It’s usually not the drone or the Earth’s curvature — it’s lens distortion.

Many drones use very wide-angle lenses, and some images are made with fisheye-style lenses. These lenses capture a huge field of view, which is useful from the air, but they often bend straight lines near the edges of the frame into curves. That’s why the horizon can appear bowed even at low altitude.

This effect is strongest when the horizon is away from the center of the image. With many wide lenses, lines near the edges curve toward or away from the center depending on framing, so the horizon may bend downward or upward.

Drones are commonly sold with wide lenses because they make it easier to capture a broad aerial scene without needing precise aiming, large/heavy telephoto lenses, or a bigger, more stable drone. If you look at drone photos made with narrower or telephoto lenses, you’ll see much less of this curvature.