Structure visible to the naked eye:
The lens acts as the lens in a lens system; in short, its main function is to focus light. Why focus light? For example, when you use a magnifying glass to start a fire on a sunny day, you'll find that the sunlight is focused onto a single point through the magnifying glass. In other words, to capture such a large area with a small piece of light, you have to use a lens to focus the light.
Lens internal structure
focal length
Focal length is the distance between the center point of the lens and the point on the film plane where a sharp image is formed. The focal length determines the angle of view; a smaller focal length results in a larger angle of view and a wider field of observation, while a larger focal length results in a smaller angle of view and a narrower field of observation. Based on whether the focal length is adjustable, lenses can be divided into two main categories: prime lenses and zoom lenses.
Imaging conditions: Focal length < Image distance < 2 times focal length
aperture
The aperture is a device used to control the amount of light passing through the lens and entering the sensor inside the camera body; it is usually located inside the lens.
Depth of field
Depth of field (DOF) is the range of sharp images that are visible before and after the point of focus.
Aperture, lens, and distance to the subject are important factors affecting depth of field.
1. The larger the aperture (f-number), the shallower the depth of field; the smaller the aperture (f-number), the deeper the depth of field.
2. The longer the focal length, the shallower the depth of field, and vice versa.
3. The closer the subject, the shallower the depth of field; the farther the subject, the deeper the depth of field.
exposure
It refers to the amount of light that enters the lens and shines on the image sensor during the photography process, which is controlled by the combination of aperture, shutter speed, and ISO.
Field of view
In optical engineering, the field of view is also called the field of view. The size of the field of view determines the field of view range of an optical instrument.
Generally, the larger the field of view, the wider the field of view; the smaller the field of view, the narrower the field of view.
The field of view varies with focal length; the closer the focal length, the larger the field of view, and the farther the focal length, the smaller the field of view.
resolution
Resolution represents a lens's ability to record detail, measured in lines per millimeter (lp/mm) as the number of black and white lines that can be distinguished. Higher resolution lenses produce sharper images.
Numerical aperture
Numerical aperture is equal to the product of the refractive index *n* of the medium between the object and the objective lens and the sine of half the objective lens aperture angle (a²). The formula is NA = n * sin a/2. Numerical aperture is closely related to other optical parameters; it is directly proportional to resolution and magnification. In other words, numerical aperture directly determines lens resolution; a larger numerical aperture results in higher resolution, and vice versa.
Back burn
To be precise, back zoom is a camera parameter referring to the distance from the camera's interface plane to the sensor. However, it's a crucial parameter when selecting lenses for in-line scanning lenses or large-format cameras, as it directly impacts lens configuration. Even cameras from different manufacturers, despite having the same interface, may have different back zoom levels.
Function of the joint ring
Adding or not adding a lens stop will not affect the camera's focal length; focal length is an inherent property of the lens.
Camera extension tubes can increase the distance between the focal plane and the lens, allowing the lens to capture objects closer, similar to macro photography, resulting in larger images.
distortion
Generally speaking, lens distortion is actually a general term for the inherent perspective distortion of optical lenses, which is distortion caused by perspective. This distortion is very detrimental to the image quality of a photograph, but because it is an inherent characteristic of lenses (convex lenses converge light rays, concave lenses diverge light rays), it cannot be eliminated, but can only be improved.
pincushion distortion
Pincushion distortion, also known as pincushion image distortion, is a phenomenon caused by the lens causing the image to "shrink" towards the center. It is most noticeable when using telephoto lenses or the telephoto end of zoom lenses.
Barrel distortion
Barrel distortion, also known as barrel image distortion, is a distortion phenomenon caused by the physical properties of the lens and the structure of the lens elements, resulting in an image that appears as a barrel-shaped expansion. Barrel distortion is most easily noticeable when using wide-angle lenses or the wide-angle end of zoom lenses.
Telecentric lens
Telecentric lenses are primarily designed to correct parallax in traditional industrial lenses. They maintain consistent image magnification within a certain object distance range, which is crucial for applications where the measured objects are not on the same plane. Due to their unique parallel optical path design, telecentric lenses have long been favored in machine vision applications with stringent requirements for lens distortion correction.
The characteristic of a telecentric lens is that almost all the light entering the lens is strongly reflected light.
Imaging effect of non-telecentric lens:
Telecentric lens imaging effect:
Telecentric lenses primarily compensate for the following characteristics of non-telecentric lenses;
1) The difference in magnification occurs because the objects being measured are not on the same measuring plane;
2) Large lens distortion
3) Parallax refers to the change in magnification of an object as the object distance increases;
4) The lens resolution is not high;
5) Uncertainty in the position of image edges due to the geometric characteristics of the visual light source.
Advantages of telecentric and non-telecentric systems respectively:
Advantages of ordinary lenses: low cost, practicality, and wide range of uses.
Disadvantages of ordinary lenses: magnification will change, and there is parallax.
Applications of ordinary lenses: Imaging large objects.
Advantages of telecentric lenses: constant magnification, does not change with depth of field, and no parallax.
Disadvantages of telecentric lenses: high cost, large size, and heavy weight.
Applications of telecentric lenses: in metrology, CCD-based measurements, and microcrystals.
When inspecting an object, it is best to use a telecentric lens in the following six situations:
1) When it is necessary to detect objects with thickness (thickness > 1/10 FOV diameter);
2) When it is necessary to detect objects that are not on the same plane;
3) When the exact distance from the object to the lens is unclear;
4) When it is necessary to inspect objects with apertures and in three dimensions;
5) When low distortion and near-perfectly consistent image brightness are required;
6) When the defect can only be detected under parallel illumination in the same direction.
