Note, unless otherwise specified, all focal lengths are in terms of the 35mm image format.
Recently, there has been a flurry of new high performance ultra-wide angle lenses introduced to the consumer market. The imaging properties of these lenses are very different from other photographic lenses; the technique used with these lenses is also very different. Because they have not been easily available for very long, many people have not had a chance to use one, so I thought an ‘update’ about this type of lens may be of value. There are many online guides explaining how to compose a photograph using an ultrawide, for example:
These lenses have fields of view significantly wider than your natural vision, which for each eye is about 55 degrees, roughly equivalent to a 45mm focal length lens. Until fairly recently (around 2005), high performance camera lenses with focal lengths shorter than 18mm were generally difficult to find. Today, interchangeable camera users have access to many ‘rectilinear’ lenses with fields of view exceeding 110 degrees.
The distinction between wide angle and ultrawide angle is fuzzy, but a good rule of thumb is that if the lens focal length is shorter than the short side of the sensor (24mm for 35mm format), the lens is called ‘ultrawide’. ‘Fisheye’ lenses have fields of view up to and exceeding a full hemisphere. The primary distinction between a fisheye and an ultrawide is that an ultrawide lens is designed to minimize distortion, while the fisheye (including design variants such as orthographic projection lenses and f-theta lenses) is designed to incorporate significant amounts of distortion. In images acquired with an ultrawide lens, straight lines (ideally) stay straight- ultrawides are ‘rectilinear’ lenses.
The shortest focal length currently available (2017) is a 10mm “hyperwide” lens sporting a field of view of 130 degrees. Several companies provide 11mm focal length lenses, others offer 12mm, and 14mm/15mm lenses are now fairly commonplace. There are even several ultrawide zoom lenses and an ultrawide tilt-shift lens. To give you a sense of how the field of view varies with focal length, see this image:
While not indicated, a 50mm lens would only see the central group of 2 bookshelves and staircase.
Historically, wide-angle lens design began in 1862 and (in my opinion) reached a zenith with the Hypergon and Metrogon/Topogon lenses.
Note, however, these lenses were designed for large format film. Ultrawide angle lenses have several design issues unique to short focal lengths including falloff, flare, fabrication, filters, and (f)chromatic (f)aberrations.
Many design constraints for ultrawides are principally driven by the relatively large distance between the lens mounting flange and the sensor. On most cameras, this distance is longer than the lens’ back focal length, and the general lens design is known as a ‘retrofocus’ design, sort of the opposite of a telephoto lens design. Lenses placed closer to the image plane in general are easier to design and fabricate. Because of this, wide-angle lenses were developed for rangefinder cameras (and can be used on mirrorless cameras) well before SLR cameras. Rangefinder camera users had several choices for many decades. However, to the best of my knowledge, the only viable SLR ultrawides for many years were Nikon’s 15mm and 13mm lenses introduced in the late 1970s and early 1980s and discontinued shortly thereafter.
While some development of APC-format ultrawide lenses occurred in the early 2000s, modern high-performance 35mm format ultrawides didn’t really appear until around 2010 when Nikon, Canon, and Zeiss each introduced new high-performance fixed and zoom ultrawide lenses. More recently, additional companies have introduced high-performance ultrawide angle lenses for full frame 35mm cameras, with focal lengths going all the way down to 10mm (Voightlander hyperwide). There are several technical reasons why ultrawide lens design has lagged behind telephoto lenses.
Once reason is the size and shape of the front element. The shorter the focal length, the more strongly curved the front element must be- similar to fisheye lens design. As extreme examples, Sigma’s 14/1.8 lens has a 80mm aspherical front element, while the Nikkor 13/5.6 front element measures 110mm across. These types of front lens elements are incredibly difficult to manufacture and awe-inspiring to observe.
Strongly curved front elements are one reason why ultrawide lenses can be especially susceptible to flare/glare. Lenses designed for mirrorless/rangefinder mounts have smaller diameters, as the lens is placed physically closer to the sensor, which helps to reduce flare.
A second technical difficulty is correcting transverse chromatic aberration due to dispersion of those large, highly curved, lens elements. In general, the designs of all of these new lenses were enabled by the introduction of new types of optical glass (extra low dispersion and anomalous partial dispersion glass types) by the major foundries.
Falloff, the optical throughput as a function of image height, occurs because the entrance pupil, the projection of the aperture stop into object space, does not remain constant as the location in object space move off the optical axis: at more oblique incident angles, the round hole appears as an elliptical hole. Correcting for falloff becomes more difficult as the field of view increases. Two non-optical approaches include: a ‘star fan’ some Hypergons came supplied with, and center filters.
One potential problem unique to digital sensors is caused by the Bayer filter: ultrawide lenses generate rays that enter the sensor at extreme angles due to the large field of view. The Bayer filters may not perform the same way as it does for rays that enter at more moderate angles. As a result, there can be chromatic effects around the periphery of the image independent of any uncorrected transverse color aberration.
Similarly, the huge front element generally makes use of filters difficult or not possible. Aside from the increased size of filter, there often is no place for a filter in front of the lens- some 3rd party filter makers (Coker, etc) have developed contraptions allowing use of large square filters, and the old Nikkors have a slot in the rear of the lens. Using polarizers will create a strong effect in the sky (whether this is good or bad depends on your point of view). Again, the smaller diameter lenses associated with rangefinder/mirrorless have more filter options.
What’s new for SLR users is also true for other camera mounts: for C-mount cameras, for example, there are now 1.3mm focal length lenses with 135 degree field of view which are equivalent to the 10mm hyperwide.
How low can the designers go? It’s unclear, because I’m not sure how the front element scales with the focal length. Seeing how large the Nikkor 6mm/2.8 and Coastal Optics (Jenoptik) 7.45mm/2.8 front elements are gives some idea- just note that those are fisheye lenses, so the front element is quite a bit smaller than what would be required for a rectilinear lens of similar focal length. For comparison, the large format 60mm Hypergon has a 135 degree field of view, scaling this to a 35mm format gives a 7mm equivalent focal length- the 10mm hyperwide approaches the 60mm Hypergon in field of view, but provides improved throughput.
In summary, the recent explosion of high performance ultrawide lenses provides photographers a hugely expanded range of choices at the ‘other’ end of focal length.