PICTURE ANGLE
On the focusing screen placed behind the lens perpendicularly to its axis, the central part of the image is sharp, but with distance from the center of the image, its illumination and sharpness decrease. It comes from here, that the beam of rays falling obliquely on the lens is diaphragmed not only by the diaphragm, but also through the edges of the lens barrel and into the image less light. This phenomenon is called vignetting.
Deterioration of image quality at the edges occurs due to insufficient removal of optical errors with oblique beams, especially with chromatic aberration, astigmatism or field curvature.
Area, which includes a circle with a still sufficiently sharp and bright part of the image, is called the image field. Hence the conclusion, that the diagonal of the photo format cannot be larger than the diameter of the lens image field.
The angle with the vertex at the main image point of the lens and encompassing the image field is called the angle of the field or image angle.
In commonly used lenses, the angle of view is in the range of 40°—50°. Telephoto lenses have a small field and the image angle is approx. 10°—25°, while in wide-angle lenses it reaches 90°.