In This Chapter
Understanding the focal length multiplication factor
Buying lenses for the long term
Lens choices
Zoom versus prime lenses
Working with different types of lenses
Fine-tuning lens performance
Exploring lens accessories
Learning lens lingo
One of the first questions that photographers ask after using the Rebel T1i/500D for a while is, "What lens should I buy next?" Because there are so many factors that must be considered with a lens purchase, the best way to answer that question is to evaluate your shooting preferences and needs, know what lenses are available, know your budget, and then study lens reports published online and in photography magazines. This chapter helps you with the evaluation process.
It's important to remember that your images are as good as the lens that you use. With a high-quality lens, pictures have stunning detail, high resolution, and snappy contrast. Conversely, low-quality optics produce marginal picture quality. Thus, it's a good strategy to invest in the best lens that you can afford. Over time your investment in lenses will far exceed the purchase price of the camera. If you make careful decisions on lens purchases, the lenses will pay off for years to come in getting great image sharpness and quality and in building a solid, long-lasting photography system.
This chapter looks at the lenses available to help you make decisions about lenses you can add to your system to enhance the type of photography you most enjoy.
One of the first things to know about the Rebel T1i/500D is that the image sensor is 1.6 times smaller than a traditional 35mm film frame. The Rebel's cropped sensor size affects the angle of view of all lenses you use. The angle of view is how much of the scene, side to side and top to bottom, that the lens includes in the image. For example, a 15mm fisheye lens has a 180-degree angle of view. By contrast, a 200mm lens has a scant 12-degree angle of view.
In short, the angle of view for all lenses you use on the T1i/500D is reduced by a factor of 1.6 times at any given focal length, producing an image that is equal to that of a lens with 1.6 times the focal length. That means that a 100mm lens on a 35mm film camera becomes the equivalent of a 160mm on the T1i/500D. Likewise, a 50mm lens becomes the equivalent of an 80mm lens, which is equivalent to a short telephoto lens on a full-frame 35mm size. A 24mm lens on a 35mm camera is roughly equivalent to a 38mm lens on the Rebel.
This focal length multiplication factor works to your advantage with a telephoto lens because it effectively increases the lens's focal length (although technically the focal length doesn't change). And because telephoto lenses tend to be more expensive than other lenses, you can buy a shorter and less expensive telephoto lens and get 1.6 times more magnification at no extra cost.
The focal length multiplication factor works to your disadvantage with a wide-angle lens because the sensor sees less of the scene when the focal length is magnified by 1.6x. But, because wide-angle lenses tend to be less expensive than telephoto lenses, you can buy an ultrawide 14mm lens to get the equivalent of an angle of view of 22mm.
You may be wondering if the change in angle of view also affects the depth of field. Given that telephoto lenses provide a shallow depth of field, it seems reasonable to assume that the focal length multiplication factor would produce the same depth-of-field results on the T1i/500D that a longer lens gives. That isn't the case, however. Although an 85mm lens on a full-frame 35mm camera is equivalent to a 136mm lens on the T1i/500D, the depth of field on the T1i/500D matches the 85mm lens, not the 136mm lens. This depth-of-field principle also holds true for enlargements. The depth of field in the print is shallower for the longer lens on a full-frame camera than it is for the T1i/500D.
Note
Depth of field is discussed in detail in Chapter 9.
In the Canon lens lineup, there are two types of lenses: EF, and EF-S lenses. The T1i/500D is compatible with all EFand EF-S-mount lenses, but the EF-S lenses are not compatible with full-frame Canon EOS dSLRs such as the EOS 5D Mark II and the EOS 1Ds Mark III. The EF-S lens mount is specially designed to have a smaller image circle, or the area covered by the image on the sensor plane. EF-S lenses can be used only on cameras with a cropped frame such as the T1i/500D, the 50D, and 40D among others because of a rear element that protrudes back into the camera body.
Figure 8.1. This image shows the approximate difference in image size between a 35mm film frame and the T1i/500D. The smaller image size represents the T1i/500D's image size.
On the other hand, the EF lens mount is compatible across all Canon EOS cameras regardless of image sensor size, and regardless of camera type, whether digital or film.
This distinction becomes important as you consider your long-term strategy for building a lens system. As you consider buying lenses, think about whether you want lenses that are compatible with both a full-frame camera and a cropped sensor. Consider also that as your photography career continues, you'll most likely buy a second camera body or move to another Canon EOS camera body. And if your next EOS camera body has a full-frame sensor, then you'll want the lenses that you've already acquired to be compatible with it. Both of these considerations point to buying EF-mount lenses. Certainly the EF-S lenses are often more affordable, but their use is limited only to cropped-sensor cameras.
The best starting point for considering new lenses is to gain a solid understanding of the different types of lenses and their characteristics. Only then can you evaluate which types of lenses best fit your needs. The following sections provide a foundation for evaluating lenses by category and by characteristics.
Lenses are categorized by whether they zoom to different focal lengths or have a fixed focal length — known as prime lenses. Within those two categories, lenses are grouped by focal length (the amount of the scene included in the frame) in three main categories: wide angle, normal, and telephoto. And within those categories are macro lenses that serve double-duty as either normal or telephoto lenses with macro capability. Here is a brief overview of lens categories.
Note
Lenses are categorized according to their focal length on a full-frame 35mm size sensor rather than on the smaller sensor size of the Rebel.
Wide-angle lenses. These lenses offer a wide view of a scene. Lenses shorter than 50mm are considered wide angle on full-frame 35mm image sensors. With the 1.6 multiplication factor on the Rebel, a wide-angle focal length begins at approximately 30mm and runs through the 15mm fisheye lens. A wide-angle lens offers extensive depth of field, particularly at narrow apertures such as f/8, f/11, and so on. Depth of field is the range in front of and behind the subject that is in acceptably sharp focus.
Normal lenses. A normal lens offers an angle of view and perspective very much as your eyes see the scene. On full-frame 35mm cameras, a 50mm lens is considered a normal lens. However, with the focal length conversion factor of 1.6x on the T1i/500D, a 35mm lens is closer to the normal focal length. Normal lenses provide extensive depth of field (particularly at narrow apertures of f/8, f/11, and so on) and are compact in size and versatile.
Telephoto lenses. These lenses offer a narrow angle of view, enabling close-ups of distant scenes. On full-frame 35mm cameras, lenses with focal lengths longer than 50mm are considered telephoto lenses. On the Rebel T1i/500D, telephoto is anything longer than 35mm. Telephoto lenses offer shallow depth of field, providing a softly blurred background, particularly at wide apertures such as f/5.6. And, of course, they offer a closer view of distant scenes and subjects.
Macro lenses. These lenses are designed to provide a closer lens-to-subject focusing distance than nonmacro lenses. Depending on the lens, the magnification ranges from half-life-size (0.5x) to 5x magnification. Thus, objects as small as a penny or a postage stamp can fill the frame, while nature macro shots can reveal breathtaking details that are commonly overlooked or are not visible to the human eye. By contrast, nonmacro lenses typically allow maximum magnifications of about one-tenth life-size (0.1x). Macro lenses are single focal-length lenses that come in normal and telephoto focal lengths.
Within the basic focal-length lens categories, you can choose between zoom and prime (also called single focal-length) lenses. The most basic difference between zoom and prime lenses is that zoom lenses offer a range of focal lengths in a single lens while prime lenses offer a fixed, or single, focal length. There are additional distinctions that come into play as you evaluate which type of lens is best for your shooting needs.
Zoom lenses, with their variable focal length, are versatile because they offer multiple and variable focal lengths in a single lens. Available in wide-angle and telephoto ranges, zoom lenses can maintain focus during zooming. To keep the lens size compact, and to compensate for aberrations with fewer lens elements, most zoom lenses use a multigroup zoom with three or more movable lens groups.
Some zoom lenses are slower than single focal-length lenses, and getting a fast zoom lens usually comes at a higher price. In addition, some zoom lenses have a variable aperture, which means that the minimum aperture changes at different zoom settings (discussed in the following sections).
The obvious advantage of a zoom lens is the ability to quickly change focal lengths and image composition without changing lenses. In addition, only two or three zoom lenses are needed to encompass the focal range you use most often for everyday shooting. For example, carrying a Canon EF-S 17-55mm f/2.8 IS USM lens and a Canon EF 55-200mm f/4.5-5.6 II USM lens, or a similar combination, provides the focal range needed for everything from landscape to portrait to some wildlife photography.
A zoom lens also offers the creative freedom of changing image composition with the turn of the zoom ring — all without changing your shooting position or changing lenses. Most midpriced and more expensive zoom lenses offer high-quality optics that produce sharp images with excellent contrast. As with all Canon lenses, full-time manual focusing is available by switching the button on the side of the lens to MF (Manual Focus).
Although zoom lenses allow you to carry around fewer lenses, they tend to be heavier than their single focal-length counterparts. Midpriced, fixed-aperture zoom lenses also tend to be "slow," meaning that with maximum apertures of only f/4.5 or f/5.6, they call for slower shutter speeds that, in turn, limit your ability to get sharp images when handholding the camera, provided that the lens does not have image stabilization (IS), a technology that is detailed later in this chapter.
Some zoom lenses have variable apertures. A variable-aperture lens of f/4.5 to f/5.6 means that at the widest focal length, the maximum aperture is f/4.5 and at the telephoto end of the focal range, the maximum aperture is f/5.6. In practical terms, this limits the versatility of the lens at the longest focal length for shooting in all but bright light unless you set a high ISO. And unless you use a tripod and your subject is stone still, your ability to get a tack-sharp picture in lower light at f/5.6 will be questionable.
Note
For a complete discussion on aperture, see Chapter 9.
Figure 8.2. Wide-angle zoom lenses such as the Canon EF 16-35mm f/2.8L USM lens are ideal for the smaller sensor size of the T1i/500D.
More expensive zoom lenses offer a fixed and fast maximum aperture, meaning that with maximum apertures of f/2.8, they allow faster shutter speeds that enhance your ability to get sharp images when handholding the camera in low light. But the lens speed comes at a price: the faster the lens, the higher the price.
While you hear much less about prime or single focal-length lenses, they are worth careful evaluation. With a prime lens, the focal length is fixed, so you must move closer to or farther from your subject, or change lenses to change image composition. Canon's venerable EF 50mm f/1.4 USM lens and EF 100mm f/2.8 Macro USM are only two of a full lineup of Canon prime lenses.
Unlike zoom lenses, prime lenses tend to be fast with maximum apertures of f/2.8 or wider on nontelephoto lenses and on some telephoto lenses. Wide apertures allow fast shutter speeds that enable you to handhold the camera in lower light and still get a sharp image. Compared to zoom lenses, single focal-length lenses are lighter and smaller. In addition, many photographers believe that single focal-length lenses are sharper and provide better image quality overall than zoom lenses.
Within the categories of zoom and prime lenses, lenses are grouped by their focal length. While some lenses cross group lines, the groupings are still useful for talking about lenses in general. Each type of lens has specific characteristics that you can use creatively to render images as you envision them. So as you read about the characteristics, consider how you can use each lens creatively and perhaps out of the traditional context.
Figure 8.3. Single focal-length lenses such as the EF 50mm f/1.4 USM lens are smaller and lighter and provide excellent sharpness, contrast, and resolution when used on the T1i/500D.
A wide-angle lens is a versatile lens for capturing subjects ranging from large groups of people to sweeping landscapes, as well as for taking pictures in places where space is cramped. The distinguishing characteristic of wide-angle lenses is the range of the angle of view. Within the Canon lens lineup, you can choose angles of view from the 15mm fisheye lens, which offers a 180-degree angle of view, to the 35mm lens, which offers a 63-degree angle of view, not counting the 1.6x focal-length multiplication factor.
With wide-angle lenses, the Rebel's 1.6x focal-length multiplier works to your disadvantage. For example, with the EF 28-135mm f/3.5-5.6 IS lens, the lens translates to only 44mm on the wide end and 216mm on the telephoto end of the lens. The telephoto range is excellent, but you don't get a wide angle of view with this lens. Thus, if you often shoot landscapes, cityscapes, architecture, and interiors, a priority will be to get a lens that offers a true wide-angle view. Good choices include the EF 16-35mm f/2.8L II USM (approximately 26 to 56mm with the 1.6x multiplier), the EF-S 10-22mm f/3.5-4.5 USM lens (approximately 16 to 35mm with the multiplier), or the EF 17-40mm f/4L USM lens (approximately 27 to 64mm with the multiplier).
Figure 8.4. This old wagon is always a draw for photographers, and the Canon EF 24-70mm f/2.8L USM lens provided excellent sharpness in this image. Exposure: ISO 100, f/2.8, 1/1000 second.
When you shoot with a wide-angle lens, keep these lens characteristics in mind:
Extensive depth of field. Particularly at small apertures from f/8 to f/32, the entire scene, front to back, will be in acceptably sharp focus. This characteristic gives you slightly more latitude for less-than-perfectly focused pictures.
Narrow, fast apertures. Wide-angle lenses tend to be faster (meaning they have wider apertures) than telephoto lenses. As a result, these lenses are good choices for shooting in lower light scenes.
Distortion. Wide-angle lenses can distort lines and objects in a scene, especially if you tilt the camera up or down when shooting. For example, if you tilt the camera up to photograph skyscrapers with a wide-angle lens mounted, the lines of the buildings tend to converge toward the center of the frame and the buildings appear to lean backward (also called keystoning). You can use this wide-angle lens characteristic to creatively enhance some compositions, or you can move back from the subject and keep the camera parallel to the main subject to help avoid the distortion.
Perspective. Wide-angle lenses make objects close to the camera appear disproportionately large. You can use this characteristic to move the closest object visually forward in the image, or you can move back from the closest object to reduce the effect. Moderate wide-angle lenses are popular for portraits, but if you use a wide-angle lens for close-up portraiture, keep in mind that the lens exaggerates the size of facial features closest to the lens, which can be unflattering.
Telephoto lenses offer a narrow angle of view, enabling close-ups of distant scenes. On the T1i/500D, the focal-length multiplier works to your advantage with telephoto lenses. Factoring in the 1.6x multiplier, a 50mm lens is equivalent to 80mm, or a short telephoto lens. And because telephoto lenses are more expensive overall than wide-angle lenses, you get more focal length for your money when you buy telephoto lenses for the T1i/500D.
Figure 8.5. Telephoto lenses are larger and heavier than wide-angle and normal lenses, but having a sharp and versatile telephoto zoom lens is indispensable. This 70-200mm lens also features image stabilization, which helps counteract camera shake when handholding the camera.
Telephoto lenses offer an inherently shallow depth of field that is heightened by shooting at wide apertures. Choose a telephoto lens to take portraits and to capture distant subjects such as birds, buildings, wildlife, and landscapes. Short telephoto lenses such as 85mm and 100mm are ideal for portraits, while long lenses (200mm to 800mm) allow you to photograph distant birds, wildlife, and athletes. When photographing wildlife, long lenses also allow you to keep a safe distance from the subject.
When you shoot with a telephoto lens, keep these lens characteristics in mind:
Shallow depth of field. Telephoto lenses magnify subjects and provide a limited range of sharp focus. At wide apertures, such as f/4, you can reduce the background to a soft blur. Because of the shallow depth of field, there is no latitude for anything except tack-sharp focus. All Canon lenses include full-time manual focusing that you can use to fine-tune the camera's autofocus.
Narrow coverage of a scene. Because the angle of view is narrow with a telephoto lens, much less of the scene is included in the image. You can use this characteristic to exclude distracting scene elements from the image.
Slow speed. Midpriced telephoto lenses tend to be slow; the widest aperture is often f/4.5 or f/5.6, which limits the ability to get sharp images without a tripod in all but bright light unless the lens has image stabilization. And because of the magnification, even the slight movement when handholding the camera and lens or in subject movement is exaggerated.
Perspective. Telephoto lenses tend to compress perspective, making objects in the scene appear stacked together.
Normal lenses offer an angle of view and perspective very much as your eyes see the scene. On full-frame 35mm cameras, 50mm lenses are considered normal lenses. However, on the T1i/500D, a normal lens is 28mm to 35mm when you take into account the focal-length multiplier. And, likewise, the 50mm lens is equivalent to an 80mm lens on the T1i/500D.
When you shoot with a normal lens, keep these lens characteristics in mind:
Natural angle of view. On the T1i/500D, a 28 or 35mm lens closely replicates the sense of distance and perspective of the human eye. This means the final image will look much as you remember seeing it when you made the picture.
Little distortion. Given the natural angle of view, the 50mm lens retains a normal sense of distance, especially when you balance the subject distance, perspective, and aperture.
Macro lenses are designed to provide a closer lens-to-subject focusing distance than nonmacro lenses. Depending on the lens, the magnification ranges from half life-size (0.5x) to 5x magnification. Thus, objects as small as a penny or a postage stamp can fill the frame, while nature macro shots can reveal breathtaking details that are commonly overlooked or are not visible to the human eye. Macro lenses are single focal-length lenses that come in normal and telephoto focal lengths.
Figure 8.6. Canon offers several macro lenses, including the EF 180mm f/3.5L Macro USM (left), which offers 1x (life-size) magnification and a minimum focusing distance of 0.48m/1.6 ft. Also shown here is the Canon EF 100mm f/2.8 Macro USM lens.
Normal and telephoto lenses offer macro capability. Because these lenses can be used at their normal focal length as well as for macro photography, they do double-duty. Macro lenses offer one-half or life-size magnification or up to 5x magnificaion with the MP-E 65mm f/2.8 1-5 Macro Photo lens.
Based on focal length and magnification, choose the lens that best suits the kinds of subjects you most often photograph. I often use the 100mm, f/2.8 Macro USM lens as a walk-around lens because much of my work lends itself to a short telephoto focal length and macro work.
Referred to as TS-E, tilt-and-shift lenses allow you to alter the angle of the plane of focus between the lens and sensor plane to provide a broad depth of field even at wide apertures and to correct or alter perspective at almost any angle. This allows you to correct perspective distortion and control focusing range.
Tilt movements allow you to bring an entire scene into focus, even at maximum apertures. By tilting the lens barrel, you can adjust the lens so that the plane of focus is uniform on the focal plane, thus changing the normally perpendicular relationship between the lens's optical axis and the camera's focal plane. Alternately, reversing the tilt has the opposite effect of greatly reducing the range of focusing.
Shift movements avoid the trapezoidal effect that results from using wide-angle lenses pointed up — to take a picture of a building, for example. Keeping the camera so that the focal plane is parallel to the surface of a wall and then shifting the TS-E lens to raise the lens results in an image with the perpendicular lines of the structure being rendered perpendicular and with the structure being rendered with a rectangular appearance.
TS-E lenses revolve within a range of plus/minus 90 degrees making horizontal shift possible, which is useful in shooting a series of panoramic images. You can also use shifting to prevent having reflections of the camera or yourself in images that include reflective surfaces, such as windows, car surfaces, and other similar surfaces.
All of Canon's TS-E lenses are manual focus only. These lenses, depending on the focal length, are excellent for architectural, interior, merchandise, nature, and food photography.
For anyone who's thrown away a stack of images blurred from handholding the camera at slow shutter speeds, Image Stabilization is a welcome addition to lenses. Image Stabilization (IS) is a technology that counteracts some or all of the motion blur that can happen when you handhold the camera. If you've shopped for lenses lately, then you know that IS comes at a premium price. IS lenses are pricey because they give you from 1 to 4 f-stops of additional stability over non-image-stabilized lenses — and that means that you may be able to leave the tripod at home.
With an IS lens, miniature sensors and a high-speed microcomputer built into the lens analyze vibrations and apply correction via a stabilizing lens group that shifts the image parallel to the focal plane to cancel camera shake. The lens detects camera motion via two gyro sensors — one for yaw and one for pitch. The sensors detect the angle and speed of shake. Then the lens shifts the IS lens group to suit the degree of shake to steady the light rays reaching the focal plane.
But what about when you want to pan or move the camera with the motion of a subject? Predictably, IS detects panning as camera shake and the stabilization then interferes with framing the subject. To correct this, Canon offers two modes on IS lenses. Mode 1 is designed for stationary subjects. Mode 2 shuts off IS in the direction of movement when the lens detects large movements for a preset amount of time. So when panning horizontally, horizontal IS stops but vertical IS continues to correct any vertical shake during the panning movement.
Stabilization is particularly important with long lenses, where the effect of shake increases as the focal length increases. As a result, the correction needed to cancel camera shake increases proportionately.
To see how the increased stability pays off, consider that the rule of thumb for handholding the camera and a non-IS lens is 1/[focal length]. For example, the slowest shutter speed at which you can handhold a 200mm lens and avoid motion blur is 1/200 second. If the handholding limit is pushed, then shake from handholding the camera bends light rays coming from the subject into the lens relative to the optical axis, and the result is a blurry image. But when you factor in IS, then you can handhold the camera and lens at 1/100 or 1/60 second or even a slower shutter speed and get a sharp image.
IS lenses come at a higher price, but they are very effective for low-light scenes and telephoto shooting.
Depending on the lens you're using, there may be some light falloff within the lens that causes the corners of the image to be darker than the center area of the frame. The corner darkening is referred to as vignetting. Vignetting is most likely to appear in images shot with wide-angle lenses, at a lens's maximum aperture, or when an obstruction such as the lens barrel rim or a filter reduces light that reaches the frame corners.
You can correct for lens light falloff in the camera if you shoot JPEG images. If you shoot RAW images, you can make the correction in Canon's Digital Photo Professional program during RAW image conversion. For JPEG images, the correction is applied by enabling the Peripheral Illumination Correction option. Once the Peripheral Illumination Correction option is enabled, the camera automatically applies correction for any of the lenses that have correction data registered in the camera. The Rebel ships with lens correction data for 25 Canon lenses. If the lens you're using isn't among the 25, you can register correction data for your lens using the EOS Utility, a program included on the EOS Digital Solution Disk.
Note
If you're using a non-Canon lens, be sure to turn off Peripheral Illumination Correction. On the Rebel T1i/500D, Peripheral Lens Illumination Correction is turned on by default.
If you use Peripheral Illumination Correction for JPEG images, the amount of correction applied is just shy of the maximum amount. If you shoot RAW images, you can, however, apply the maximum correction in Digital Photo Professional. Also, the amount of correction for JPEG images decreases as the ISO sensitivity setting increases. If the lens does not communicate distance information to the camera, then less correction is applied.
Here's how to turn on or turn off Peripheral Illumination Correction. Because Peripheral Illumination Correction is turned on by default, do these steps only if you are using a non-Canon lens, or if you do not want the correction for some other reason.
Set the camera to the JPEG image quality setting that you want.
Press the Menu button, and then turn the Main dial to highlight the Shooting 1 (red) menu.
Press the down cross key to highlight Peripheral illumin. correct., and then press the Set button. The Peripheral illumin. Correct. screen appears with the attached lens listed, and whether correction data is available. If correction data is unavailable, then you can register correction data using the Canon EOS Utility program.
By default, Peripheral Illumination Correction is turned on. If you're using a non-Canon lens, then press the down cross key to select Disable, and then press the Set button.
There are a variety of ways to increase the focal range and decrease the focusing distance to provide flexibility for the lenses you already own. These accessories are not only economical, but they also extend the range and creative options of existing and new lenses. Lens accessories can be as simple as using a lens hood to avoid flare; adding a tripod mount to quickly change between vertical and horizontal positions without changing the optical axis or the geometrical center of the lens; or adding a drop-in or adapter-type gelatin filter holder. Other options include using lens extenders, extension tubes, and close-up lenses.
For relatively little cost, you can increase the focal length of any lens by using an extender. An extender is a lens set in a small ring mounted between the camera body and a regular lens. Canon offers two extenders, a 1.4x and 2x, that are compatible only with L-series Canon lenses. Extenders can also be combined to get even greater magnification.
For example, using the Canon EF 2x II extender with a 200mm lens doubles the lens's focal length to 400mm, and then you can apply the 1.6x focal length conversion factor to get the equivalent of a 640mm lens. Using the Canon EF 1.4x II extender increases a 200mm lens to 280mm or 448mm with the 1.6x conversion factor.
Extenders reduce the amount of light reaching the sensor. The EF 1.4x II extender decreases the light by 1 f-stop, and the EF 2x II extender decreases the light by 2 f-stops. In addition to being fairly lightweight, the obvious advantage of extenders is that they can reduce the number of telephoto lenses you carry.
The 1.4x extender can be used with fixed focal-length lenses 135mm and longer (except the 135mm f/2.8 Softfocus lens) and with zoom lenses including the 70-200mm f/2.8L, 70-200mm f/2.8L IS, 70-200mm f/4L, 70-200mm f/4L IS USM, and 100-400mm f/4.5-5.6L IS zoom lenses. With the EF 2x II, autofocus is possible if the lens has an f/2.8 or faster maximum aperture and compatible IS lenses continue to provide stabilization for two shutter speeds less than 1/focal length in seconds.
Extension tubes are close-up accessories that provide magnification increases from approximately 0.3 to 0.7, and can be used on many EF lenses, though there are exceptions. Extension tubes are placed between the camera body and lens and connect to the camera via eight electronic contact points. The function of the camera and lens is unchanged, and you can combine extension tubes for greater magnification.
Canon offers two extension tubes, the EF 12 II and the EF 25 II. Magnification differs by lens, but with the EF12 II and standard zoom lenses, it is approximately 0.3 to 0.5. With the EF 25 II, magnification is 0.7. When combining tubes, you may need to focus manually.
Figure 8.8. Extenders, such as this Canon EF 1.4x II mounted between the camera body and the lens, extend the range of L-series lenses. They increase the focal length by a factor of 1.4x in addition to the 1.6x focal length multiplication factor inherent in the camera.
Extension tube EF 25 II is not compatible with the EF 15mm f/2.8 Fisheye, EF 14mm f/2.8L USM, EF 20mm f/2.8 USM, EF 24mm f/1.4L USM, EF 16-35mm f/2.8L USM, EF 17-40mm f/4L USM, EF 20-35mm f/3.5-4.5 USM, EF 24-70mm f/2.8L USM, MP-E 65mm f/2.8 1-5x Macro Photo, TS-E 45mm f/2.8, and EF-S 18-55mm f/3.5-5.6 (at wide angles). Extension tube EF12 II is not compatible with the EF 15mm f/2.8 Fisheye, EF 14mm, f/2.8L USM, and MP-E 65mm f/2.8 1-5X Macro Photo lens.
Additionally, you can use screw-in close-up lenses. Canon offers three lenses that provide enhanced close-up photography. The 250D/500D series uses a double-element design for enhanced optical performance. The 500D series features single-element construction for economy. The working distance from the end of the lens is 25cm for the 250D, and 50cm for the 500D.
As you shop for lenses and read lens reports, the terms can be very confusing. There is no need to memorize these terms, but when you are considering lenses, come back to this section of the book to look up the terms you're reading about or hearing from a salesperson. In that way, you can make a more informed decision and know what elements are important for the type of lens you're considering.
Here are key terms for lens construction and technology.
EF/EF-S lens mount. This designation identifies the type of mount that the lens has and the camera accepts. The EF lens mount provides not only quick mounting and removal of lenses, but it also provides the communication channel between the lens and the camera body. The EF mount is fully electronic and resists abrasion, shock, and play. The EF system does a self-test using a built-in microcomputer so that you're alerted of possible malfunctions of the lens via the camera's LCD display. In addition, if you use lens extenders, the exposure compensation is automatically calculated. The EF-S lens mount is designed specifically for cropped image-sensor cameras such as the T1i/500D. These lenses tend to be more affordable with good optical resolution. Because EF-S lenses have a shorter back focus than EF lenses, the EF-S lenses won't be compatible should you eventually buy a full-frame EOS camera.
USM. When you see USM, it indicates the lens features a built-in ultrasonic motor with a very quiet focusing mechanism. The motor is powered by the camera; however, because the lens has its own focusing motor, you get fast focus. USM lenses use electronic vibrations created by piezoelectric ceramic elements to provide quick and quiet focusing action with near instantaneous starts and stops.
In addition, lenses with a ring-type ultrasonic motor offer full-time manual focusing without the need to first switch the lens to manual focus. This design is offered in the large-aperture and super-telephoto lenses. A second design, the micro-ultrasonic motor, provides the advantages of this technology in the less expensive EF lenses.
L-series lenses. Canon's L-series lenses feature a distinctive red ring on the outer barrel, or in the case of telephoto and super-telephoto lenses, are distinguished by Canon's well-known white barrel. The distinguishing characteristics of L-series lenses, in addition to their sobering price tags, are a combination of technologies that provide outstanding optical performance. L-series lenses include one or more of the following technologies and features:
UD/Fluorite elements. Ultralow Dispersion (UD) glass elements help minimize color fringing or chromatic aberration. This glass also provides improved contrast and sharpness. On the other hand, Fluorite elements, which are used in super-telephoto L-series lenses, reduce chromatic aberration. Lenses with UD or fluorite elements are designated as CaF2, UD, and/or S-UD.
Aspherical elements. This technology is designed to help counteract blurred images that happen as a result of spherical aberration. Spherical aberration happens when wide-angle and fast normal lenses cannot resolve into a sharp point of focus light rays coming into the lens from the center with those coming from the edge. An aspherical element uses a varying curved surface to ensure that the entire image plane appears focused. These types of optics help correct distortion in ultrawide-angle lenses as well. Lenses with aspherical elements are designated as AL.
Dust, water-resistant construction. The L-series EF lenses stand up well to inclement weather and heavy use. L-series lenses have rubber seals at the switch panels, exterior seams, drop-in filter compartments, and lens mounts to make them both dustand water-resistant. Moving parts, including the focusing ring and switches, are also designed to keep out environmental contaminants.
Image stabilization. Lenses labeled as IS lenses offer image stabilization, which is detailed earlier in this chapter. IS lenses enable you to handhold the camera at light levels that normally require a tripod.
Macro. Macro lenses enable close-up focusing with subject magnification of one-half to life-size and up to 5x with the MP-E 65mm f/2.8 Macro Photo lens.
Full-time manual focusing. An advantage of Canon lenses is the ability to use autofocus, and then tweak focus manually using the lens's focusing ring without switching out of autofocus mode or changing the switch on the lens from the AF (Autofocus) to MF (Manual Focusing) setting.
Inner and rear focusing. Lenses' focusing groups can be located in front of or behind the lens diaphragm, both of which allow for compact optical systems with fast AF. Lenses with rear optical focusing, such as the EF 85mm f/1.8 USM, focus faster than lenses that move their entire optical system, such as the EF 85mm f/1.2L II USM.
Floating system. Canon lenses use a floating system that dynamically varies the gap between key lens elements based on the focusing distance. As a result, optical aberrations are reduced or suppressed through the entire focusing range. In comparison, optical aberrations in nonfloating system lenses are corrected only at commonly used focusing distances. At other focusing distances, particularly at close focusing distances, the aberrations appear and reduce image quality.
AF Stop. The AF Stop button, offered on several EF IS super-telephoto lenses, allows you to temporarily suspend autofocusing of the lens if an obstruction comes between the lens and the subject to prevent the focusing from being thrown off.
Diffractive optics. Diffractive optics (DO) are created by bonding diffractive coatings to the surfaces of two or more lens elements. The elements are then combined to form a single multilayer DO element designed to cancel chromatic aberrations at various wavelengths when combined with conventional glass optics. Diffractive optics result in smaller and shorter telephoto lenses without compromising image quality.