The “focal length” of a lens is the distance between the “reference point for imaging by the lens” and the “focus of parallel light coming from a point source at infinity on the optical axis”, which is a value unique to the lens. It doesn’t change. The “point that serves as the reference for imaging by the lens” means “the point that can be regarded as the center of the lens”. The focal length is the most important item for showing the lens specifications because it determines the field of view and angle of view . It is written as a lower “f”.
Aperture (stop, aperture stop)
The photographic lens has parts with “holes” for adjusting the light entering the lens. This “hole” or “part with a hole” is called an “aperture”. Basically, a round hole shape diaphragm is good, but there are also polygonal hole diaphragms such as hexagons due to the structure for changing the hole size.
The aperture plays an important role in determining the quality of the image imaged by the lens. For example, a large aperture can capture a large amount of light, and a small aperture can reduce the amount of light. Also, the way the image is blurred changes depending on the position and size of the aperture.
In imaging with a lens, the ray that passes through the center of the aperture is called the “main ray”, but since it shows the basic imaging state of the lens as the luminous flux when the aperture is stopped down the most, it is the reference ray at that angle of view.
Of the “diaphragms”, the ones that can continuously change the size of the holes around the optical axis are called “iris” or “iris diaphragm”. The “iris” is the “iris” of the eyeball, and has a structure that is convenient for adjusting the amount of light.
The iris is composed of several “aperture blades”, but since it is optically advantageous for the aperture shape to be close to a circle, the number of diaphragm blades is large for lenses aiming for high performance. On the other hand, low-cost lenses have few diaphragm blades, and there are also two or three lenses.
F number, F value, aperture ratio
The “F number”, “F value”, or “diameter ratio” is used as an index to indicate the amount of light that the shooting lens can capture or the brightness of the image that is focused by the shooting lens. These show the same thing, and are calculated by the ratio of the focal length to the size of the aperture. The F value is written in uppercase “F” to distinguish it from the lowercase “f” that indicates the focal length.
The smaller this value is, the brighter the lens is, and this is because it can capture a lot of light. On the other hand, a lens with a large value is called a “dark lens”. Bright lenses can shoot with faster shutter speeds and shorter exposure times, but they are relatively expensive due to the difficulty of correcting aberrations in terms of optical design.
Generally, lens specifications are indicated by “focal length” and “brightest F-number”, but this “brightest F-number” is referred to as “open F-number”, “open aperture”, and “maximum aperture ratio”.
In a general shooting lens
, the F number is indicated by a geometric progression of √2 such as F1.4, F2, F2.8, F4, F5.6 …, and the value becomes one step larger (√2 times). The amount of light in the image is halved for each. With “iris” type lenses, the F number can be adjusted with the aperture ring (aperture ring), and halving the amount of light (1/2) is called “one aperture stop”.
The aperture can adjust not only the brightness but also the range of focus called “depth of focus”, and when you stop down, the range of focus becomes longer (deeper). However, since the effect of the diffraction phenomenon called “small aperture blur” increases when the aperture is stopped down, it is not possible to take advantage of the resolution with a camera with a small pixel pitch, so be careful not to stop down too much.
A general photographic lens is composed of a combination of multiple “single lenses” made by processing a glass plate into a spherical surface, but each glass material (optical glass) absorbs light to some extent or reflects light on the lens surface. It is not completely transparent with a “transmittance” of 100%.
“F number” is an index that determines the amount of light (brightness) that can be taken in by the lens, but since the transmittance of the glass material is not taken into consideration, it is not possible for different shooting lenses to have the same image brightness even with the same F number. Not limited. For this reason, some lenses for strict exposure are displayed with a “T number” that takes “transmittance” into consideration in the “F number” to clarify the brightness of the image.
Large-aperture, large-aperture lens
A “bright lens” with a small “F number” is called a “large aperture” or “large aperture lens”.
In recent consumer digital cameras, the number of large-diameter lenses that demand greater “blurring” is increasing.
Shooting from a short distance (close-up distance) is called “close-up shooting” or “close-up” in order to shoot a large subject, and a “macro lens” is used.
The periodic length of light that travels through space is called the “wavelength.”
Since “wavelength” also affects the sensitivity of the image sensor and the quality of the captured image, machine vision uses ultraviolet light (ultraviolet light) and infrared light (infrared light) in addition to visible light.
Light in the wavelength range visible to the human eye is called “visible light”. The range of “visible light” is not clear, and it is said to be about 400 to 760 nm.
Light in the visible light range appears to be separated into rainbow colors depending on the wavelength, but the shorter wavelength side is violet (violet), and the longer the wavelength, the more blue, green, yellow, orange, and red appear.