Focal length and angle of view

The focal length photographers and manufacturers regard as 'normal' for a camera is approximately equal to the diagonal of the camera's picture format. In other words:

• For a 6 × 7inch rollfilm camera, a lens of about 80–105 mm would be considered normal. For a 35 mm (24 × 36 mm) camera, between 35 mm and 50 mm focal length is normal.
• For APS (17 × 30) picture size cameras, the normal lens is 25 mm. And for a digital camera with a tiny 4.8 × 6.4 mm sensor, the lens would only be 6–10 mm focal length.

The shorter the focal length the smaller the image the lens produces. But a lens of short focal length used with a small-format camera gives the same angle of view as a lens of longer focal length used in a bigger camera. You are just scaling everything up or down. All your combinations above therefore give an angle of view of about 45°, and so each camera set up to photograph the same distant subject will include about the same amount of the scene.

Fundamental of photography: picture structuring, lights, intensity, distance, focusing, exposure, printing