A camera in automatic mode will evaluate the entire scene and choose the exposure automatically. If the entire scene appears too dark, the camera will increase the exposure. If the entire scene is too bright, it will decrease the exposure.
If the intended object is small, bright, and on a dark background like a sunset, changing zoom will affect the overall brightness of the entire scene. The automatic mode might choose an incorrect exposure. It is the source of much flat-Earth misinformation & can be corrected using manual exposure.
In the illustration, we used a Nikon P1000 to photograph a lamp on a dark background, from wide-angle (24mm) to telephoto (860mm), using both manual and full-automatic exposure (green square mode). We used the same exposure the full auto mode gave us for the shots in the manual mode.
The wider the zoom, the less the lamp occupies the frame, and the darker the overall scene. The autoexposure compensated it by increasing the exposure. As a result, the lamp becomes overexposed, with a noticeable glare around it. The glare is not part of the lamp but is only a result of an overexposed light source against a dark background.
On the other hand, the longer the zoom, the more the lamp occupies the frame. The overall scene becomes brighter, and the autoexposure system compensates by reducing the exposure. As a result, we can observe the actual image of the lamp at the expense of the surrounding that becomes invisibly dark.
We can lock the exposure using manual exposure, and the light source appears the same even though we are changing the zoom.
- Exposure (photography) – Wikipedia