The \u201cfocal length\u201d of a lens is the distance between the \u201creference point for imaging by the lens\u201d and the \u201cfocus of parallel light coming from a point source at infinity on the optical axis\u201d, which is a value unique to the lens. It doesn\u2019t change. The \u201cpoint that serves as the reference for imaging by the lens\u201d means \u201cthe point that can be regarded as the center of the lens\u201d. The focal length is the most important item for showing the lens specifications because it determines the field of view and angle of view\u00a0.\u00a0It is written as a lower \u201cf\u201d.<\/p>\n
The photographic lens has parts with \u201choles\u201d for adjusting the light entering the lens.\u00a0This \u201chole\u201d or \u201cpart with a hole\u201d is called an \u201caperture\u201d.\u00a0Basically, 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.<\/p>\n
The aperture plays an important role in determining the quality of the image imaged by the lens.\u00a0For example, a large aperture can capture a large amount of light, and a small aperture can reduce the amount of light.\u00a0Also, the way the image is blurred changes depending on the position and size of the aperture.
\nIn imaging with a lens, the ray that passes through the center of the aperture is called the \u201cmain ray\u201d, 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.<\/p>\n
Of the \u201cdiaphragms\u201d, the ones that can continuously change the size of the holes around the optical axis are called \u201ciris\u201d or \u201ciris diaphragm\u201d.\u00a0The \u201ciris\u201d is the \u201ciris\u201d of the eyeball, and has a structure that is convenient for adjusting the amount of light.
\nThe iris is composed of several \u201caperture blades\u201d, 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.\u00a0On the other hand, low-cost lenses have few diaphragm blades, and there are also two or three lenses.<\/p>\n
The \u201cF number\u201d, \u201cF value\u201d, or \u201cdiameter ratio\u201d 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.\u00a0These show the same thing, and are calculated by the ratio of the focal length to the size of the aperture.\u00a0The F value is written in uppercase \u201cF\u201d to distinguish it from the lowercase \u201cf\u201d that indicates the focal length.
\nThe smaller this value is, the brighter the lens is, and this is because it can capture a lot of light.\u00a0On the other hand, a lens with a large value is called a \u201cdark lens\u201d.\u00a0Bright 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.<\/p>\n
Generally, lens specifications are indicated by \u201cfocal length\u201d and \u201cbrightest F-number\u201d, but this \u201cbrightest F-number\u201d is referred to as \u201copen F-number\u201d, \u201copen aperture\u201d, and \u201cmaximum aperture ratio\u201d.<\/p>\n
In a general\u00a0shooting lens<\/a>, the F number is indicated by a geometric progression of \u221a2 such as F1.4, F2, F2.8, F4, F5.6 \u2026, and the value becomes one step larger (\u221a2 times). The amount of light in the image is halved for each. With \u201ciris\u201d type lenses, the F number can be adjusted with the aperture ring (aperture ring), and halving the amount of light (1\/2) is called \u201cone aperture stop\u201d.<\/p>\n The aperture can adjust not only the brightness but also the range of focus called \u201cdepth of focus\u201d, and when you stop down, the range of focus becomes longer (deeper).\u00a0However, since the effect of the diffraction phenomenon called \u201csmall aperture blur\u201d 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.<\/p>\n A general photographic lens is composed of a combination of multiple \u201csingle lenses\u201d 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 \u201ctransmittance\u201d of 100%.<\/p>\n \u201cF number\u201d 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, such as\u00a0cinema lenses<\/a>, are displayed with a \u201cT number\u201d that takes \u201ctransmittance\u201d into consideration in the \u201cF number\u201d to clarify the brightness of the image.<\/p>\n A \u201cbright lens\u201d with a small \u201cF number\u201d is called a \u201clarge aperture\u201d or \u201clarge aperture lens\u201d. Shooting from a short distance (close-up distance) is called \u201cclose-up shooting\u201d or \u201cclose-up\u201d in order to shoot a large subject, and a \u201cmacro lens\u201d is used.<\/p>\n The periodic length of light that travels through space is called the \u201cwavelength.\u201d Light in the wavelength range visible to the human eye is called \u201cvisible light\u201d.\u00a0The range of \u201cvisible light\u201d is not clear, and it is said to be about 400 to 760 nm. Read more:<\/p>\nT-number<\/h4>\n
Large-aperture, large-aperture lens<\/h4>\n
\nIn recent consumer digital cameras, the number of large-diameter lenses that demand greater \u201cblurring\u201d is increasing.<\/p>\nClose-up<\/h4>\n
Wavelength<\/h4>\n
\nSince \u201cwavelength\u201d 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.<\/p>\nVisible light<\/h4>\n
\nLight 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.<\/p>\n