EF 24mm f/1.4L II USM
Development objectives for the EF-S 18-200mm f/3.5-5.6 IS
The EF-S 18-200mm f/3.5-5.6 IS (Photo 1) is an extended-range zoom lens developed specifically for the EOS 50D and other APS-C sensor EOS Digital series* cameras. The lens combines a long 11-times zoom range, spanning wide angle thru telephoto focal lengths, built-in image stabilization and superb cost performance. The primary aims of the lens’s development are outlined below.
* EOS 50D, EOS 40D, EOS 30D, EOS 20D, EOS 20Da, EOS Kiss X2, EOS Kiss F, EOS Kiss Digital X, EOS Kiss Digital N, and EOS Kiss Digital (current as of January 2009).
Feature 1: Excellent image stabilization
This lens features a dedicated image stabilizer unit with the same structure as the unit on the EF-S 18-55mm f/3.5-5.6 II USM and the EF-S 55-250mm f/4-5.6 IS lens. (The compensation optics barrel is held in place with three springs.) An optimized algorithm controls the drive of the compact, lightweight, high-performance image stabilizer unit, delivering a stabilization gain of about four steps in shutter speed.* The lens is also able to distinguish automatically between pan shooting and camera shake in normal shooting, and adjusts the IS mode accordingly. This feature permits for optimal image-stabilized shooting in both situations and frees the user from concerns about mode selection.
*Generally, the effective limit for stable unaided handheld shooting is a shutter speed of about 1/focal length [x 1.6 for APS-C sensor cameras] seconds.
Feature 2: Outstanding optical performance
The zoom mechanism is constructed so that all five lens groups move when zooming. This design gives an 11 times zoom ratio, covering focal lengths from wide angle thru telephoto, and image stabilization. The inclusion of two UD (ultralow dispersion) lens elements and two precision molded glass aspherical elements provides excellent aberration correction, particularly of chromatic and spherical aberrations, and ensures great image quality at all focal lengths (figures 1 and 2).
The lens’s focal length range is equivalent to 29 - 320mm in the 35mm-film format.
Feature 3: Optimal optics for digital SLRs
The lens configuration and coatings have been designed to minimize the flaring and ghosting found with digital SLRs due to surface reflections from the imaging sensor. The configuration and coatings also provide excellent color balance.
Other features
Fast autofocusingA high-speed CPU and an optimized AF algorithm control the drive of the compact motor for fast autofocusing.
Minimum focusing range of 0.45mThe inner-focusing system allows for a minimum focusing range of 0.45m at all zoom focal lengths. The maximum magnification is 0.24 (at 200mm).
Circular aperture produces beautiful bokehThe lens features an electro-magnetic diaphragm with a circular aperture that produces attractive bokeh (blurring effects).
Zoom-ring lockWhen the lens is attached to a camera on a strap, the zoom-ring sleeve may slide out under its own weight if the lens is pointed down. Therefore, we added a zoom-ring lock to hold the zoom ring in the wide-angle position (the shortest position) to prevent the end of the lens from striking something and being damaged. The zoom-ring lock lever is located at the rear near the mount.
Redesigned switch shape prevents accidental mode changesThe focus mode switch has been reshaped to prevent the switch from being accidentally moved while shooting or transporting.
Improved operability and high-grade exterior finishThis lens has been furnished with the same zoom-ring grip pattern and finish as our high-end models, giving a firmer feel and high-grade exterior finish.
Environmentally friendly designThe lens optics use only lead-free glass in consideration of the environment.
Furthermore, the lens design meets the environmental requirements of the European Union's RoHS Directive (Directive on the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment).
Development objectives for the EF 24mm f/1.4L II USM
The EF 24mm f/1.4L II USM (Photo 2) is a high-performance L series wide-angle lens developed as the replacement for the EF 24mm f/1.4L USM. Professionals and prosumers have raved about the EF 24mm f/1.4L USM since it was released in 1997. Completely redesigned, the EF 24mm f/1.4L II USM delivers even better performance and higher image quality.
Feature 1: Brightest in its class* and better image quality than the previous model
The lens uses a new optical design with a class-leading maximum aperture of f/1.4 to give better image quality than previous models. The use of two precision molded glass aspherical elements corrects aberrations, particularly distortion and field curvature, which occur frequently in wide angle lenses. Furthermore, two UD elements minimize lateral chromatic aberration and improve image quality across the entire image plane, especially near the edges.
Furthermore, stable image quality is assured from infinity to macro distances with a floating focusing system that moves the two focusing lens groups asymmetrically to limit aberration fluctuations during focusing (figures 3 and 4).
*Among interchangeable SLR lenses with a 24mm focal length. Current as of December 2008.
Feature 2: Newly developed special anti-reflective coating
We applied a newly developed special anti-reflective coating, called Subwavelength Structure Coating (SWC), on the rear surface of the front lens element. SWC minimizes the flare and ghosting that commonly occurs when light strikes the lens at a large angle of incidence.
Feature 3: Superior waterproof and dustproof construction
Waterproof and dustproof construction (Figure 5) at the mount, switches, switch panel, and focusing ring prevents dust and moisture from entering the lens, making it suitable for taking pictures in harsh conditions.
*Be sure to use the lens with a weatherproof/dustproof body.
Other features
Fast, silent autofocusingThis lens model achieves the same fast, silent autofocusing as its predecessor thanks to a ring-USM drive, high-speed CPU, and optimized AF algorithm.
Manual focusing in AF modeThe full-time mechanical manual focus construction allows the user to continue focusing manually without leaving the AF mode after one-shot AF operation.
Redesigned switch shape prevents accidental mode changesThe focus mode switch has been reshaped to prevent the switch from being accidentally moved while shooting or transporting.
Circular aperture produces beautiful bokehThe lens features an electro-magnetic diaphragm with a circular aperture that produces attractive bokeh (blurring effects).
Oversized, petal-shaped hood provides effective light blockingThe inner wall of the oversized, petal-shaped EW-83K hood has been given a flocking treatment for better anti-reflective performance, greatly cutting harmful reflections (Photo 3).
Environmentally friendly designThe lens optics use only lead-free glass in consideration of the environment. Furthermore, the lens design meets the environmental requirements of the EU’s RoHS Directive.
SWC, a newly developed special coating for photographic lenses
SWC (Subwavelength Structure Coating) is a newly developed coating that greatly suppresses the occurrence of reflected light in photographic lenses.
Because SWC has a completely different structure from conventional vapor-deposited coatings, it can greatly reduce the flare and ghosting that conventional coatings cannot control (Photo 4).
SWC’s anti-reflective principleA lens’s surface reflects light because the refractive indexes on each side of the glass-air interface are quite different. Ordinary lenses are coated with a very thin film that suppresses reflections by making use of light interference effects at the film-glass interface. The problem with vapor-deposited coatings is that they cannot achieve ideal interference when either the incidence angle or exit angle is large. Consequently, the coating’s anti-reflection performance also declines.
If, however, we could fashion a coating in which the refractive index changes continuously between air and glass, light would not reflect at this layer since reflections are caused by the refractive index discontinuity at an interface. Thus, all light would move from air to glass or from glass to air. This is the principle behind SWC’s anti-reflective property (Figure 6).
The structure of SWCWe’ve known since the 1960s that the eyes of moths eliminate reflections due to a film with microscopic bumps on the surface of the eye. These microscopic bumps mitigate the refractive index differences between air and the lens of the eye, effectively removing the air-eye interface.
We took this principle one step farther for SWC by developing a method for forming a structure of nano-wedges, which are smaller than the wavelength of visible light, on the lens surface. By arranging the wedges on the lens surface with their pointed ends sticking up in the air, as shown in Figure 7, we are able to form a continuous refractive index gradient from the tips of the wedges to their bases. As a result, the incident light is drawn, virtually sucked in, from the tips of the wedges down to their bases without creating reflections.
The benefit of SWCWe used SWC on the rear surface of the front lens element of the EF 24mm f/1.4L II USM. Light enters and exits this surface at a large angle because of its large curvature. For this reason, conventional coatings are not able to sufficiently suppress reflections, giving rise to the ghost shown in Photo 5.
SWC, on the other hand, keeps reflections to an extremely low level, so that almost no ghost is apparent in Photo 6.
Canon will use SWC on more EF lenses, particularly lens elements with large curvatures, as part of our continuing pursuit of lenses with better image quality and better performance.