Infinity Focus

To an optical device, like a camera, there is little difference between an airplane at 20000 ft and the Moon at 384400 km (238855 mi). The aperture of the lens is far too small compared to the distance of either object. Focusing on either object will make the other object in focus, too.

Flat-Earthers claim that because both objects appear in focus in a photograph, then the Moon must be close to the airplane. In reality, both can be considered at infinity. And it will be easy to make both objects to be in focus simultaneously.

Depth of field (DoF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image. An object outside the depth of field will not appear in focus. These factors determine the DoF:

• Focal length. The longer the focal length, the smaller the DoF.
• Distance to object. The farther the object, the higher the DoF.
• Aperture of the lens. The bigger the aperture, the smaller the DoF.
• Resolution of the final output. The larger the resolution, the smaller the DoF.

The angular size of the Moon is about 0.52°. The angular wingspan of the plane is half of that or about 0.26°. The wingspan of a De Havilland Dash 8 Q400 is 28.4 m. Therefore, the distance to the plane from the photographer is about 28.4 m / sin(0.26°) = 6258 m.

DoF can be calculated using any DoF calculator on the Internet. If we were to use a full-frame camera with a 24-megapixel sensor, a 400mm lens with the aperture of f/5.6, and the final output of 10 inches, focusing on an airplane at 6000 m would give use the DoF between 815 m to infinity. And if we focus the camera toward the Moon at 384400 km away, then the DoF will be between 943 m to infinity.

Even if the distance between the Moon and the plane is very far, it is not hard to make both objects to appear in focus in the same image.