Short Focus and Retro Focus Lenses
The focal length for a lens that's Wide Angle is quite a bit shorter than for a regular lens focal length for the picture size created using the same camera. Another way to say, wide angle lenses, let you capture depth in an image and broaden the viewing angle.
In both photography and in cinematography, a lens is called wide-angle when its focal length is significantly less than what a normal lens focal length produces for the picture size created by a camera, whether the distance is determined by the image frame film plane using film format cameras or the
photo sensor dimensions for digital cameras.
A wide-angle photo of steps illustrating perspective distortion created by the distance which the photo was shot. The front stairs seem to have a forward slant.
In a still photograph, a specific camera format normal lens contains a focal length nearly the same as the diagonal length of the photo frame or digital image sensor length. In cinematography, a "normal" lens is thought to be somewhat longer
A simple formula for determining an angle of view of any particular lens that creates a rectilinear photo. In addition to showing a wider viewing angle, a picture created using a lens that's wide-angle becomes more predisposed to forms of perspective distortion than one created by using a normal lens, as they are typically utilized much closer to a subject.
35mm Wide-angle lens format
On a 35mm full-frame camera containing a 36mm x 24mm format, its diagonal measurement is 43.3mm while typically a normal lens by most manufacturers full frame cameras is considered to be 50mm. Also typically, a lens with a focal length which is 35mm or shorter is thought to be wide-angle.
On an APS-C image sensor camera this figure is decreased
to 30mm due to the "crop factor" of the sensor. Typical wide-angle lens sizes
for full-frame 35mm cameras are 14, 18, 21, 24, 28 and, 35mm. Many lenses
contained in this focal range will create an image that's somewhat rectilinear
coming the film plane (although some barrel distortion levels are typical here).
An extreme wide-angle lens, one which does not create a rectilinear image is
called fisheye lenses. Typical 35mm focal lengths are 6 to 8mm (which create a
circular image). Lenses featuring 14 - 16mm focal lengths may either be fisheye
or rectilinear or designs. Wide-angle lenses are made in either
fixed-focal-lengths and zoom editions. Lenses producing rectilinear images for
35mm cameras come in focal lengths beginning at 12mm, including zooms with
ratios of 2:1 which also start at 12 mm.
Ultra Wide Angle Lenses
- Ultra wide-angle lenses have focal lengths which are shorter in length than the short part of the film or image sensor.
- For APS-C this any lens less than 15mm which is thought to be ultra wide angle.
- On 35mm film or full-frame sensors any lens which is shorter than 24mm
- On 6x45 any lens lesser than 41mm
- On 6x6 or 6x7 any lens which is shorter than 56mm
Digital camera considerations
Equivalent focal length for APS-size digital cameras is
multiplied using a crop factor. Typically interchangeable-lens APS-C digital cameras contain image sensors that are not as large as the film format found full-frame 35mm cameras. Generally, the measurements of these image sensors are the APS-C cropped image size, i.e., in the range of 24mm x 16mm. Therefore, for any particular focal length, its
angle of view will not be as wide than it found within a full-frame film or image sensor camera due to the fact that the smaller image sensor only "sees" a smaller part of the image cast by the lens. The camera makers supply a crop factor (typically called a focal-length multiplier or sometimes labeled a field-of-view factor) to demonstrate the smaller size of the image sensor than for a 35mm film frame.
For instance, one typical factor happens to be 1.5 (the Nikon DX format
along with some others), while many cameras feature 1.6 crop factors (typical
Canon EOS DSLR cameras), 1.7 (for Sigma DSLR cameras) along with both (the Four-thirds-formats). The 1.5 is an indication that the lens angle of view when mounted on this camera is equal to a focal length that's 1.5 times greater than a 35mm full-frame camera, the very reason crop factor is additionally called a focal-length multiplier.
For example, a 28mm lens mounted on a DSLR containing a 1.5 crop factor has an angle of view identical to a 42 mm lens mounted on a 35mm full-frame. To figure the focal length for a lens on a 35mm digital camera to provide an equivalent viewing angle as one mounted on a 35mm full-frame camera, a focal length for a full-frame lens needs to be divided into the crop factor. For instance, to compare an equivalent angle of view of a 30mm lens mounted on a 35mm full-frame, to a digital camera containing a 1.5 crop factor, a 20mm lens should be used.
Lens makers have stepped up to this issue by building wide-angle lenses that have greatly reduced focal lengths to fit these cameras. By doing so, they decrease the image diameter cast to slightly larger than the diagonal measurements of the photo sensor. Thus giving the designers more wiggle room in providing the necessary optical corrections to economically create high quality images using these reduced focal lengths,
particularly when a lens is a zoom. For example zoom lenses starting at 10mm minimum focal lengths are made by several manufacturers. At 10mm, these lenses offer the same angle of view at a 1.5 crop factor as a 15mm lens mounted on a full-frame camera.
There are two totally different categories of wide-angle lenses: short-focus and retro focus.
Typical short-focus wide-angle lens cross-section.
Typical retrofocus wide-angle lens cross-section.
A short-focus lens is typically made from multiple glass elements with shapes that are typically symmetrical in both front of and in back of the diaphragm. As the lens focal length increases, the distance from the rear lens element to the photo sensor or film plane also increases, which makes wide-angle, short-focus lenses undesirable for using with SLR cameras except if they are being used with their reflex mirrors locked in the up position. Short-focus lenses are extensively employed for large format view cameras.
A retro focus lens takes care of this proximity issue by using an asymmetrical layout which lets the rear element be a further distance from the image sensor than its effectual focal length would indicate. For instance, it is typical for a rear element of an 18 mm retro focus lens of to be in excess of 25mm from the image sensor, making it possible to build wide-angle lenses to fit SLR cameras.