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When capturing images in visual applications, it is common to encounter scenarios where the object being photographed is not on a single plane. This brings us to the concept of depth of field. This article will discuss the definition of telecentric lens depth of field, the related parameters, and common misconceptions about it.
Depth of field refers to the distance between the farthest and nearest planes that appear clear in an image. In other words, within the telecentric lens depth of field, objects at different working distances can be imaged "clearly".
However, this definition is somewhat misleading because it does not clearly define what constitutes a "clear image". According to the optical conjugate relationship, there can only be one perfectly sharp object plane. As an object moves away from this optimal plane, the image will inevitably become blurrier. Therefore, the key to using the concept of telecentric lens depth of field lies in how "clearly" is defined.
Unlike other parameters, telecentric lens depth of field is rarely listed in a lens specification sheet. This is because it is influenced by four variables in its formula. For a given lens, the telecentric lens depth of field will differ depending on the working distance and aperture size.
The longer the focal length, the smaller the depth of field.
The smaller the F-number (larger aperture), the smaller the depth of field.
The shorter the working distance (higher magnification), the smaller the depth of field.
When it comes to their impact on depth of field, working distance has the most significant influence, followed by aperture size, and then focal length.
To achieve a greater depth of field, it is advisable to use a lens with a longer working distance, a smaller F-number, and a shorter focal length. Additionally, using a large sensor with a compatible lens can also improve the telecentric lens depth of field. It is important to note that depth of field is not a design specification of a telecentric lens but rather a result of the lens's design. Lens designers cannot directly optimize a lens based solely on depth of field as a target parameter.
When comparing the telecentric lens depth of field of lenses from different manufacturers, it is essential to ensure that the focal length, F-number, and working distance are kept identical.
If one lens shows better depth of field than another under the same test conditions, the reason could be attributed to differences in working distance, aperture, or focal length. It is advisable to check whether the aperture is falsely labeled. If all three parameters are confirmed to be the same, the lens with a better depth of field may simply have superior imaging quality. In such cases, the resolution difference, rather than the telecentric lens depth of field itself, is likely the primary factor.
While reducing the aperture size beyond the lens's optimal aperture can indeed increase the depth of field, it also amplifies the effects of diffraction. As the aperture closes further, diffraction becomes more pronounced, leading to a significant decline in image quality. In applications requiring a large depth of field, it is crucial to strike a balance between image quality and telecentric lens depth of field. Blindly pursuing a greater depth of field may result in a scenario where a lens with a 5-megapixel resolution only delivers the quality of a 1-megapixel lens, wasting the hardware investment.
Within the same depth of field range, a telecentric lens can achieve higher measurement accuracy than an FA lens, thanks to its telecentric optical path. Even when the object deviates from the optimal working distance and the image becomes blurred, the gradient transition of the grayscale remains uniform. The central position of the grayscale transition from black to white does not shift. As a result, despite the blurriness, the detected edge position remains stable, ensuring the measurement accuracy is not significantly compromised. This is often the reason why people claim that telecentric lenses have a larger depth of field. In reality, it is not that the telecentric lens depth of field is larger, but rather that it provides more accurate measurements.
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