Canon lens technologies

Canon’s lens technology leads the industry in producing supersharp, high-resolution pictures. Optical engineering plays a fundamental role in the design of a dSLR camera, and consequently, lenses must also be engineered to even greater levels of precision. Through the development and use of such technologies as diffractive optics, fully electronic mounts, and image stabilization, just to name a few, Canon has stayed at the forefront of the digital market.

FLUORITE AND UD GLASS

How light is refracted depends on the wavelength of the light; this means that where the lens focuses can actually change depending upon specific colors/wavelengths. When the different wavelengths, or colors, are focused at different points, the resulting color can be rendered inaccurately in a phenomenon known as chromatic aberration, and it’s especially a problem with longer (telephoto) lenses. Lenses often contain a nonchromatic element that helps to correct chromatic aberration — think of it as a sort of filter. This solution, however, is limited to being used for only two primary (spectral) colors, so without additional optical engineering, at least some chromatic aberration may still exist.

The mineral fluorite, it turns out, has very low refractive and dispersion qualities — something beyond that of plain optical-quality glass. As a result, Canon has developed a way to integrate fluorite with the optical glass in the manufacturing process. This glass, then, is much better at refracting the three primary colors (red, green, and blue) accurately and producing a much higher quality image.

Canon also uses a similar concept in its UD glass lenses. The UD glass is a proprietary type of optical material exhibiting properties similar to that of fluorite-integrated glass. This comes in regular and super-UD versions, the latter of which is even more efficient and requires less glass to correct chromatic aberration.

ASPHERICAL LENSES

Most lenses are spherical, meaning their surfaces comprise parts of spheres. Generally speaking, these very accurately mimic the curvature of the human eye. They take a three-dimensional image and convert it into a two-dimensional, flat image on your camera’s sensor. However, with wider-angle lenses, a purely spherical lens is insufficient to avoid distortion — especially at the outer edges of the image. To remedy this, an aspherical lens was developed, which is, as indicated by the name, not purely spherical in order to correct the areas of the glass where light is not refracted accurately.

Canon uses extremely high precision machines that mold aspherical lenses to exact specifications; in some cases, the glass is then treated with an ultraviolet-hardened resin film to perfect the shape and eradicate spherical flaws.

Distortion and lens flare are both minimized with aspherical lenses, along with contrast being maximized. A good number of Canon’s mainstream lenses incorporate aspherical qualities, including such popular models as the EF 24-70mm f/2.8L and the EF 14mm f/2.8L II USM.

Of course, in some cases spherical aberrations can be used to artistic effect even if they do not provide a completely natural or accurate representation of the image being photographed. Take, for example, the image in 4-9, where slightly distorted edges help to pop out the color and dimensionality of the main subject.

Taken in Tokyo's famous Tsukiji fish market, these shellfish almost jump off the page when taken with a fisheye lens, emphasizing their color and texture even more. The areas around the side of the image are clearly distorted by the spherical 15mm lens. Taken with an EOS-1D Mark lln with an EF 15mm f/2.8 Fisheye lens, ISO 640, 1/80 second, f/4.5.

DIFFRACTIVE OPTICS

Lenses do not form perfect images, and there is always some degree of distortion or aberration introduced by the lens that causes the image to be an imperfect replica of the object. Careful design of the lens system for a particular application ensures that the aberration or distortion is minimized. Telephoto and super-telephoto lenses are especially susceptible to chromatic aberration, which is an optical color defect. Diffractive optics (DO) technology, which was designed to correct these problems in telephoto lenses, uses the principle of diffraction, which is to change the direction of a light wave’s path, to create lenses with a strong dif-fractive element. They also have optical qualities that help to correct color fringing. If you are not sure how to tell if the diffraction of your lens is good, try examining the straight edges of a subject in a photo you’ve taken. If there is a crisp clear edge without prismatic color fringing, then your lens has good diffractive optics. Figure 4-10 shows how different colors can be refracted through the lens in different ways, resulting in fringing.

Prismatic color fringing occurs when lenses fail to diffract different colors precisely, as shown here. Note how the focal plane is where all parts of the lens — from the center to the edges — must accurately point rays of light to produce a crisp image.

While only a few lenses today use Canon’s DO technology (the EF 400mm f/4 DO IS USM and the EF 70-300mm f/4.5-5.6 DO IS USM), none of their professional lenses exhibits significant distortion. However, the longer the telephoto shot, the more susceptible your shots will be to various aberrations of long-range shots such as fringing and blur.

The net result of Canon’s optical engineering and technology are lens designs that are lighter and smaller with higher image quality. DO technology takes this a step farther, meaning lenses can be even more efficient than comparable focal-length lenses that incorporate conventional glass optical elements.

FLOATING SYSTEM

Lesser-quality lenses often produce acceptable results when focused in a mid-range, common focusing distances. However, they can and do produce image aberrations at extreme focal ranges — especially the closer ones. This is generally the result of inaccuracies between lens elements that work well when positioned together for common ranges, but are less accurate when at their least-common configurations.

Canon lenses employ a floating system of optical elements, which automatically adjusts gaps between lens elements in relation to the focusing distance. This works to correct any aberrations at any range. This also limits the telescoping distance of the lens elements, which helps to eliminate them from fluctuating in their position. Because closer ranges are more severely affected by lens aberrations, the floating system is particularly helpful with macro and wide-angle lenses.

ULTRASONIC MOTORS (USM)

An ultrasonic motor is one that operates using the principle that a stator (elastic body) subject to vibration results in friction that turns the rotor in a specified direction. This very precise, quiet technology also generates a nearly insignificant amount of camera shake — making it perfect for driving the lens’s autofocus feature. Lacking gears, it is direct-drive and very efficient, which means it consumes very little power from the camera’s battery. Canon has two different USM types, one that is used in lenses with very wide apertures and super-telephoto models (a ring USM) and another designed for compact lenses (micro USM).

IMAGE STABILIZER

No one likes blurry shots, and they so often seem to occur with the images that you think are going to be the best ones. However, an unsteady hand, the wind, shooting from a vehicle, or any number of things can cause the camera to shake and result in a blurred image. Being able to hold a camera steady is a skill in which many photographers pride themselves. It’s been said that an average amateur photographer can hold a camera steady for a photo taken down to about 1/125 second; a more experienced semipro or enthusiast, down to about 1/60 second; and a pro photographer down to 1/30 second. This is assuming optimal conditions with a normal or wide-angle lens, both of which are more forgiving than a telephoto lens.

If you’ve ever watched a space shuttle launch on TV, the video cameras being used have very long telephoto lenses. They have to be tilted as the rocket soars towards space, and the longer the focal range, the less steady the image becomes — which is very visible. It’s the same with still photography: The longer the lens, and the more you have to move to get the shot, the more likely it is to be blurred by camera shake.

Shooting at a faster shutter speed, when possible, is one way to eliminate camera shake. Another is to use a tripod or monopod. Of course, that’s not always possible, and to aid in keeping the images steady, Canon has developed a very sophisticated, gyroscopically driven image-stabilization system used in a number of its long prime lenses and several zoom models. These lenses are identified with an IS, for example, EF 28-300mm f/3.5-5.6L IS USM.

This in-lens system (which, by the way, you can hear quietly whirring if you put your ear to an IS lens with your finger depressing the focus button) allows perfectly stable images to be taken down to as slow as 1/15 second for most pro photographers, and for the less experienced shooters down to 1/30 second. Most IS lenses include settings such as optimized modes for image stabilization during a panning (side-to-side) motion. The super-telephoto lenses, such as the EF 300mm up to 600mm models, also employ a mechanism that enhances IS functioning while mounted to a tripod. I tend to use the IS feature of my EF 70-200mm lens all the time, even for fast shutter-speed shots — just for added security that long-range images will be as crisp as possible, such as in 4-11.

I took this photo while standing in the Saudi Arabian Desert in Qatar where conditions are harsh and footing is unstable. My 1D Mark II was well suited for this environment, with its exceptional protection from the elements, and my EF 70-200mm f/2.8L lens's IS system served me well even while fully extended at its maximum focal length. Taken at ISO 50, 1/800 second, f/5.6.

ELECTRONIC MOUNT

Until the advent of electronic control of lenses, they operated mechanically using a control mechanism that connected to the camera at the mounting ring. The mechanics were more subject to failure and also caused considerably more noise and rattle, which can affect sensitive exposures. The mechanical controls have since been replaced by a series of electronic connections, shown in figure 4-12, that communicate with the camera and carry information to mechanical elements in the lens controlled by microprocessors.

This is a photo of the back of a Canon lens where it mounts to the camera body. Note the electronic contacts, through which information passes to and from the camera.

As many as 50 different information signals pass between camera and lens, covering and controlling everything from aperture and focus to different types of image stabilization as well as minimal focusing distances. Because the standard Canon lens mount is 54mm (which is amply large), special
lenses with extra-large apertures and controls can easily be accommodated, too, such as tilt-and-shift models.

DUST- AND WATER-RESISTANT CONSTRUCTION

Like the Mark III camera series, certain longer-range Canon lenses are constructed to be dust- and water-resistant. Specifically, the EF300mm f/2.8L USM, EF 400mm f/2.8L IS USM, EF 500mm f/4L IS USM, and EF 600mm f/4L IS USM lenses all feature rubber linings for key points where unwanted elements can enter the camera. Moving parts, such as the focusing ring and switches, are sealed, as are switch panels, exterior seams, and drop-in filter compartments. You might notice that the extra rubber sealing rings leave very fine abrasion marks on the outside of your camera’s lens mount; Canon assures customers that this has no adverse effect on operation.

TILT-AND-SHIFT LENSES

Canon tilt-and-shift lenses, designated with a TS-E in the product name, are designed to let you control the angle of the plane of focus in a lens, which allows you to manipulate perspective in an image. For example, if you stand at the bottom of a building and take a photo pointed upward, the straight vertical lines of the walls appear to converge. Taken from farther away, they appear much straighter. Photographers can use tilt-and-shift lenses to virtually eliminate convergence and control perspective in their photographers, which is especially useful for applications such as architectural photography.

A TS-E lens has control knobs for physically altering the X and Y axes of its main elements, which float so that they can be positioned for optimal effect. Tilting the lens changes the angle of the plane of focus between the lens and film plane, while shifting the lens adjusts its optical axis in parallel.

No autofocus is available for these lenses.

Category: Science of Lenses

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