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Five Factors in choosing the Right Lens for your Imaging System

Collection of light from the scene and project onto the image sensor.

  • Choosing the right lens for the size of the sensor ensures that light is directed by the lens onto the entire surface of the sensor so that it is fully illuminated.

  • Ensure the image circle of the lens matches or is larger than the sensor in order to prevent unwanted, shading, vignetting or dark image areas and will keep the image quality consistent. 

  • Lenses and image sensors are described in either inch format 1/2″,
    2/3″ etc. or by a direct measurement in millimeters

  • The maximum image circle is determined by the sensor, therefor a larger
    lens does not provided any additional value and is generally an unnecessary

 Sensor size

Collection of light from the scene and project onto the image sensor.

  • Right lens should always be selected based on the sensor’s resolution, the pixel size.
  • Resolution for a lens and its resolving power are not the same when selecting a lens.
  • To calculate the pixel size, the following formula can be used:
    Pixel size = sensor width × sensor height √ resolution
  • For Example,  Kowa JC5M2 Series lens described below is five Megapixels. We can work out the resolving power in terms of pixels. *Excerpt from a data sheet sensor surface = 8.8 mm × 6.6 mm = 58.1 mm2, pixel surface = 58.1 mm2         58.1mm/ 5 MP = 11.6 µm2. With square pixels: pixel edge length = 3.45 µm

Working distance and field of view

Every lens has an optimal operating distance range.

  • Often referred to in data sheets as the “recommended working distance”

  • If the lens is mounted at the indicated distance to the object being captured (or scene being observed),the maximum image quality will be achieved and a lack of definition is avoided.

  • Another important factor to consider when choosing a lens is the size of the object, field of view (FOV), and is a measurement of the area to be captured by the camera at the selected working distance.

  • The FOV of a lens and sensor combination is determined by a lens’ focal length which is expressed in millimeters and can be calculated using the formula
  • For example, to observe a 2-meter high object from 5 meters with a sensor that is 6.6 millimeters high, a camera with a focal length of 16mm     (6.6 / 2000 × 5000 = 16.5) will be needed.

Optimum illumination and aperture

Ambient light, its intensity, color temperature or specific wavelength will all impact a lens selection.

  • Inside the lens, a variable aperture controls the amount of light that falls onto the sensor.

  • Just as with the pupil in the human eye, varying the aperture will control the amount of light that can pass through on to the sensor. 

  • The term F-number describes this variable aperture. 

  • This dimensionless number is the ratio of the focal length to the effective aperture diameter (entrance pupil,Dep) of the lens.

Types of lens

  • Lenses are generally classified according to the size of their thread or “mount”.

  • Industrial applications commonly use C-mount lenses, which have
    a thread of 1″ diameter.

  • These are suitable for sensors up to 4/3″ or for sensor diagonals up to 22.5 mm. For larger sensors, full format or longer linear array sensors, F-mount lenses are used.

  • CS-mount lenses, like C-mount lenses, have a thread diameter of 1″ and have a short “back focal distance” (distance between the sensor and the lens)

  • S-Mount or M12 lenses are also popular. They are often only a few millimeters to several centimeters high. For smaller sensors (1/2″, 1/3″ or less), an S-mount lens will be well suited.

  • M12 lenses on the other hand, generally have short focal length lenses, which are often referred to as a “fish-eye” lens. They have a set aperture and are suitable for
    covering wide fields of view of 160° or even 180°. M12 lenses are frequently
    used in mobile devices, and in the automotive and security sectors.

  • “Ruggedized” lenses are another type of specialized lens used in the industrial sector. They have been designed for particularly harsh environments where there is a large degree of movement or vibration. These “shock-proof ” lenses can achieve accurate and stable results even in applications that require subpixel-accurate measurements, such as 3D measuring systems for car body construction and prototype production.
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