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In thermal and night vision devices equipped with laser rangefinder (LRF), users often encounter an issue that when increase the magnification to high levels, the LRF indicator (a yellow box used to mark the target for range measurement) is no longer visible on the display. This document offers a technical explanation of why this occurs.

Understanding the Field of View (FOV) and Magnification

To explain this issue, it's important to understand the definitions of magnification and field of view (FOV) of an imaging device.

· Magnification: Primarily refers to the digital or optical enlargement of the image received by the device. When magnification increases, it enlarges the central portion of the existing field of view onto the screen, rather than adding new content or expanding the visible area.

· Field of View (FOV): The FOV represents the observable area that the device can display. As magnification increases, the device’s field of view becomes narrower, reducing the visible area. Conversely, when magnification decreases, the field of view expands, showing more of the scene.

In summary, increasing the magnification allows for clearer observation of details in the center of the image, but the overall field of view narrows, causing peripheral image to gradually disappear.

Why the Indicator Disappears

1. The physical location of LRF

In an ideal scenario, the rangefinder indicator should be precisely located at the optical axis center of the device (x=0, y=0). However, due to the typical installation of the rangefinder module adjacent to the lens, there often exists a physical deviation between the location of the rangefinder indicator and the optical center. Such misalignment is challenging to avoid during the design and manufacturing processes, resulting in the rangefinder indicator not being perfectly aligned with the optical axis.

2. Narrowing FOV at High Zoom

As the magnification increases, the device's field of view narrows accordingly. When zooming in the image, the device only displays the central portion of the image, while areas near the edges of the field of view are cropped out. If the rangefinder indicator is already off-center in the initial field of view, it will move further away from the center of the screen as the magnification increases, eventually disappearing from the visible area. This explains why the rangefinder indicator vanishes on the screen at high magnifications. This phenomenon is similar to using a telescope to zoom in on an object, where only the central portion of the field can be seen, and the surrounding parts are cropped out.

Conclusion

The disappearance of the rangefinder indicator at high magnifications is due to the physical positioning of the LRF and the narrowing of the display area as zoom increases. Since the LRF position and indicator is not perfectly centered, it will shifts out of view at higher zoom levels. To mitigate this, users should be advised to utilize the rangefinder at lower zoom settings where the indicator remains visible on the screen, ensuring accurate distance readings. This phenomenon arises from the inherent physical limitations of the device's design, rather than device malfunction.