mainstream type is to fix the aperture ring (aperture ring) with a "lock lever" (knob screw) or a "lock screw" (small screw).
Autofocus (AF: autofocus)
A system that automatically adjusts focus using sensors, control systems, motors, etc. is called "autofocus".
The "autofocus" method is roughly divided into the "passive method" that uses the image captured by the image sensor and the "active method" that uses an external sensor such as an infrared sensor.
Mount, lens mount
The part where the optical lens is attached to the camera is called the "mount" or "lens mount". Also known as a “camera mount”. It is also called a "mechanical interface" in the sense of a mechanical connection. By adopting the mount structure, various optical lenses can be used for the camera, but they cannot be used as they are without the mechanism and optical compatibility. For example, the mount structure includes "screw type", "flange type", "bayonet type", and "relay ring type", and optically "image circle" and "image size" and "flange type". Back "matching is required. ( There are also mounts such as "S mount" whose flange back is not fixed.) Currently, mounts called "C mounts" are traditionally the mainstream for machine vision cameras, but the image size of the image sensor used is used. On the other hand, various mounts are adopted. Recommended mount in accordance with the image size have been proposed in the international standardization has been achieved.
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