Selecting the Best Lens for Desired Field of View
Which lens should I use for my 3D scanning application?
When scanning an object in 3D, it is important to determine if the hardware you are using is best suited for the size of the object you are scanning. Lenses in particular will influence the 3D scanner’s field of view. Field of view is the observable area that an optical instrument is able to capture. Lenses with different focal length will create different field of view. The most common focal lengths for lenses are 9mm, 12.5mm, 16mm, and 25mm lenses. Sometimes 6mm and 35mm lenses are also used.
The field of view for a particular 3D scanner depends largely on the two factors: 1) lens being used 2) the distance between the 3D scanner and the object. There is no perfect lens to use; it just depends on how much you want the 3D scanner to capture from a certain distance. At the same distance a lens with a small focal length (mm) allows you to see a wider field of view whereas one with a large focal length allows you to see a narrower field of view but with more detail.
To determine the field of view use the calculator here: http://www.machinevision.ca/fieldofviewcalculator
You will need to provide:
- Focal length: Focal length is the distance from the camera sensor to the lens, expressed in mm.
- Distance to the object: Distance is how far the lens is from the object.
- Image sensor: Equipment that converts an optical image to an electric signal. At 3D3 Solutions, our 3D scanners use image sensors that are ½”.
Here are some general rules we use in our 3D scanning lab:
- 9mm lens: Scans a torso or a full human body. We use this type of lens when we don’t have a lot room to scan and works well for scanning objects with a large area.
- 12mm lens: Scans human faces approximately 50cm away from the scanner.
- 16mm lens: Scans small objects such as a computer mouse well from approximately 30cm away from the scanner.
- 25mm lens: Scans smaller objects such as dental impressions
- 35mm lens: Scans tiny objects such as coins
It might appear that it would be better to use a lens with a smaller focal length. However, there is an inverse relationship between focal length and lens distortion. For example, the smaller the mm, the larger the distortion you tend to get when scanning an image. All lenses have a certain amount of distortion that is unavoidable but our 3D scanning software, FlexScan3D, compensates for lens distortion. While there is popularity in using zoom lenses, they have a tendency to create more distortion and are more expensive than non-zoom lenses. We only recommend using non-zoom lenses whenever possible.
For 3D scanning using a two camera system (DUO Scan in the case of FlexScan3D), you will also need to move the cameras further apart as your effective field of view grows. Generally, the cameras’ distance from each other affects 15% of the field of view. Smaller focal length lenses require your cameras to be further apart than lenses with larger focal length. For a small scanning window at 10cm, you normally need to have the cameras 1.5cm apart. For a 2 meter scanning window, your cameras should be at least 30cm apart.
If you purchase your own lenses for your 3D scanner, another factor to consider is each lens has an effective focal range. Focal range is the distance from a lens to its focus. Some are set to focus on objects from 1 meter to 2 meters, while others are set to focus from 1cm to 10cm. Most scanning projects require a focal range of 5cm to approximately 1.5 meters but experimenting with different settings is key.
You will need to do some testing for your particular application in order to get the desired field of view and therefore it is recommended that users should purchase a variety of lenses for various scanning applications.
