A key lens in the landscapers kit is a wide angle, unhappily the challenge of designing a wide angle lens is much greater than the 50mm focal length. The reason comes down to lens optical aberrations and phenomena that become even more problematic as the angles of incident light become steeper for marginal and oblique rays of light gathered which are exacerbated by the wider angles of view. The following optical aberrations are influenced by the angle of view - coma, field curvature, astigmatism, distortion and chromatic aberration, some of the aberrations have a power law relationship, which means that as the angle of view doubles the distortion increases by a factor of eight. Some optical aberrations are influenced by the aperture size - spherical aberrations and coma. Spherical aberrations have a power law relationship so doubling the aperture e.g from f4 to f2.8 increases the influence by a factor of 8, alternatively it is easier to design an f4 lens than an f2.8 lens as the influence of spherical aberrations are reduced by a factor of 8.
It is fair to state that in the L lens line up Canon has the 24mm focal length pretty well covered and there's also a number of lenses that been upgraded to deliver improved optical performance across the image. The italic text in the optical design description is taken from Canon's marketing literature and the year of release in brackets. I've included the F1.4 fast WA as that is where the optical designers face the most challenges and coincidentally where some of the best marketing information on optical design resides.
24mm f1.4 II IS L (2008-)
Optical design : 13 lens elements in 10 groups. 'It features two high-precision, large-diameter aspherical lenses for sharpness across the sensor, even in peripheral areas of full-frame sensors. Lens elements have a newly-designed anti-reflective SWC (Sub Wavelength Coating) that departs from conventional coatings by using an extremely fine structure that minimizes ghosting and flaring across the lens surface, regardless of the angle with which light enters or exits. It also features two UD lens elements to minimize chromatic aberrations,'
Angle of view horizontal, vertical, diagonal - 74, 53, 84
Weight : 0.65 Kg
Dimensions : 93.5 mm in diameter and 86.9 mm long.
Focus - auto/manual
Retail price ~ £1,700
24mm f1.4 L (1991-2008)
24mm f3.5 TS II L (2009-)
Optical design : 16 lens elements in 11 groups. '.. includes high-precision asphercial front element keeps distortion, common in wide angle lenses, to an absolute minimum, even at the edges of the frame, also feature multiple (3 no off) UD elements to reduce chromatic aberration.' and 'Designed with UD glass to minimize and compensate for chromatic aberrations and a specially coated aspherical element for the highest possible glare-free image quality ..'
Angle of view horizontal, vertical, diagonal - 74, 53, 84
Weight : 0.78 Kg
Dimensions : 88.5 mm in diameter and 106.9 mm long.
Focus - manual
Retail price ~ £1,450
24mm f3.5 TS L (1991-2009)
Zooms covering the 24mm focal length
11-24mm f4 L (2015 -)
Optical design : 16 lens elements in 11 groups. 'Its new optical design features Super UD and UD lenses that correct chromatic aberration. Four aspheric lens elements help to minimize distortion from the center of the image to the periphery and throughout the zoom range. A Subwavelength Coating (SWC) and Air Sphere Coating (ASC) significantly reduce flare and ghosting.'
11mm Angle of view horizontal, vertical, diagonal ~ 117, 95, 126
24mm Angle of view horizontal, vertical, diagonal - 74, 53, 84
Weight : 1.18 Kg
Dimensions : 108 mm in diameter and 132 mm long.
Focus - auto/manual
Retail price ~ £2,800
24-70mm f2.8 II L (2012-)
Optical design : 18 lens elements in 13 groups. 'With a redesigned optical system that includes two Ultra-low Dispersion (UD) and one Super UD aspheric elements to minimise chromatic aberration and colour blurring, the lens delivers consistently sharp, high-contrast images. Each lens element also features Canon’s optimised Super Spectra coatings to reduce ghosting and flare and ensure excellent colour balance. Additionally, a fluorine coating minimises the amount of dust, dirt and fingerprints that adhere to the front and rear of the lens, helping to maintain superior image quality.'
24mm Angle of view horizontal, vertical, diagonal - 74, 53, 84
70mm Angle of view horizontal, vertical, diagonal ~ 29, 19, 35
Weight : 0.805 Kg
Dimensions : 88.5 mm in diameter and 113 mm long.
Focus - auto/manual
Retail price ~ £1,400
24-70mm f2.8 L (2002-2012)
24-70mm f4 IS L (2012-)
Optical design : 16 lens elements in 12 groups. '... includes three types of aspherical lens elements (ground, replicated and glass moulded). Two UD lens elements virtually eliminate chromatic aberration, while Super Spectra lens coatings suppress flare and ghosting.'
24mm Angle of view horizontal, vertical, diagonal ~ 74, 53, 84
70mm Angle of view horizontal, vertical, diagonal ~ 29, 19, 35
Weight : 0.6 Kg
Dimensions : 83.4 mm in diameter and 93 mm long.
Focus - auto/manual
Retail price ~ £700
16-35mm f2.8 II L (2007-)
Optical design : 16 lens elements in 12 groups, 'completely redesigned the optics on the EF 16-35mm f/2.8L II USM to deliver higher contrast levels and improved resolving power. To ensure corner-to-corner sharpness and contrast throughout the focal range, the lens includes three types of aspherical lens elements (ground, replicated and glass moulded). Two UD lens elements virtually eliminate chromatic aberration, while Super Spectra lens coatings suppress flare and ghosting.'
16 mm Angle of view horizontal, vertical, diagonal ~ 98, 74, 108
35 mm Angle of view horizontal, vertical, diagonal - 54, 38, 63
Weight : 0.64 Kg
Dimensions : 88.5 mm in diameter and 111.6 mm long.
Focus - auto/manual
Retail price ~ £1,150
16-35mm f2.8 L (2001-2007)
16-35mm f4 IS L (2014-)
Optical design : 16 lens elements in 12 groups. '... includes two Ultra-low Dispersion (UD) elements to minimise chromatic aberration and eliminate colour blurring. Super Spectra Coatings reduce ghosting and flare, while fluorine coatings on the front and rear elements prevent dust and water drops sticking to each element.'
16 mm Angle of view horizontal, vertical, diagonal ~ 98, 74, 108
35 mm Angle of view horizontal, vertical, diagonal - 54, 38, 63
Weight : 0.615 Kg
Dimensions : 82.6 mm in diameter and 112.8 mm long.
Focus - auto/manual
Retail price ~ £740
24-105 f4 IS L (2005-)
Optical design : 18 lens elements in 13 groups. 'Constructed with one Super-UD glass element and three aspherical lenses, this lens minimizes chromatic aberration and distortion. The result is excellent picture quality, even at wide apertures.'
24mm Angle of view horizontal, vertical, diagonal - 74, 53, 84
105 mm Angle of view horizontal, vertical, diagonal ~ 19, 13, 23
Weight : 0.67 Kg
Dimensions : 83.5 mm in diameter and 107 mm long.
Focus - auto/manual
Retail price ~ £700
17-40 f4 L (2003-)
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Optical design : 12 lens elements in 9 groups. 'Superior optics are assured by the use of three Aspherical lens elements, in addition to a Super UD (Ultra-low Dispersion) glass element. Optical coatings are optimized for use with digital cameras.'
17 mm Angle of view horizontal, vertical, diagonal ~ 93, 70, 104
40 mm Angle of view horizontal, vertical, diagonal ~ 49, 34, 57
Weight : 0.475 Kg
Dimensions : 83.5 mm in diameter and 96.8 mm long.
Focus - auto/manual
Retail price ~ £550
Now it also happens that Sigma are releasing lenses that cover the 24mm focal length in the EF mount too and some might allege that they've released lenses that target Canon's older lenses.
Sigma 24mm f1.4 (2015-)
Optical design : 15 elements in 11 groups 'One of the elements required to achieve high rendering performance is the correction of chromatic aberration. Particularly, for axial chromatic aberration that is hard to correct even during image processing, the lens development stage was vital in ensuring minimized distortion. The SIGMA 24mm F1.4 DG HSM | Art incorporates FLD ("F" Low Dispersion)* glass and SLD (Special Low Dispersion) glass elements to minimize chromatic aberration of magnification which is mainly visible around the edge of the image. Moreover the power layout ensures the correction of axial chromatic aberration. Without any color blur, it achieves high image quality throughout the entire focusing range, thus achieving sharp and high contrast image rendering.'
It is not possible to compensate for distortion that is often observed with wide-angle lenses just by changing the aperture value. Therefore, the lens development stage was vital in ensuring minimized distortion. The SIGMA 24mm F1.4 DG HSM | Art adjusted the incidence angle of the light source from the surface of the first lens and positioned each aspherical glass element to optimize the power layout at respective positions. This has helped it succeed in minimizing distortion throughout the entire image.
Weight : 0.665 Kg
Dimensions : 85 mm in diameter and 90.2 mm long.
Focus - auto/manual
SRP - £799.99
Retail price ~ £700
Sigma 24-105mm f4 OS A (2013- )
'Flare and ghosting were thoroughly measured and monitored from an early stage in the lenses development to establish an optical design which is resistant to strong incidental light such as backlight. The Super Multi-Layer Coating reduces flare and ghosting and provides sharp and high contrast images even in backlit conditions. Attaching the matched petal type hood, which is supplied, will provide extra protection from flare and ghosting.'
'This is a lens that represents the concept of our “Art” line, and it meets the highest standard of our quality test. For those standard lenses with a high zoom ratio, they tend to show astigmatic aberration, field curvature, distortions and color aberration. In order to compensate for these, various types of high-performance lenses, such as FLD, SLD and glass-molded aspheric lenses including double-sided aspheric lens, have been included into the optical system. In addition, it suppresses chromatic aberration very effectively at telephoto-end, and achieves superior image quality throughout the zoom range. This lens overcomes low peripheral brightness that is common for those lenses with similar specifications. Also, the wide filter size 82mm is contributing for superior optical performance.'
24mm Angle of view horizontal, vertical, diagonal - 74, 53, 84
105 mm Angle of view horizontal, vertical, diagonal ~ 19, 13, 23
Weight : 0.885 Kg
Dimensions : 88.6 mm in diameter and 109.4 mm long.
Focus - auto/manual
SRP ~ £849.99
Retail price ~ £680
Aside from a multitude of lens reviews and test measurement data, there's plenty of anecdotal evidence to suggest that each lens upgrade and new lenses introduced have raised the bar of optical performance, the lens descriptions and diagrams, show the investment by Canon in their L lenses and Sigma in their Art lenses, in design and manufacturing. The take home lesson would be that optical performance has been achieved with the use of more aspheric and exotic glass elements, surface coatings and an increased number of lens elements and groups to deliver improved optical performance.
My knowledge of optical theory is at a rudimentary level, but reading optical discussions and the 50mm focal length is considered an optimal focal length to design lenses with a large aperture, small dimensions and optical excellence for the 35mm format. In Part 1 of this series the Zeiss Otus 55/1.4 physical dimensions were listed and people who appear to know about optical matters speculated that Zeiss's designers had achieved world class optical excellence wide open by increasing the dimensions beyond the minimally required optical relationships incurring a weight and size penalty. The consensus of informed opinion was that Zeiss increased the size to keep the lens at a realistic price point for the intended market and demand. Comparing old/new EF mount lenses a case could be made for increased physical sizes in the newer lenses.
So for a landscape photographer wanting to invest in an EF mount lens covering the 24mm focal length there appears to be number of candidate lenses currently in the market.
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