The job of the lens is to project an image of the outside world onto the surface of film or digital image sensor. This projected image is brightest and sharpest in the center of the image. In other words, there is fall off so we look at the corners to see if the coverage will be adequate. Normally, the shorter the focal length, the closer the lens must be to film or sensor. Thus making a short lens with good coverage is challenging. For this reason, most cameras are principally fitted with a “normal” focal length lens. This will be a lens with a focal length about equal to the corner to corner measurement of the format frame (the diagonal measure).
For the full frame (Fx) the format measures 24mm height by 36mm length. The diagonal measure of the full frame is 43mm. Fitted with a 43mm focal length, the angle of view delivered is 31° height, 45° width, 53° diagonal. Because a 43mm is somewhat unusual, the industry settled on 50mm as being “normal”. When a 50mm is mounted, the angles of view for the full frame is 27° height, 40° length and 47° diagonal. Again the 50mm lash-up is considered “normal” for the Fx format.
As a rule of thumb, the realm of wide angle for any format is a lens about 70% of “normal = 35mm for the Fx. Such a lens will deliver 37° height, 54° length and 63° diagonal. A 28mm delivers 46° height, 66° length and 75° diagonal.
The Compact Digital or APS (Advanced Photo “System) format is 16mm height by 24mm length with a diagonal of 30mm. When fitted with a “normal” 30mm the angle of views are: 30° height, 44° length and 51° diagonal. Wide-angle 70% of “normal” = 20mm. Fitted with a 20mm, the angles of view are: 44° height, 61° length and 72° diagonal.
Again rule of thumb: “normal” = lens about equal to diagonal measure of the format. Wide-angle about 70% of “normal” or shorter. The realm of telephoto is 200% of “normal” and longer. Portrait is a moderate telephoto 200% thru 250% of “normal”.
The problem with wide-angle lenses design is accaptable coverage of the format. In other words, too much vignette will be unacceptable. To accomplish, the lens maker likely uses a retro focus design. This is a telephoto like lens mounted backwards. This design delivers the needed wide angle of view and because the distant lens to sensor is elongated the vignette is mitigated