Contrast
The difference in brightness from the dark part to the bright part of the image is called "contrast". For example, when a black-and-white striped chart with clearly separated brightness of 0 for black and 255 for white is taken as a subject, the image formed by the machine vision lens has a dull black-and-white edge due to aberration, although there are some differences. It will be a gray gradation. In this case, the original brightness level cannot be secured, and the difference in brightness between black 50, white 205, etc. will decrease.
MTF (modulation transfer function)
There is "MTF" as an index showing the performance of the machine vision lens.
"MTF" is the "modulation transfer function characteristic" and indicates how much the lens can reproduce the contrast when the "spatial frequency" changes.
Conventionally, "resolution" has been used to indicate the resolution of a machine vision lens, but in recent years, "MTF characteristics" have become more common. Originally, the resolution and MTF cannot be simply converted, but it is judged that "if the contrast ratio in MTF is 20% or more, the resolution is achieved". Ten
Distortion, distortion
"Distortion" is one of the "Seidel's five aberrations", and is an aberration in which the shape of an object and an image do not become the same (similar shape). For example, a rectangular subject is distorted into a barrel or pincushion shape. (Refer to "2.1 Aberration")
Distortion has the characteristic of being "proportional to the cube of the angle of view", so it is noticeable in "wide-angle lenses" with a large angle of view. In this case, it shrinks toward the periphery (the optical magnification becomes smaller), resulting in barrel distortion. On the other hand, if it extends toward the periphery (the optical magnification increases), it becomes a pincushion type distortion.
Distortion is largely due to the lens type (lens configuration, placement method of each lens), so even if you stop down, it will not improve. In recent years, image processing technology has been used to correct distortion.
Peripheral illumination
"Peripheral illumination" literally means the amount of light in the peripheral part of the screen, but usually it means the "peripheral illumination ratio" that indicates the difference from the central part of the screen.
As a characteristic of a machine vision lens, the brightness of the peripheral part tends to decrease with respect to the central part of the screen, and this is called "peripheral light falloff".
The cause of this "peripheral light falloff" is the phenomenon called "Cosine 4th power rule" that darkens the image of light entering from an angle, and the phenomenon called "aperture erosion" that is kicked at the periphery of the lens in the lens barrel. There is something to do.
It should be noted that it is not caused by the
machine vision lens, and there is a phenomenon that the amount of peripheral light decreases depending on the "incident angle to the image sensor". In this case, use a lens with an "exit pupil position" corresponding to the image sensor. Will be improved.
Depth of focus
The taking lens works to connect an object as an image, but at this time, it is necessary to "image a point as a point". There is only one point that is intuitively in focus, and you can imagine that the "blurring" increases as you move away from it. However, in an actual camera system, when viewed in the depth direction along the optical axis, the range of focus is not one point, and there is a certain range that can be regarded as not out of focus, which is called "depth of focus". increase.
The "depth of focus" is determined by the F number of the machine vision lens and the pixel pitch of the image sensor. The width of the depth of focus becomes narrower for a lens with a smaller F number (brighter) or a sensor with a smaller pixel pitch.
Depth of field (DoF)
The "depth of focus" is near the image sensor inside the camera, but there is a certain range on the subject side that can be regarded as not being out of focus, and this is called the "depth of field".
Items within the "depth of field" "look in focus without blurring", so you can effectively use the depth when shooting a deep subject.
Like the "depth of focus", the "depth of field" is determined by the F number and the pixel pitch of the image sensor, but it is also related to the optical magnification. The smaller the F-number (brighter), the smaller the pixel pitch of the sensor, and the larger the optical magnification (enlargement), the narrower the depth of field. A narrow depth of field is called "shallow depth", and a wide width is called "deep depth".
The "depth of field" is calculated, but the commonly used formula using the "allowable circle of confusion diameter" calculates the "range where the points are not blurred", that is, the shallowest depth. In actual use, it is thought that it will be slightly larger than the calculated value because there is an "acceptable amount of blur" depending on the shape of the subject, required resolution, visual inspection, image processing, and other usage methods.
Allowable circle of confusion (CoC)
There is "allowable circle of confusion diameter" as a parameter used when calculating "depth of focus" or "depth of field". The "circle of confusion" here is a circular image when points are imaged by the lens, but the smallest size that the image sensor cannot recognize as blur is especially called the "acceptable circle of confusion diameter". ..
The "allowable circle of confusion diameter" is determined by the optical imaging limit of the lens called the "pixel pitch" or "Airy disk diameter" of the image sensor, and the larger the "pixel pitch" or "Airy disk diameter" is, the more " It becomes the "allowable circle of confusion diameter".
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