Lesson 1: choosing a lens (focal length)

Remember, we agreed that you’re looking to create images that are first and foremost in-focus and not too bright (overexposed) or too dark (underexposed.) Simple. First learn the rules, then learn to break them.

Lenses come in various focal lengths, the length or distance from the front of the lens to the camera’s sensor measured in millimeters.

Some lenses are primes (one focal length) and others are zooms (various focal lengths). Many photographers swear by prime lenses to achieve tack-sharp focus; other photographers swear by zoom lenses for versatility. I am both. If I’m looking to take a close up portrait of an 8-year-old witch with a spider on her veil, I use my fixed-focal-length prime 105mm (which is considered a telephoto lens; more on that in a minute).

Nikon D300, 105mm prime lens, f5.6 @ 1/250 second, ISO 200.

My workhorse lens, the one I grab most often, is a 28-70mm zoom lens (zoom because it covers all focal lengths from 28 through 70, wide angle to telephoto zoom because of the field of view it covers. Keep reading.) This lens doesn’t produce quite the tack-sharp images as my primes, but the tradeoff I get is versatility. If I’m shooting portraits of a 1-year-old, who by definition movesveryfast, it’s incredibly helpful to be able to change the focal length of my lens in one quick movement from a 70mm to a 28mm and everywhere in between as she starts to run toward me. Can’t do that so easily with a telephoto lens or a prime lens. When working with children or in fast-changing environments, switching lenses can mean missing the shot. The flexibility and versatility of a zoom can be a gift to photographers.

My 28-70mm is a wide-angle to telephoto zoom lens. My 105mm is a prime telephoto lens. Focal length numbers range from fisheye to wide angle to normal to telephoto. A fisheye lens has a field of vision covering up to 180 degrees; wide angle is between fisheye and normal; a 50mm lens is considered “normal” because it sees roughly the same field of view as the human eye; and a telephoto lens has a narrower field of view and a magnified image (think spotting scopes or binoculars.) Here’s a chart to help you visualize focal lengths:

Here’s an example of my 28-70mm lens shot at 28mm and again at 70mm without changing the distance from me to the subject:

Nikon D700, 28-70mm lens at 28mm, f4 @ 1/1250 second, ISO 400

Nikon D700, 28-70mm lens at 70mm, f4 @ 1/1250 second, ISO 400

If you are a realtor looking to photograph the interior of homes, or a pilot looking to photograph your co-pilot inside a cramped flight deck, or a tourist looking to fit the Washington monument inside your frame, you’ll need a wide angle lens. 16mm or 28mm for example; the drawback to wide angle lenses is some visual distortion. (There is software that can fix this.) Traditionally, wide angle lenses are not considered ideal for portraits. (Although, we’ll discuss this later in “first learn the rules, then learn to break them.”)

Nikon D300, 16mm f/2.8 prime lens at f5.6 @ 1/2500 second, ISO 200

Nikon D300, 16mm f/2.8 prime lens at f5.6 @ 1/50 second, ISO 800

Most portrait photographers start out at 105mm, partly because there is no unflattering distortion to facial features and partly because of this:

bokeh
noun Photography
the visual quality of the out-of-focus areas of a photographic image, especially as rendered by a particular lens : a quick, visual survey of the foreground and background bokeh of a variety of lenses.
ORIGIN from Japanese.

Nikon D200, 105mm f/2.8 prime lens at f3.2, shutter speed not recorded, ISO 100

If you didn’t know what bokeh was but you know you love images where people are sharp and the background is really, really blurry, then you know you should be looking for a telephoto lens.

There are some instances in which you can’t physically move closer to your subject, so you need a lens that can take you there. If you are looking to photograph your son on the football field, or your daughter at a swim meet, or the Pope who drives by in the Pope Mobile while you stand behind police barricades from 25 feet away, you’ll need a telephoto lens to magnify the image you want to shoot. Examples of telephoto focal lengths are 85mm, 105mm, 200mm, 400mm. The numbers go even bigger than that — Sports Illustrated photographers use behemoths from the sidelines. If I were out shooting 300lb. men trying to kill one another, I’d use behemoth too, as my favored 28-70mm would put me right in the crossfire trying to get an image (if I didn’t get thrown out of the game first for walking onto the field.)

Following is an example of shooting from the edge of the pool with my 70-200mm telephoto zoom, first at 70 and then at 200. Note: my D700 is a digital full-frame; so I lose the times-1.5 factor on focal length. 70mm is in fact 70mm. Also note that I’m at ISO 400; when shooting sports, even in sun, I bump up my ISO to 400 to give me a faster shutter speed so I can freeze action — totally unnecessary for this shot, but I had my settings ready in anticipation of shooting swimmers and divers.

Nikon D700, 70-200mm f/2.8 zoom telephoto lens at 70mm, f5 @ 1/1600 second, ISO 400

without moving, this is the same shot as above only I changed the focal length of my lens to 200mm. notice I had to adjust my shutter speed to compensate — longer focal lengths suck up more light. if my shutter speed starts to go slower than 1/250 with this lens at 200mm, then I need to start looking at bumping up my ISO.

Nikon D700, 70-200mm f/2.8 zoom telephoto lens at 200mm, f5 @ 1/800 second, ISO 400

Okay okay okay, I get it you say. I understand focal length. Next lesson.

But wait — one more thing. There a really, really important exposure equation that relates to focal length, and if you don’t learn it straight-up, you’ll spend a lot of money on a telephoto lens and come back asking why your pictures are consistently blurry. Here’s the magic equation to keep in mind:

Your shutter speed must be equal to or faster than your focal length to avoid camera shake. What does this mean? If you are hand-holding your camera with a 200mm telephoto lens, you must use a shutter speed of 1/200 or faster to avoid blurry images. Handholding a 400mm lens? Shutter speed must be 1/400 or faster. Handholding a 50mm lens? Shutter speed must be 1/50 or faster. Of course, you can use a tripod or a monopod to keep your camera still to compensate for this law (notice the Sports Illustrated photogs on the sidelines with their monopods, partly for this reason.) This might not make sense right away, but when we get to shutter speed it’ll start to click (pun intended?) For now, just remember this:

Focal length affects shutter speed choices.

Buying tips: Remember, you can’t put a Canon lens on a Nikon body or a Pentax lens on a Sony body. Lens mounts have electronic connectors that relay information from body to lens. There are some third party lenses that are built specifically for Nikon or Canon bodies, Sigma lenses, for example. But again, you can’t put a Sigma lens for Nikon mount on a Canon body. You can save money on third party lenses but be sure to do your research to make sure your lens is compatible with your camera body.

Some digital SLRs have smaller sensors than others. Check your manual. If your digital SLR has a “full frame” sensor, then the focal length of your lens will be equal to the focal length number printed on the lens and you can skip to the next paragraph. (85mm = 85mm on a full frame body, for example.) I wasn’t always shooting with a full frame digital SLR, and the way I got around this annoyance was to multiply the focal length of the lens by 1.5 to determine roughly how a lens would perform on my camera body. So my 85mm prime lens suddenly became equal to 127.5mm, which means my 200mm was suddenly 300mm. This is GREAT if you’re looking to make telephoto lenses more telephoto without spending money! However, this same law can prove bothersome if you work a lot in the fisheye or wide angle range. Some manufacturers make digital lenses designed specifically for digital bodies that are not full-frame. I don’t personally own any of these lenses, so I can’t show you any pictures from them.

Okay, so now you have some rough guidelines for choosing the focal length of your lens. You know you need a roughly wide angle/normal/telephoto lens for the kind of images you want to shoot, and you might try to find a zoom that could kill two birds with one stone. You start looking for lenses in respective categories. And now you’re confused again, because after the focal length numbers, there are ADDITIONAL numbers stamped on each lens, and you don’t know what they mean other than to make some of the lenses ghastly more expensive than others even though the focal length is exactly the same. What gives?

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