Development objectives for the EF 100mm f/2.8L Macro IS USM
Professionals and advanced amateurs alike have been demanding lenses that keep pace with the evolution of digital cameras and the progress of photographic technology.
One constant request has been for a high-resolution, mid-telephoto macro lens capable of 1x magnification shooting without using tripods or other attachments. There is also a strong demand for a macro IS lens with good lens stabilization performance at macro distances.
The Canon lens line already has the high-resolution EF 180mm f/3.5L Macro USM as part of our L series and the popular EF 100mm f/2.8 Macro USM lens, but neither of these or our other macro lenses provide the novice with straightforward macro-photography. The reason for this is that the effect of camera shake on photos increases exponentially as the lens magnification increases. Simple vibration detection methods do not provide enough compensation at high magnifications and image stabilization diminishes. This is why macro photography has always required a tripod.
For the EF 100mm f/2.8L Macro IS USM (Photo 1), we thoroughly studied the components of camera shake before developing an image stabilization system that works in the macro range using what we call “Hybrid IS” technology. With this technology and lens, we have addressed the hopes of professionals, advanced amateurs, and even ordinary users by making hand-held macro photography possible. The following sections describe the characteristic features of this lens.
(1) Analysis of camera shake at macro distances
There are several camera motions that degrade images including angular shake, lateral shake, and focus blurring (Figure 1).
Conventional image stabilization techniques primarily correct angular shake. Angular shake is blurring caused by an angular change (yawing) at the end of the lens with the camera or the lens base acting as the fulcrum. The resulting blurring amount in the image plane is expressed approximately as f × tanθ, where f is the lens's focal length in millimeters and θ is the angle of movement. Given a constant amount of angular shake when shooting, then the impact on the photo is proportional to the focal length. Thus, the longer the focal length, the easier blurring can occur.
Lateral shake is image degradation caused by vibrations of the entire camera or lens either horizontally or vertically. When the camera or lens is vibrated by an amount Δ, the impact of lateral shake in the image plane is a multiple of the lens magnification. (The amount of blurring in the image plane is equal to the lens magnification × Δ.)
Regular lenses not used for macro photography have much lower magnifications than macro lenses. Even the highest magnifications are in the range of 0.24x (the EF 300mm f/4L IS USM at a range of 1.5 meters) or 0.23x (EF 35mm f/2 at a range of 0.25 meters). The lateral shake effect at these magnifications is only one-fourth of that of a macro lens with 1x magnification. Lateral shake is even less noticeable in normal photography. Shooting with a 100mm lens at a range of 5 meters decreases the lateral shake to one-fiftieth of a macro lens.*1
*1. Shooting at a distance of about 50x the focal length is considered ideal for portrait photography.
Lateral shake effects virtually disappear with more distant subjects and smaller magnifications (Figure 2).
This is why regular IS lenses are able to use just an angular velocity sensor (commonly known as a vibration gyroscope) to detect angular shake and still obtain satisfactory stabilization results.
(2) The Hybrid IS system
In macro photography, however, and particularly at 1x magnification, 0.1mm of shake on the photographer's side translates to 0.1mm (100 μm) of blurring in the image (Figure 3).
As a result, the total camera shake visible in an image is a combination of lateral shake and angular shake (Figure 4).
Conventional image stabilization techniques have been unable to account for lateral shake in the macro photography range and, thus, a solution has long been awaited for the lack of image stabilization at macro distances.
For this new macro lens, we have developed a new Hybrid IS system algorithm (Figure 5), which incorporates three techniques to correct lateral shake:
(1) pairing an angular velocity sensor with an acceleration sensor
(2) analyzing the correlation between the two sensors' detection signals
(3) calculating the amount of image plane blur from the correlation of the shake signals
We also optimized how the correction optics are driven in the IS unit in response to the data from the Hybrid IS system. This results in effective image stabilization in the macro range, even at 1x magnification.
The Hybrid IS system and the optimized IS unit deliver a stabilization gain of more than two f-stops in shutter speed at 1x magnification compared to shooting without IS. Of course, the Hybrid IS system still gives the same stabilization gain that normal IS systems provide of about four f-stops*2 in shutter speed at normal shooting distances compared with (1/focal length).
*2. The slowest shutter speed for blur-free hand-held shooting is generally considered to be [1/focal length] seconds. Therefore, this shutter speed is used as a reference for measuring the stabilization gain.
Photo 2 shows a number of test photos taken by our designers.
To simulate an amateur photographer, the photos were taken hand held*3 with the focus on the coin's edge. Although it is hard to recognize on the page, when expanded you can clearly see that the outlines look sharp and crisp when shot with Hybrid IS on. Incidentally, we recommend that the camera's AF mode be set to AI Servo for macro photography.
*3. Both elbows were braced when shooting to limit camera shake.
(3) High performance optics and supporting mechanisms for best-in-class image quality
Conventional macro lenses use a four-group structure in a + - + - arrangement. The second and third groups move for focusing. On this lens, however, we added a positive lens group between the second group and the fourth group (the former third group) and split the former fourth group into two groups. This means there are six lens groups arranged as + - + + - +. Furthermore, we use one half of the former fourth group as the image stabilization group and the other as the aberration balance regulation group. This redesign has the benefit of limiting image curvature fluctuations due to focusing and minimizing performance degradation near the edges. Using a UD lens element in the first lens unit reduces by half the lateral chromatic aberrations that macro photography is susceptible to and dramatically improves MTF at the periphery.
We also took a long look at the barrel technologies that support the optics. We decided to use a ceramic ball support in the image stabilization unit (see page 22 in Canon Technology Highlights 2009*4) for very sensitive responses to even the slightest camera shake.
*4. Canon Technology Highlights 2009 is available from the Canon Web site.
The lens's inner-focusing AF system, ring-USM drive, high-speed CPU, and optimized AF algorithms enable fast, easy autofocusing. The focusing distance range switch has three positions, including a macro range, for fine focusing adjustments.
We also redesigned the aperture mechanism, developing a new nine-blade electromagnetic diaphragm with an optimized blade shape. As a result, the aperture shape remains an almost perfect circle in the macro photography position not only at maximum aperture but also when stopped down as much as two f-stops. This aperture design, coupled with the improved periphery image performance due to the aforementioned optics redesign, allows photographers to enjoy superb bokeh even to the edges of the frame.
In macro photography, the effective aperture gets smaller as the magnification increases. With this lens, then, [Therefore, with this lens] the exposure must be corrected when using a handheld exposure meter (see Table 1 for the correction amounts).
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(4) Other characteristics
We used the latest simulation technology to strengthen the body construction so the lens can withstand rough usage by professionals and advanced amateurs. We successfully improved the lens's ruggedness while keeping weight increase to a minimum. Dust-resistant and water-resistant construction was employed for the mount assembly, the switch panel, the switches, the focus ring, and other moveable parts to keep dust and water from entering the lens.
Naturally, the lens only uses eco-friendly lead-free glass.
Using accessories
The ET-73 lens hood fits this lens. For tripod work, we recommend using the D (B) tripod mount ring developed specifically for this lens instead of using the camera's mount. Like similar mounts for telephoto lenses, this mount ring attaches directly to the lens, leaving the camera free to be turned. This allows for more flexible shooting while keeping the optical axis directly on the subject.
We recommend using a macro ring light for macro photography because built-in camera flashes tend to be partially blocked by the lens. The Macro Ring Lite MR-14EX and the Macro Twin Lite MT-24EX can be used with the Macro Lite Adapter 67 (all sold separately).
To support as many photographic styles as possible, we have assembled a wide variety of tools, including various filters, extension tubes, gelatin filter holders III/IV, protect filters (67mm), and polarizing filters (67mm).
This concludes our look this month at the characteristic features of the EF 100mm f/2.8L Macro IS USM lens. Table 2 compares this lens's specifications with those of the EF 100mm f/2.8 Macro USM lens and the EF 180mm f/3.5L Macro USM lens.
Try out this lens for yourself and experience the thrill of high-definition macro photography.
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