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Optical Basic Concepts - Resolution

Source:Shenzhen Kai Mo Rui Electronic Technology Co. LTD2026-04-16

Resolution is defined as the minimum distance between two object points (or image points) that can be distinguished. This is referred to as the object-side resolution of the lens, denoted as Resolution(Object), or the image-side resolution, denoted as Resolution(Image). The unit is µm. Resolution(Object) = 0.61 × Wavelength / NA = 1.22 × Wavelength × Fno. Resolution(Image) = Lens Magnification × Resolution(Object) Resolving power is defined as the number of black-and-white line pairs within 1 mm, denoted as Resolving power. The unit is line/mm or lp/mm. Resolving power = 1 / Resolution(Image) When referring to resolution, most engineers think of camera resolution while ignoring that lens resolution also plays a critical role in the final imaging performance. In fact, optimal imaging can only be achieved when lens resolution matches camera resolution. The figure below intuitively shows the influence of lens-camera matching on imaging performance.

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So how can lens resolution match camera resolution?
We know that the object is imaged onto the camera sensor.
The minimum distance between two object points imaged onto the camera sensor through the lens system can only be distinguished when it is as shown in Figure c.
If the distance between the two imaged object points on the sensor is as shown in Figure a or Figure b, the two points cannot be distinguished.
Therefore, when image-side resolution of the lens = 2 × pixel size, the lens resolution and the camera are perfectly matched.
For example, if the camera pixel size is 5 μm and the lens magnification is 0.5×:
Resolution(Object) = 2 × 5 μm ÷ 0.5 = 20 μm
At this value, the lens and camera are perfectly matched.
  • If Resolution(Object) < 20 μm: the lens is over-qualified, and camera resolution becomes the system bottleneck.
  • If Resolution(Object) > 20 μm: the camera is over-qualified, and lens resolution becomes the system bottleneck.

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In addition, engineers often confuse the object-side resolution of the lens with the accuracy of the product.
Accuracy refers to the difference between the measured value and the true value.For example, if the true value of a product is 1.0 mm and the required accuracy is ±5 μm, the product is qualified as long as the measured value falls within the range of 0.995 mm to 1.005 mm.
In machine vision, the camera resolution (e.g., 2 megapixels) is usually calculated according to the customer's FOV (Field of View) and accuracy requirements.Once the camera is selected, its pixel size (e.g., 4.65 μm) is also determined.When selecting a lens, we can choose one whose resolution matches the camera using the method described above, so as to meet the accuracy requirements of the system.
If the customer specifies that a target with an object-side size of 5 μm needs to be observed, the object-side resolution of the lens must be < 10 μm.


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