The function of the camera lens is to collect reflected light from a scene and to form a focused image on the camera's sensor plane. The lenses that have been chosen and fitted to your Vicon Vantage cameras offer optimal data quality and provide several view angle choices/options that are appropriate for your requirements. The type of lens that will provide optimum performance for a particular application depends on factors such as the field of view (FOV), image circle, aperture and depth of field, and any lens filter fitted. For descriptions of these, see Camera lens characteristics, which is included to provide you with a basic understanding of the most important functional lens characteristics.
The proprietary Vicon lenses designed for Vicon Vantage cameras have been custom-built for motion capture. They have a large image circle to ensure that the entire image—not just the center—is evenly illuminated. Other CMount C-Mount lenses recommended for Vantage cameras have been selected based on this criteria for image format size suitable for high-resolution sensors.
Other commercially available lenses are not necessarily designed to work with high resolution cameras; they tend to have a limited resolving power, and are more suitable for sensors with a resolution of less than two megapixels. As a result, when capturing with markers close together, the lens can have difficulty resolving the gap between the markers, causing merging even when the focus is optimal. For further details of Vantage camera lens specifications, see Vicon Vantage Vicon Vantage camera technical specifications.xref
The following topics describe the factors to consider in choosing a lens typethat affect lens types:
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The most important factor in determining the lens to use with a that distinguishes Vicon Vantage camera lenses is the total area that the camera can see, using a given lens. This is called the Field of View (FOV).
The focal length determines the angle of view (AOV) through the lens, as shown in the following diagram:
Normally, the AOV is calculated based on the known focal length and sensor size. This in turn determines the FOV, based on the distance (L) to the object being captured, as shown in the following diagram.
The sensor area used for capturing data through each Vicon Vantage camera determines the maximum FOV available for a particular combination of camera and lens. The area of the sensor is determined by its horizontal width (h) and vertical height (v).
The capture frame rate for each Vicon Vantage camera can be configured in the Vicon application software. The configured frame rate will also affect the field of view. Higher camera frame rates may make windowing of the sensor necessary, such that the sensor area used for capture is reduced in comparison to lower camera frame rates. For full details, see Vicon Vantage Vicon Vantage camera technical specifications. xrefIf you are using Vantage+ firmware or later, you can also select High Speed mode in your Vicon application software to increase the camera frame rate with little or no reduction in the field of view.
Another characteristic of the camera sensor is the image circle. This is the sharp circular image that the camera lens casts onto the sensor, as shown in the following image.
The following table gives the formulae used to calculate the angle of view (AOV) and field of view (FOV) for camera and lens combinations. These formulae require the dimensions of the sensor area used for each of the Vantage cameras at different frequencies.
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|Horizontal AOV (°)||Vertical AOV (°)||Horizontal FOVWidth H (m)||Vertical FOVHeight V (m)|
|2 x Tan-1 (h/2f)||2 x Tan-1 (v/2f)||h x L/f||v x L/f|
f = focal length of the lens (mm)
h = horizontal width of sensor (mm)
v = vertical height of sensor (mm)
L = distance from the lens to the object (m)
For sensor size details, see Vicon Vantage camera range.
As an example, consider a Vantage V16 camera with an 18mm lens operating at 120fps at a distance of 9m from the target markers. Given the V16 sensor size (18.43mm (H), 18.43mm (V)), you would calculate:
|Horizontal AOV:||2 x Tan-1 (18.43/(2 * 18)) = 54.22°|
|Vertical AOV:||2 x Tan-1 (18.43/(2 * 18)) = 54.22°|
|Horizontal FOV at 9m:||0.0183 x (9/0.018) = 9.15 m|
|Vertical FOV at 9m:||0.0183 x (9/0.018) = 9.15 m|
These formulae take into account only the optical components of the camera and lens, thus they calculate the maximum possible theoretical Field of View from this combination. Poor illumination of the volume by camera strobes, less than optimal aperture or gain settings, or poor marker surface quality might all reduce the FOV in which a camera can recognize a marker below this maximum theoretical level.
When using these formulae, note that for frame rates above the full-resolution rates given for Vantage cameras in Vicon Vantage camera performance comparison, the sensor area is reduced due to windowing, as described in Field of View.
Another significant factor in choosing a lens type that differentiates lens types is the aperture (also known as the f-stop value, f-number, and F#).
The aperture is the ratio of the focal length of the lens to the diameter of the lens opening, which determines the amount of light that can pass through the lens. Smaller f-stop values (e.g. f2, f2.8) represent wider apertures that allow more light to pass through, while larger f-stop values (e.g. f11, f16, f22) represent narrower apertures that allow less light to pass through. Typically, faster lenses have larger diameter optics that can pass more light.
Each fstop f-stop value changes the lens opening from the next fstop f-stop by a factor of 2. For example, decreasing the aperture from f11 to f8 allows twice as much light to pass through. Increasing the aperture from f11 to f16 allows half as much light to pass through.
The aperture and magnification affect the depth of field, that is, the portion of the image that has sharp focus. Immediately surrounding this area, there is a region in which the image remains in focus. Outside of this area, moving towards or away from the lens, the focus becomes progressively less sharp and the image appears out of focus. Thus, as the aperture and focal length decrease, the depth of field increases. Conversely, as the aperture and focal length increase, the depth of field decreases.
Camera lens filters
To optimize the performance of the Vantage cameras, each lens is fitted with an optical filter that attenuates wavelengths of light other than the narrow passband required to pass the light emitted by the light-emitting diodes (LEDs) from the Vantage strobe unit and reflected back from the markers to the camera.
When the cameras are used outdoors, try to position the cameras to minimize the ambient light as the filter cannot distinguish ambient light of the same wavelength as that of the strobe.