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Ultra-wide-angle lenses have focal lengths of 20mm or less. It is difficult to design lenses with good optical performance at these angles, and to make them render straight lines. ‘Fisheye’ lenses have even shorter focal lengths, but here the battle to reproduce lines as straight has been abandoned, and these lenses produce an almost spherical view of the world – hence the term ‘fisheye’. It is important to check the specifications of lenses in the 15-20mm range, because some will be genuine wide-angles and some will be fisheyes.

Ultra-wide-angle lenses can be difficult to use for a number of reasons. To avoid dramatically converging verticals (keystoning), it is important to keep the camera level when shooting, and many architectural photographers will use a tripod and a spirit level as they set up their shot to ensure that this is the case.

Another disadvantage of ultra-wide-angle lenses is that it may not be possible to use filters with them, either. The front element of the lens may well be so curved that it protrudes outwards too far, and the angle of view will be so wide that many filter fittings will intrude on the edges of the frame.