Lens Selection - Important factors to consider
Camera system is incomplete and not usable without the right lens systems. As we know, lens system is responsible for focussing the light from the scene onto the imager surface. The right lens enables us to get the images or videos with the right clarity, information and depth.
So, how do we choose the right lens?
The main features on the lens that needs to be understood are:-
1. Lens format
2. Focal length
3. Field of View
4. Lens Mount
Lens format indicates the lens’ match with the image sensor format. Ideally, we need to ensure that the sensor image format and the lens format matches. However, there are scenarios where we do not get exact match when we bring in lens specification requirement such as FOV, Focal length, resolution etc. When we do not get an exact match, then we should follow the below thumb rule
→ Larger Format lenses can be used on smaller format sensor.
→ Smaller format lenses cannot be used on larger format sensor.
<IMAGE LENS SENSOR CIRCLE>
For example, should you be choosing the IMX290 Sony sensor based camera module, then we have an understand that the sensor size is 1/2.8”. The lens we choose should be of lens format of either 1/2.8” or larger such as 1/2.5” or 1/2.3” etc.
Another example we can look at is the AR0521 On Semiconductor sensor based camera module. The sensor size in this case is 1/2.5”. We should work on selecting a lens that has lens format of either 1/2.5” or larger such as 1/2.3” or 1/2" etc.
Focal length is an optical property of the lens. It indicates the distance between the optical center of the lens and the imager plane. As you know, lens is not a single piece of glass. Rather, it is a combination lens elements which enables us to focus the light and cut down on distortions. The point where the light rays converges to form a sharp image is termed as optical center.
<IMAGE - FOCAL LENGTH>
Focal length is an indicator of FOV. The higher the focal length, the narrower the FOV and vice versa.
Field of View
When we discuss about cameras and lens, we come across two terms that are sometimes used interchangeably. They are angle of view and field of view. We would like to make it clear that these terms have 2 different meaning when we look at the camera system as a one entity.
Angle of view indicates is optical terminology. It indicates the angle that can be covered by the lens assembly when capturing a scene. It does not change with respect to the camera system.
However, Field of View is a camera specification. The field of view indicates the region of the scene that can be captured onto the sensor image after being focused by the lens. Angle of view and field of view are the same when the lens format and the sensor format matches. However, then the lens format is greater than the sensor format, we find that field of the view of the camera is lower that angle of view of the lens. The vice versa is not recommended as it results in wastage of sensor area.
Let’s say, we choose a sensor with lens format of 1/3” and angle of view of 80 degrees. When we use it with a sensor with sensor format of 1/3”. We are looking at Field of View of 80 degrees. However, when we use the same lens with a sensor of 1/3.2”, then we are looking at Field of View lower than 80 degrees.
When we read through lens specifications, we come across the spec termed Lens Mount. We find terms such as C Mount, CS Mount, M12 or S mount, M8, M7, M6 and M4. These are based on the barrel diameter of the lens. Each of these standards have portfolio of lenses catering to different FOVs, Focal lengths, Depth of field etc. The most commonly used standards are the C Mount, CS mount and S Mount. The M6 format are commonly used on the mobile phone or tablet focused camera modules.
We shall be able to guide you to the right lens mount standards that fit your mechanical requirements.
This covers the major criteria that defines which lens to use for a particular solution. Engineers at Regami solutions shall be able to assist you with more details.