Wide Angle Lens; Wide-angle lens; Wide angle
A wide-angle lens has a focal length that is considerably smaller than the focal length of a ‘normal’ lens. In the case of lenses used with film, or digital image sensor cameras, a ‘normal’ lens approximates to the field of view of the human eye. In simple terms that would be a 50mm lens for a normal 35mm film or full-frame digital image sensor.
All Lenses are compared to an old SLR standard for 35mm film. Today we have 35mm digital image sensors. These are used in “full-frame” cameras. However, they should also be considered against the smaller ‘cropped sensor’ or ‘APS-C’ camera of most DSLRs. A 35mm focal length or less normally indicates that the lens is a wide-angle lens. Many wide 35mm camera angle lenses have focal lengths around 24 – 35mm. For APS-C (cropped) sensors however, focal lengths at less than 25mm are getting into the wide-angle range.
At focal lengths less than 24mm a very wide angle class of lenses is created. These are called ultra-wide angle lenses. These are around 24mm to around 18mm. In this case, depending on the camera, they show some distortions at the periphery of the lens and especially in the corners. Furthermore they exhibit a tendency to create fish-eye shots.
The degree to which a wide-angle lens distorts the image (particularly at the periphery) is dependent on what corrective elements (individual glass elements in the lens) are included in the internal lens groups. To keep the lenses’ image as rectilinear (rectangular in its final shape) as possible several corrective glass elements are usually applied. The success of this correction is dependent on the lens specification. Normally these corrective elements increase the cost of a lens, sometimes considerably. Top-end lenses therefore normally have much better rectilinear correction.
Digital cameras with very small sensors (compact cameras for example and some point-and-shoot models) can have wide-angle lenses fitted of around 8mm, possibly 6mm. These are not normally removable lenses and are specially designed for working at this short focal length. Such focal lengths are not suitable to use in DSLRs as the internal clearance is insufficient between the back of the lens and the sensor. Such short focal lengths normally preclude the use of a mirror such as the one found between the lens and sensor in a DSLR.
A wide angle lens creates a larger image circle on the sensor plane than a standard lens. The larger image circle enables the wider field of view which gives the lens is notable lateral field of view. Typically, a wide-angle lens would see an additional ten degrees of the scene either side of the normal field of view of the human eye.
Long focal length lenses (greater than 50mm) appear to magnify the subject. This creates an apparent compression of distance and a shallower depth of field than a wide-angle lens. The opposite is true of the wide-angle lens. They tend to magnify distance between a foreground subject and a background subject, exaggerating the size of the foreground objects and minimising the size of the background ones.
The apparent magnification of the distance from foreground to background has two effects. It makes massive distant objects appear to be very much reduced in mass. Secondly, parallel lines appear to converge at vanishing point much more quickly than they would with a standard lens. This exaggerates the collective appearance of clouds trending into the scene, lines pointing to the centre of the scene, tall objects loom forward when close to the camera and lean backwards when in the distance and so on. Thus, wide-angle lenses are often prized for their ability to focus the viewers eye into the centre of the scene.
A full-frame 35mm camera with a full sensor-plane size of “36 mm by 24 mm”, has a sensor diagonal of 43.3 mm which is the same film standard size of the older film SLRs. Legacy associations have become customary in the industry standardising the normal lens at 50 mm. The legacy focal length 35 mm has come to be considered wide-angle to ensure compatibility with the same standard sizes of lenses for 35mm cameras which used film.
For 35mm DSLRs the same legacy conventions apply to the creation of ultra-wide-angle lenses. The latter are normally assumed to be of shorter focal length than 24mm.