A telephoto lens is a type of lens in which the physical length is shorter than the focal length. A telephoto lens has a narrow field of view, and as a result, the curvature of the Earth is less pronounced if taken using a telephoto lens compared to another lens with a wider field of view.
Flat-Earthers like to bring up pictures taken from the ISS that show a practically flat horizon, and use them as evidence of inconsistency. In reality, the images were taken using a telephoto lens with a narrow field of view.
In the illustration, on the left side is a picture of Earth taken using an ultra-wide-angle lens Nikon 17-35mm at 17mm. Such a lens gives us a wide field of view: 93° horizontally and 70° vertically. As a result, it covers a lot of the Earth, and Earth’s curvature is displayed prominently.
On the other hand, the picture on the right side was taken using a telephoto lens Sigma 50-500mm at its maximum focal length of 500mm. It has a narrow field of view, only 4.12° horizontally, and 2.75° vertically. As such, only a small part of the horizon is visible, and Earth’s curvature is far less pronounced.
Camera’s field of view is an important factor that determines the visibility of Earth’s curvature in a photograph. The wider the field of view, the more prominent the curvature is visible.