Avoiding Potential Wide Angle Problems

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Wide-angle lenses have a few quirks that you'll want to keep in mind when shooting so you can avoid falling into some common traps. Here's a checklist of tips for avoiding common problems:

■ Symptom: converging lines. Unless you want to use wildly diverging lines as a creative effect, it's a good idea to keep horizontal and vertical lines in landscapes, architecture, and other subjects carefully aligned with the sides, top, and bottom of the frame. That will help you avoid undesired perspective distortion. Sometimes it helps to shoot from a slightly elevated position so you don't have to tilt the camera up or down.

■ Symptom: color fringes around objects. Lenses are often plagued with fringes of color around backlit objects, produced by chromatic aberration, which comes in two forms: longitudinal/axial, in which all the colors of light don't focus in the same plane; and lateral/transverse, in which the colors are shifted to one side. Axial chromatic aberration can be reduced by stopping down the lens, but transverse CA cannot. Both can be reduced by using lenses with low diffraction index glass (or UD elements, in Canon nomenclature) and by incorporating elements that cancel the chromatic aberration of other glass in the lens. For example, a strong positive lens made of low dispersion crown glass (made of a soda-lime-silica composite) may be mated with a weaker negative lens made of high-dispersion flint glass, which contains lead.

■ Symptom: lines that bow outward. Some wide-angle lenses cause straight lines to bow outwards, with the strongest effect at the edges. In fisheye (or curvilinear) lenses, this defect is a feature, as you can see in Figure 6.9. When distortion is not desired, you'll need to use a lens that has corrected barrel distortion. Manufacturers like Canon do their best to minimize or eliminate it

Figure 6.9 Many wide-angle lenses cause lines to bow outwards towards the edges of the image; with a fisheye lens, this tendency is considered an interesting feature.

(producing a rectilinear lens), often using asphericallens elements (which are not cross-sections of a sphere). You can also minimize barrel distortion simply by framing your photo with some extra space all around, so the edges where the defect is most obvious can be cropped out of the picture.

■ Symptom: dark corners and shadows in flash photos. The Canon EOS 40D's built-in electronic flash is designed to provide even coverage for lenses as wide as 17mm. If you use a wider lens, you can expect darkening, or vignetting, in the corners of the frame. At wider focal lengths, the lens hood of some lenses (my 17mm-85mm lens is a prime offender) can cast a semi-circular shadow in the lower portion of the frame when using the built-in flash. Sometimes removing the lens hood or zooming in a bit can eliminate the shadow. Mounting an external flash unit, such as the mighty Canon 580EX can solve both problems, as it has zoomable coverage up to 114 degrees with the included adapter, sufficient for a 9mm rectilinear lens (14mm equivalent). Its higher vantage point eliminates the problem of lens-hood shadow, too.

■ Symptom: light and dark areas when using polarizing filter. If you know that polarizers work best when the camera is pointed 90 degrees away from the sun and have the least effect when the camera is oriented 180 degrees from the sun, you know only half the story. With lenses having a focal length of 10mm to 18mm (the equivalent of 16mm-28mm), the angle of view (107 to 75 degrees diagonally, or 97 to 44 degrees horizontally) is extensive enough to cause problems. Think about it: when a 10mm lens is pointed at the proper 90-degree angle from the sun, objects at the edges of the frame will be oriented at 135 to 41 degrees, with only the center at exactly 90 degrees. Either edge will have much less of a polarized effect. The solution is to avoid using a polarizing filter with lenses having an actual focal length of less than 18mm (or 28mm equivalent).

Using Telephoto and Tele-Zoom Lenses

Telephoto lenses also can have a dramatic effect on your photography, and Canon is especially strong in the long-lens arena, with lots of choices in many focal lengths and zoom ranges. You should be able to find an affordable telephoto or tele-zoom to enhance your photography in several different ways. Here are the most important things you need to know. In the next section, I'll concentrate on telephoto considerations that can be problematic—and how to avoid those problems.

■ Selective focus. Long lenses have reduced depth-of-field within the frame, allowing you to use selective focus to isolate your subject. You can open the lens up wide to create shallow depth-of-field, or close it down a bit to allow more to be in focus. The flip side of the coin is that when you want to make a range of objects sharp, you'll need to use a smaller f/stop to get the depth-of-field you need. Like fire, the depth-of-field of a telephoto lens can be friend or foe. Figure 6.10 shows a photo of a piece of antique glassware that couldn't be removed from its display shelf, so I photographed it using a short tele-photo lens and wider f/stop to de-emphasize the other glassware in the background.

■ Getting closer. Telephoto lenses bring you closer to wildlife, sports action, and candid subjects. No one wants to get a reputation as a surreptitious or "sneaky" photographer (except for paparazzi), but when applied to candids in an open and honest way, a long lens can help you capture memorable moments while retaining enough distance to stay out of the way of events as they transpire.

■ Reduced foreground/increased compression. Telephoto lenses have the opposite effect of wide angles: they reduce the importance of things in the foreground by squeezing everything together. This compression even makes distant objects appear to be closer to subjects in the foreground and middle ranges. You can use this effect as a creative tool.

Figure 6.10 A wide f/stop helped isolate this antique glassware from the other items on the display shelf.

■ Accentuates camera shakiness. Telephoto focal lengths hit you with a dou-ble-whammy in terms of camera/photographer shake. The lenses themselves are bulkier, more difficult to hold steady, and may even produce a barely perceptible see-saw rocking effect when you support them with one hand halfway down the lens barrel. Telephotos also magnify any camera shake. It's no wonder that image stabilization is popular in longer lenses.

■ Interesting angles require creativity. Telephoto lenses require more imagination in selecting interesting angles, because the "angle" you do get on your subjects is so narrow. Moving from side to side or a bit higher or lower can make a dramatic difference in a wide-angle shot, but raising or lowering a tele-photo lens a few feet probably will have little effect on the appearance of the distant subjects you're shooting.

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100 Photography Tips

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