Machine Vision Lens

Prime lenses
Telencentric lenses
Large-format line scan camera lenses
line-scan telecentric lenses
Customized lneses

Introducing Machine Vision Lens

Unlike photography where there are macro, wide angle, telephoto and standard lenses, in Machine Vision the choice is mainly limited to standard lenses with fixed focal lengths (brighter) or variable focal lengths (zoom) and telecentric lenses (which limit the effects of parallax in the perspective image).

Aperture

The aperture is simply the size of the diameter of the diaphragm that opens to allow light to pass through the lens. The larger it is, the more light passes through and vice versa. The aperture of the diaphragm, which is expressed in < f >, also affects another fundamental concept, the depth of field (area of sharpness in the image). Beyond this area, the rest of the image becomes more or less blurred.

The larger the aperture, the smaller the area of sharpness and vice versa.

The smaller the aperture, the larger the area of sharpness.

Where it gets a little more complicated, is that the aperture of the diaphragm is the result of a ratio 1/X yet on the machine vision lens, it is written in X. Result, a small aperture is equivalent to large numerical values and vice versa. So when we talk about large aperture, it is a < f > small and vice versa. It's just gymnastics to integrate. What you need to remember is that the larger the aperture you choose (a < f > small), the shallower the depth of field, i.e. only a small part of the image will be in focus, the one you have focused on. Conversely, the smaller the aperture you choose (a large < f >), the greater the depth of field, meaning that a large part of the image will be in focus, the foreground as well as the background.

Small aperture :

f/5,6

f/6,7

f/8

f/11

f/16

f/22

Wide aperture :

f/1,4

f/1,8

f/2,8

f/3,5

f/4,5

But be careful: the more the diaphragm is closed (small aperture and large depth of field), the more the exposure time of the camera (shutter) must be important or the more the ambient lighting must be powerful to be able to let pass the same quantity of light. For machine vision applications, objects are generally in motion, the image capture must be fast to acquire a clear image. It is therefore advisable to use as much as possible very high luminosity lighting to have a large depth of field and a short exposure time (shutter).

For a machine vision lens it is customary to announce the maximum aperture.

Example for a fixed focal length lens: 17mm f:1.4 means that the maximum aperture is f:1.4 ("very bright lens")

Example for a machine vision zoom lens: 70-200mm 3.5-4.5 means that the maximum aperture for a 70mm focal length is 3.5 and that the maximum aperture for a 200mm focal length is 4.5.

The focal length

The focal length allows you to adjust the field of view according to the distance of observation. The greater the focal length of the lens, the closer we get to the subject without changing our observation position. Conversely, the shorter the focal length of the lens, the further away from the subject we are without changing our position of observation. The lenses with long focal lengths are called "telephoto" (focal length greater than 80 mm) and the lenses with short focal lengths are called "wide angle" (focal length less than 70 mm)

Long focal lengths = tight framing, close subject, crushed perspective and shallow depth of field.

Short focal lengths : wide framing, distant subject, stretched perspective and large depth of field.

In industrial vision, the focal lengths used are mainly 8mm, 16mm, 25mm and 35mm because the camera or cameras must be close to the product to be inspected. Some more specific applications may require a lens with focal lengths below 8 mm (4.8 mm, 6 mm etc.) or above 35 mm (50 mm, 100 mm etc.).

The focal length written on the machine vision lenses depends on the size of the sensor, in photography we often speak of "24x36" or "35 mm" equivalent.

Telecentric lenses

With a conventional lens, an object that is closer to the lens (short working distance) will appear wider than the same object that is further away (long working distance). Telecentric lenses are lenses with no perspective or parallax error. The absence of perspective error makes them ideal for metrology applications. Telecentric objectives generally offer better image quality and have a magnification range of 0.08 to 6x.

Extension rings

The extension rings also called extension tubes or spacers, are fixed between the lens and the camera. They exist in different sizes: 0.5 mm, 1 mm, 2 mm, 5 mm, 10 mm, 20 mm and 40 mm and can be combined (e.g. BA of 6 mm = 1 BA of 5 mm + 1 BA of 1 mm). They allow the camera-lens to be physically closer to the object to be controlled while remaining sharp on it, therefore the field of view is significantly reduced. Indeed, the objectives focus on an object from a minimum distance of work which can correspond to a field of vision wider than necessary and / or a distance and a size too large.

The extension rings reduce the minimum working distance, reduce the field of view, reduce the size of the vision system.

The mounts

The lens mount used for industrial cameras is usually a C mount. It connects to a camera via a 1''x32 thread and has a collar distance of 17.5 mm. The CS mount (or C Short) has the same physical thread as a C mount but has a flange distance of 12.5 mm. The F mount or 42/72 is used for large sensors such as matrix or line scan cameras of high resolution.