Lens distortion is a common phenomenon in photography that usually occurs when using wide-angle lenses to take photos.
A wide-angle lens, simply put, is a lens with a shorter focal length that allows you to capture a larger area in your photo.
Wide-angle lenses are more commonly used in landscape and architectural photography due to their ability to capture a larger area of the scene (and hence capture wider / taller objects such as large buildings or mountains).
In shooting this mosque building, I used a Canon 10-18 mm lens, and the angle of distortion was clearly visible.
