Zoom or Prime

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Zoom lenses have changed the way serious photographers take pictures. One of the reasons that I own 12 SLR film bodies is that in ancient times it was common to mount a different fixed focal length prime lens on various cameras and take pictures with two or three cameras around your neck (or tucked in a camera case) so you'd be ready to take a long shot or an intimate close-up or wide-angle view on a moment's notice, without the need to switch lenses. It made sense (at the time) to have a half-dozen or so bodies (two to use, one in the shop, one in transit, and a couple backups). Zoom lenses of the time had a limited zoom range, were heavy, and not very sharp (especially when you tried to wield one of those monsters handheld).

That's all changed today. Lenses like the razor-sharp Canon EF 28-300mm f/3.5-5.6L IS USM can boast 10X or longer zoom ranges, in a package that's about 7-inches long, and while not petite at 3.7 pounds, quite usable handheld (especially with IS switched on). Although such a lens might seem expensive at $2,200-plus, it's actually much less costly than the six or so lenses it replaces.

When selecting between zoom and prime lenses, there are several considerations to ponder. Here's a checklist of the most important factors. I already mentioned image quality and maximum aperture earlier, but those aspects take on additional meaning when comparing zooms and primes.

■ Logistics. As prime lenses offer just a single focal length, you'll need more of them to encompass the full range offered by a single zoom. More lenses mean additional slots in your camera bag, and extra weight to carry. Just within Canon's line alone you can select from about a dozen general purpose prime lenses in 28mm, 35mm, 50mm, 85mm, 100mm, 135mm, 200mm, and 300mm focal lengths, all of which are overlapped by the 28-300mm zoom I mentioned earlier. Even so, you might be willing to carry an extra prime lens or two in order to gain the speed or image quality that lens offers.

■ Image quality. Prime lenses usually produce better image quality at their focal length than even the most sophisticated zoom lenses at the same magnification. Zoom lenses, with their shifting elements and f/stops that can vary from zoom position to zoom position, are in general more complex to design than fixed focal length lenses. That's not to say that the very best prime lenses can't be complicated as well. However, the exotic designs, aspheric elements, low-dispersion glass, and Canon's new diffraction optics (DO) technology can be applied to improving the quality of the lens, rather than wasting a lot of it on compensating for problems caused by the zoom process itself.

■ Maximum aperture. Because of the same design constraints, zoom lenses usually have smaller maximum apertures than prime lenses, and the most affordable zooms have a lens opening that grows effectively smaller as you zoom in. The difference in lens speed verges on the ridiculous at some focal lengths. For example, the 18mm-55mm basic zoom gives you a 55mm f/5.6 lens when zoomed all the way out, while prime lenses in that focal length commonly have f/1.8 or faster maximum apertures. Indeed, the fastest f/2, f/1.8, f1/4, and f/1.2 lenses are all primes, and if you require speed, a fixed focal length lens is what you should rely on. Figure 6.7 shows an image taken with a Canon 85 mm f/1.8 Series EF USM Telephoto Lens.

■ Speed. Using prime lenses takes time and slows you down. It takes a few seconds to remove your current lens and mount a new one, and the more often you need to do that, the more time is wasted. If you choose not to swap lenses, when using a fixed focal length lens you'll still have to move closer or farther away from your subject to get the field of view you want. A zoom lens allows you to change magnifications and focal lengths with the twist of a ring and gen- Figure 6.7 An 85mm f/1.8 lens was perfect for this handheld photo of erally saves a great deal of time. bluesman Tab Benoit.

Categories of Lenses

Lenses can be categorized by their intended purpose—general photography, macro photography, and so forth—or by their focal length. The range of available focal lengths is usually divided into three main groups: wide-angle, normal, and telephoto. Prime lenses fall neatly into one of these classifications. Zooms can overlap designations, with a significant number falling into the catch-all wide-to-telephoto zoom range. This section provides more information about focal length ranges, and how they are used.

Any lens with an equivalent focal length of 10mm to 20mm is said to be an ultra-wide-angle lens; from about 20mm to 40mm (equivalent) is said to be a wide-angle lens. Normal lenses have a focal length roughly equivalent to the diagonal of the film or sensor, in millimeters, and so fall into the range of about 45mm to 60mm (on a full-frame camera). Telephoto lenses usually fall into the 75mm and longer focal lengths, while those from about 300mm-400mm and longer often are referred to as super-telephotos.

Using Wide-Angle and Wide-Zoom Lenses

To use wide-angle prime lenses and wide zooms, you need to understand how they affect your photography. Here's a quick summary of the things you need to know.

■ More depth-of-field. Practically speaking, wide-angle lenses offer more depth-of-field at a particular subject distance and aperture. (But see the sidebar below for an important note.) You'll find that helpful when you want to maximize sharpness of a large zone, but not very useful when you'd rather isolate your subject using selective focus (telephoto lenses are better for that).

■ Stepping back. Wide-angle lenses have the effect of making it seem that you are standing farther from your subject than you really are. They're helpful when you don't want to back up, or can't because there are impediments in your way.

■ Wider field of view. While making your subject seem farther away, as implied above, a wide-angle lens also provides a larger field of view, including more of the subject in your photos. Table 6.1 shows the diagonal field of view offered by an assortment of lenses, taking into account the crop factor introduced by the EOS 40D's smaller-than-full-frame sensor.

■ More foreground. As background objects retreat, more of the foreground is brought into view by a wide-angle lens. That gives you extra emphasis on the area that's closest to the camera. Photograph your home with a normal

Table 6.1 Field of View at Various Focal Lengths

Diagonal Field Length at Focal Length Needed to Produce Same

Table 6.1 Field of View at Various Focal Lengths

Diagonal Field Length at Focal Length Needed to Produce Same

of View


1X Crop Field of View at 1.6X Crop

107 degrees



94 degrees



84 degrees



75 degrees



63 degrees



47 degrees



28 degrees



18 degrees



12 degrees



8.2 degrees



lens/normal zoom setting, and the front yard probably looks fairly conventional in your photo (that's why they're called "normal" lenses). Switch to a wider lens and you'll discover that your lawn now makes up much more of the photo. So, wide-angle lenses are great when you want to emphasize that lake in the foreground, but problematic when your intended subject is located farther in the distance.

■ Super-sized subjects. The tendency of a wide-angle lens to emphasize objects in the foreground, while de-emphasizing objects in the background can lead to a kind of size distortion that may be more objectionable for some types of subjects than others. Shoot a bed of flowers up close with a wide angle, and you might like the distorted effect of the larger blossoms nearer the lens. Take a photo of a family member with the same lens from the same distance, and you're likely to get some complaints about that gigantic nose in the foreground.

■ Perspective distortion. When you tilt the camera so the plane of the sensor is no longer perpendicular to the vertical plane of your subject, some parts of the subject are now closer to the sensor than they were before, while other parts are farther away. So, buildings, flagpoles, or NBA players appear to be falling backwards, as you can see in Figure 6.8. While this kind of apparent distortion (it's not caused by a defect in the lens) can happen with any lens, it's most apparent when a wide angle is used.

Tilting the camera back produces this "falling back" look in architectural photos.

Tilting the camera back produces this "falling back" look in architectural photos.

■ Steady cam. You'll find that you can handhold a wide-angle lens at slower shutter speeds, without need for image stabilization, than you can with a tele-photo lens. The reduced magnification of the wide-lens or wide-zoom setting doesn't emphasize camera shake like a telephoto lens does.

■ Interesting angles. Many of the factors already listed combine to produce more interesting angles when shooting with wide-angle lenses. Raising or lowering a telephoto lens a few feet probably will have little effect on the appearance of the distant subjects you're shooting. The same change in elevation can produce a dramatic effect for the much-closer subjects typically captured with a wide-angle lens or wide-zoom setting.

The crop factor strikes again! You can see from this table that wide-angle lenses provide a broader field of view, and that, because of the 40D's 1.6 crop factor, lenses must have a shorter focal length to provide the same field of view. If you like working with a 28 mm lens with your full-frame camera, you'll need an 18 mm lens for your EOS 40D to get the same field of view. (Some focal lengths have been rounded slightly for simplification.)


The DOF advantage of wide-angle lenses is diminished when you enlarge your picture; believe it or not, a wide-angle image enlarged and cropped to provide the same subject size as a telephoto shot would have the same depth-of-field. Try it: take a wide-angle photo of a friend from a fair distance, and then zoom in to duplicate the picture in a telephoto image. Then, enlarge the wide shot so your friend is the same size in both. The wide photo will have the same depth-of-field (and will have much less detail, too).

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