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Optic

Objectiv / lens


In order for an object to be captured by a camera, a lens is required. Depending on which lens system is installed in a lens, a corresponding image is imaged by the reflecting light rays on the image plane of the camera chip. 

The mapping is influenced by several factors:

  • lens radii
  • Distance between lenses
  • Distance between lens and object (working distance)
  • Distance from lens to sensor

The relationship between the image (object image on the sensor) and the object itself is called the magnification.

How a lens works

  

The magnification is defined by the selection of the focal length of the lens. This means the ratio between the size of the image of the object to the object itself.

There are two types of lenses, the fixed focal length lens and the zoom lens.

With a fixed focal length lens, the lens systems are mounted at a fixed distance to each other. They have a stable minimum working distance (MOD). Due to the fixed focal length, these lenses have a relatively fixed magnification range.

In image processing, working distance, and magnification are usually stable and not changed, so fixed focal length lenses are mainly used.

Zoom lenses have a focusing unit, these lenses change their focal length by moving a part of the lens. Zoom lenses are often mechanically less stable and are therefore usually not suitable for exact measurements .

Criteria for optics

The choice of the right lens depends directly on the sensor of the camera used. The parameters chip size and pixel size are important with regard to the selection.

 • Sensor size - image circle diameter

    the lens must illuminate the entire sensor surface to avoid vignetting (shading) in the image.

• Pixel size - optical resolution

   The better the optical resolution of the lens, the finer the structures can be transmitted.

• Lens resolution - Image scale

To be able to better resolve details of an object and to have a clear edge recognition, these details should be mapped to approx. 4 pixels.

 

Sensor size:

The lens must be designed at least for the sensor size (always specified in inches), such as the size of the camera sensor used. However, a camera with a smaller sensor can also be used.

 

In addition, there are still terms that play an important role in connection with lenses.

 

Focal length:

The focal length of an optical system is the distance between the optical reference plane (main plane) and the point at which the light rays (infinity) are bundled in a single point (focal point).  Behind this point is the image plane on which the camera sensor is located.

  

Magnification:

The ratio of image to object size describes the magnification of the optics and is directly related to the focal length of the lens and the working distance.

 

Field depth:

Depth of field is a measure of an area in which the image of the object is considered to be sharp. This is influenced by many factors, but mainly by the aperture selected. The smaller the aperture, the greater the depth of field.

 

   


Vignetting


Homogeneity of illumination

All images have an intensity drop, starting from the center of the image to the edge. The causes of this phenomenon include the lenses. With regard to this influence, the following must be taken into account when selecting the lens.

With any lens, the image brightness drops towards the edge. The reason for this can have two causes. The first reason is the optics itself and the second reason can be the sensor.

Mechanical vignetting

With mechanical vignetting, the beam path in the edge area is mechanically dimmed by limiting mount parts of the optics. This causes a shadowing of the edges of the image. Shading usually occurs when the image circle or the size of the optics is too small for the sensor.

Vignettierung

Cos4-Vignetting

All optics are influenced by the so-called Cos4 vignetting. The reason for this is the physically caused loss of light due to longer distances and flat angles in the edge area.


Sensor shadowing

If microlenses are used, the incident light will collect and focus on the active pixel area. However, there are also conditions that allow the incident light to fall on the sensor at a flat angle and refracts part of the light so that it no longer falls on the active part of the sensor.

Sensorabschattung

 

 

 

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