This month's Technical Report introduces the EF17-40mm f/4L USM, a new L-series ultrawide-angle zoom lens released in May 2003.
In addition, we will also describe improving the compatibility of EF lenses with digital photography.
The EF17-40mm f/4L USM (Photo 1), is a compact ultrawide-angle L-series zoom lens. It has been developed as a higher-class model of the EF20-35mm f/3.5-4.5 USM.
The concepts of development are as follows:
• Excellent image quality as an L-series lens with high-cost performance
• Expanded focal length from ultra wide-angle 17mm to 40mm
• Ultrahigh image definition
• Ideal with digital SLR cameras also
• Higher-speed AF than its predecessor
• Dust-and-moisture-proof construction
Feature 1: Ultrawide-angle zoom
The optical system of the EF17-40mm f/4L USM is based on the ultrawide-angle zoom lens EF16-35mm f/2.8L USM, but incorporates new design features that give a wider 17-40mm zoom range while improving on the superb image quality. The EF17-40mm f/4L USM adopts a five-group construction with five moving groups and inner focusing by the second group.
With its 17mm wide-angle focal length, the lens ensures wider-angle shooting not only with conventional silver-halide SLR cameras but also with digital SLR cameras that have imaging element dimensions of the Advanced Photo System C size.
When used with the EOS 10D, it provides a focal length range equivalent to 27-64mm on 35mm film.
Feature 2: Ultrahigh-image quality
• Ultrahigh-image quality - the trademark of the L-series
The optical system, with 12 elements in 9 groups (see Fig. 1), uses the largest diameter high-precision molded glass (GMo) aspherical lens of any EF lens, and two replica aspherical lenses. With this, the lens achieves excellent correction for distortion and aberrations that arise during zooming, and a zoom range of 17-40mm.
The use of a Super-UD (ultra-low dispersion) lens in the tenth element minimizes the lateral chromatic aberration that tends to occur at wide-angle focal lengths. The result is high quality images with excellent resolution and high contrast throughout the entire zoom and focusing range (see Fig. 2), results which can be expected only from L-series lenses.
The darker black lines represent the MTF characteristics at maximum aperture; the lighter lines represent the characteristics at f/8. The solid lines trace the radial S (sagittal) curve, while the broken lines trace the tangential M (meridional) curve.
• Minimum focusing distance of 0.28 meters
The minimum focusing distance of 0.28m is the shortest in its class, and remains the same through the entire zoom range. It was achieved by increasing the power of the focusing lens group, and compares with 0.34m with the EF20-35mm f/3.5-4.5 USM. It also gives a maximum magnification of 0.24x at 40mm telephoto (100 x 150mm field of view).
• Optimized coatings considering use with digital SLR cameras
As with the EF16-35mm f/2.8L USM, the EF17-40mm f/4L USM features optimized lens coatings in consideration of its use with digital SLR cameras. As a result, the lens successfully realizing both appropriate color balance and minimization of the ghost and flare attributable to surface reflections from the imaging element.
• Circular aperture for attractive out-of-focus effects
The lens uses an EMD (electromagnetic diaphragm) with an aperture that remains virtually circular from f/4 to f/8, providing attractive out-of-focus effects (boke).
Feature 3: Silent, high-speed AF
In addition to the silent ring-type ultrasonic motor (USM) and enhanced inner focusing system, the autofocus (AF) algorithm is improved, resulting in even faster AF than the EF20-35mm f/3.5-4.5 USM.
Feature 4: Excellent dust-and-moisture-proof construction
By employing dust-and-moisture-proof construction (see Fig. 3) in the lens mount, switch panel, zoom ring and focusing ring, the lens can be used in harsh environments. Attaching a protection filter to front of the lens is recommended to further improve dust-and-moisture resistance in harsh environments.
• Gelatin filter holder provided at the rear
The lens has a drop-in gelatin filter holder at the rear. The holder can holds up to three 31 x 31mm gelatin filters.
• Allows manual focusing even in AF mode
The EF17-40mm f/4L USM incorporates full-time mechanical manual focusing that allows manual focusing after one-shot AF operation without leaving AF mode.
• Wide, easy-to-operate zoom and focusing rings
A wide rotating zoom ring enables fine-tuning of composition, and the wide focusing ring allows delicate adjustment of manual focusing.
• Non-rotating filter ring
Since the filter does not rotate while zooming or focusing, it allows smooth operation of circular PL (polarizing) filters. To prevent vignetting, using the Canon 77mm PL-CII filter is recommended.
• Large dedicated scalloped hood with superb light blocking
The EW-83E, a large removable scalloped hood designed specifically for this lens, has superior antireflection flocking on the inner wall. It is highly effective in keeping unwanted light out of the lens. Secure hood attachment is assured by the clearly audible click when the hood is properly attached.
Feature 6: Environmentally friendly lead-free glasses
In consideration of the environment, the optical system contains only lead-free glasses.
Others
Major specifications of the EF17-40mm f/4L USM are shown in Table 1, while the depth of field data is depicted in Table 2.
| EF17-40mm f/4L USM | ||
| Focal Length/Aperture Ratio | 17-40mm f/4 | |
| Lens Construction | 12 elements in 9 groups | |
| Angle of View | Diagonal | 104° - 57°30’ |
| Vertical | 70°30’- 34° | |
| Horizontal | 93° - 49°20’ |
|
| Minimum Focusing Distance | 0.28m (at all zoom focal lengths) | |
| Maximum Magnification/ Field of View |
0.24x, 100 x 150mm (at 40mm telephoto) | |
| Minimum Aperture | f/22 | |
| Filter Size | 77mm | |
| Gelatin Filter Holder | III : Incompatible IV : Compatible at Telephot(hood cannot be used) |
|
| Maximum Diameter/ Length | 83.5 x 96.8mm | |
| Weight | 475g | |
| Hood | EW-83E | |
| Lens Case | LP1319 | |
| Price | 120,000 yen (including hood and case, tax not included) |
|
Table 2 Depth of Field Data for EF17-40mm f/4L USM
Improving Compatibility of EF lenses to Digital Photography
• Measures against flare and ghost particular to digital photography
The image sensors in digital cameras have higher specular surface reflection than film. Due to this characteristic, when there is intense point light source within the field, the light reflects off the surface of image sensor, causing flare or ghost. To solve this issue, Canon has adopted a new approach to the optical design of EF lenses.
• Adoption of meniscus lenses
Most super-telephoto lenses have a protective glass at the front. When this glass has a flat surface, the light that reflects off the image sensor reflects back from the protective glass, occasionally resulting in spot-shaped ghost.
To eliminate this phenomenon, all of Canon's large-aperture IS super-telephoto lenses adopt a meniscus lens shaped protective glass.
The meniscus lens is spherical and has curvature with same direction on both sides. By employing the meniscus lens as protective glass, the reflected light from the image sensor forms an image in front of the image sensor, and then diffuses. The majority of the diffused light deviates from the image sensor, thus preventing ghost (see Fig. 4).
Even when using lenses without protective glass, flare and ghost may occur resulting from complex reflection of the light which reflects off the image sensor into the lens. To prevent this effect particular to digital photography, Canon has optimized the shape and coating of the lenses, including the EF17-40mm f/4L USM and the EF24-70mm f/2.8L USM.
Specifically, each lens element adopts a different design, in order to reduce the amount of repeated reflection inside the lens. Furthermore, high-transmission multi-layer optical coatings are applied to lens surfaces which are susceptible to reflection. This allows any light reflected off the image sensor to escape out of the front of the lens, thus reducing flare and ghost (see Fig. 5).
In addition, appropriate balance between multi-layer and single-layer coatings contributes to the excellent color balance.
