Lenses are kind of like shoes, you really only NEED one pair; but it is nice to have more.
The basic factors effecting lens selection are:
1) How much light they let through: This is given by the f-stop range, which is the ratio of the aperture to the focal length, given in what look like an odd series of numbers because they are the root of the square. Typically “fast” lenses have a maximum aperture of f:2.8 or lower, f:1.2, and f:1.4 are very fast. “Slower” lenses are in the f:5.6 to f:8.0 range with f:4.0 being the medium speed. The faster the lens, the more it generally costs, as the glass becomes huge with longer focal lengths. Fast wide-angle lenses tend to be smaller than long lenses as the aperture is a function of the focal length thus an f:4 600mm lens has a front element that is nearly 10 inches across, where as a f:4 20mm lens is less than a half-inch across.
2) Focusing speed: Two factors affect this. One is the method of driving the focus mechanism. Nikon lenses have two methods. One is an motor embedded within the lens, denoted by the term AF-S. The other method is a drive pin, which is driven by a motor within the camera. This second method is termed simply AF. AF-S is much faster, and quieter. It also allows instant manual focus. The AF lenses use a button on the lens to go into and out of auto-focus mode, and the drive is slower and noisier. The other factor which affects the auto-focus rate is the amount of light the lens gathers, with “faster” or low aperture (f:1.2, f:1.4, f:2.8) focusing faster than f:5.6 ones.
3) Fixed focal length or zoom: A lens of a fixed focal length is optimized for that single focal length. It has fewer elements inside. This reduces internal reflections that degrade image quality. Such lenses are ALWAYS sharper than zoom lenses, given the same quality glass. Obviously fixed lenses require a lot of purchases. Carrying a ton of fixed length lenses and switching lenses in the field is a major drawback. Zoom lenses offer a variable range of focal lengths. Generally the sharpness across the range of the lens varies, but is controlled enough to satisfy most users.
4) Focal length: This is usually measured in millimeters, though some view-camera lens manufacturers use inches. The discussion of focal length and its effect depends on the image format size. In the case of digital cameras a 35mm lens gives what is called the natural perspective. That simply means that it approximates the way the unaided eye perceives the world. It is a little tricky to make such a statement as we humans can really see a field of more than 190 degrees. In practice however, we tend to concentrate on about only 45 degrees of our field of view, and really can only see sharply over a 2 degree region. Our brains process things as a composite so we think we see much better then we do. Wide angle lenses are anything under 35mm for digital, 12mm is really wide taking an angle of view of around 100 degrees. Extreme exaggeration of perspective can be used for effect at 12mm. Lenses in the 25-45mm range exhibit much less perspective exaggeration and are nice for photos of people in small rooms. The 70-120mm range is good for single head portraits, and isolation of cluttered backgrounds. This cluttered background concept is fairly important. The longer the focal length, the less of the image is sharply in focus. With a short 12mm lens, it is possible to have everything from eight inches to infinity in focus. With a 1200mm lens you would be able to only have something at 30 to 30.2 feet in focus (yes, that’s barely 2.3 inches!). The long lens effect flattens the objects in view, and allows for very selective focus. The short lens exaggerates perspective, and keeps more things in focus.
5) Lens build quality: The physical build of the lens is more important the more you use it. The higher priced lenses tend to be made of metal as opposed to their less pricey cousins, which are generally made of plastic. Metal lenses bounce a little better (they are more likely to survive a fall), will last longer, and generally weigh a lot more. Some people prefer lightweight lenses. In truth, most people don’t use equipment as much as they think and plastic lenses are fine. Professionals tend to buy metal as they use their equipment daily and they wear better.
6) Lens Optical quality: Things like ED (extreme dispersion) glass allows lens engineers to use fewer elements in a lens. The fewer elements used, the better the lens, given equal correction. This is due to the fact that internal reflections cause a loss of contrast, and sharpness is a direct function of contrast. ED lenses cost more then non-ED lenses. Virtually all lenses are multicoated these days, and that is another way to reduce internal reflection. Generally no lens is tack sharp throughout its entire f-stop range. On the lower end of the f-stop range, the quality of the glass is the predominant factor, after about f:8 or f:11 diffraction from the aperture comes into play. In the case of zoom lenses, there are tradeoffs between linearity and sharpness throughout the zoom range. Higher priced lenses simply have a wider “sweet spot”, so to speak.
7) Shooting style: Some people are telephoto lovers, some are wide angle lovers, some choose the lens that is best for the job given the way they envision the subject. This really gets into the murky world of interpretation, art, style and all those good things. Sometimes it is a fairly clean cut choice. If, for example, I am shooting birds out in the field that I know will be 200 feet out, there is only one non-zoom lens I am bringing with me and a few tele-extenders. If however I am just touring around, I would probably bring 4 lenses. First, I’d want something really wide 12-24mm f:4 zoom, and something less wide but faster, such as a 17-35mm f:2.8. Mounted on the camera would be a 28-70mm f:2.8, which by the way is the best lens in the world, though some think it is to heavy, and for the long end a 70-200mm f:2.8.
So mix up all these factors, and remember lenses are like shoes.
Peter J. Lopez
