Digital Camera How-To: Understanding Lens Specifications

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Other than the image sensor, there's no component that's more important on your digital camera than the lens. A good lens allows you to get clear, focused shots while offering the flexibility to creatively frame your subjects. Understanding how your lens works allows you to make informed decisions on how to shoot in a variety of situations. For example, most digital camera lenses have a tough time taking a close-up shot when set for a wide angle. We'll get into why in a moment, but the result in a portrait situation is flatter, wider faces. To resolve this, simply back away from your subject and use the optical zoom on your digital camera to reframe the shot. Now the face will return to its normal proportions, as shown in Figure 1. By knowing how your lens responds to situations such as these, you can anticipate problems or even use these quirks to your advantage.

Figure 1: The first shot was taken close to the subject's face, causing distortion in the center of the image. By backing up and using the zoom lens to frame the shot, the subject's face returns to the correct proportions.

In this article, we'll explain how to understand the capabilities of your digital camera's lens. By knowing what your lens can do, you can get better pictures and avoid any optical unpleasantness. First, we'll tell you why digital camera manufacturers use a 35 mm equivalent to describe your camera's lens, even though it isn't always an accurate representation. Next, we'll define focal length and describe how it affects the field of view of your lens. Then, we'll talk about how to tell whether your lens is "fast" or "slow" and how that affects its image-capturing abilities. Finally, we'll go over optical and digital zoom, as well as special lens features such as macro modes. 35 mm vs. Digital As with most features of digital cameras, comparing them to traditional film photography can be downright confusing. Camera manufacturers usually attempt to assuage this uncertainty by using the 35 mm format as the frame of reference. This is no different with lenses, as most photographers understand what a 24 mm lens or a 100 mm lens can do. However, the calculations for these lens types are based on a 35 mm frame (36 mm x 24 mm). Most consumer-level image sensors are much smaller, some as small as one-third of an inch. Figure 2 shows the difference between different film and image sensor sizes.

Figure 2 As you can see, the size of even the largest image sensor is only a small fraction of the size of a 35 mm frame.

For example, a typical 4-megapixel 2/3-inch image sensor is only 8.8 x 6.6 mm! And, of course, these sizes vary from manufacturer to manufacturer, making this standard 35 mm "equivalent" a highly variable range. Plus, the high-tech lenses of the new breed of diminutive digital cameras makes figuring out what your lens specifications a research project. Our recommendation is to consider the manufacturer's lens description to be a working range for your digital camera lens, primarily when it comes to focal length. Let's take a more in-depth look at what's really going on with your lens. Zooming In The best place to start when evaluating your lens is its zoom range. Taking a page from video camera makers, digital cameras usually have zoom ratios listed as 2X, 3X, etc. This figure is the range of magnification the lens can cover from its widest angle to its greatest telephoto position. The amount of magnification on digital cameras is usually around 3X, or three times the size of the widest angle. Having a large zoom range gives you the flexibility to take a wide range of shots, from distant objects to close-ups. Many of the new, higher megapixel cameras are boasting 6X, 8X, and even 10X zoom ranges, which can add considerable cost to the camera. However, since most digital cameras can't use interchangeable lenses as easily as a 35 mm setup, having a large zoom range gives you a great shooting range. Figure 3 shows the difference between several popular zoom ranges.

Figure 3: A large zoom range gives you more opportunities to frame your shot.

Most digital cameras have two types of zoom: optical and digital. Optical uses the glass in the lens to magnify your scene, while digital zoom uses software inside the camera to enlarge the pixels. This procedure is exactly the same as enlarging an image in your photo editor, but much more destructive to your image as it just enlarges the pixels on your image sensor. Even though you might have a digital zoom on your camera, avoid it and use your photo editor to make any magnifications beyond the range of your optical zoom. Note: Most digital cameras feature Wide and Telephoto buttons to control your zoom range, although some of the newer models are moving back to the familiar zoom ring on the lens housing. The Tricky World of Focal Length The non-technical definition of the focal length of a digital camera lens is the distance between the lens surface and the image sensor. The larger the distance, the stronger the magnification factor of the lens. Focal length is measured in millimeters (mm), and as we mentioned, typically rated to a 35 mm equivalent. Focal length determines the angle of view for your camera. When a camera has a short focal length, it has a wider angle of view. When a camera has a longer focal length, the field of view is narrower. Focal length is listed as a range of numbers. For example, the Canon PowerShot G2 has a focal length of 34–102 mm, as illustrated in Figure 4. This gives it a zoom range of 3X, as 34 mm multiplied by 3 equals 102 mm.

Figure 4: Smaller angles give a wider field of view while larger angles produce narrower views.

When you increase the focal length of a lens by zooming in, there are a couple of things that happen. First, your depth of field decreases. This makes focusing in on your subject more important as the focal range is much smaller than a wide angle. Second, zooming in tends to flatten your image. As we mentioned earlier, close up shots taken at a wide angle tend to emphasize objects in the center of the frame. So, if you're planning to shoot a portrait, you might want to consider placing your tripod a little farther away to get the most flattering representation. Note: Ever wonder what the field of view is for your eyes? Since everyone's eyes are different, there isn't an exact number. However, 50 mm is considered a normal lens, or one that provides a field of view close to that of the human eye. F-stop on a Dime Ever hear someone refer to a lens as "fast" or "slow"? What they're referring to is the maximum f-stop your lens can support. The lower the f-stop, the better your lens performs in low-light situations and when you want to freeze motion. But wait, you say, aren't most digital cameras notoriously bad for low-light shooting? It's true, digital cameras are known to have problems getting enough light to the sensor when using higher f-stops. But improvements to image sensors and advances in lens technology have allowed digital cameras to feature much faster f-stops. Most digital cameras range from f/2 to f/16, with the lower numbers being "faster." Since adding more f-stops to a camera increases its cost (since the lens must be more complex), most have a very limited range of f-stops. If low light or action shots are important to you, look for a camera with a fast lens. Note:Lower f-stops produce very narrow depth of field. Macro-licious Since digital cameras typically have very wide lenses, it makes it easier for lens manufacturers to add macro-focusing ability, meaning the camera can focus at a very close distance. There are a few things to remember when using the macro mode on your digital camera. Using a flash is discouraged, as you'll be so close to your subject that the bright flash is almost sure to produce hotspots. Also, be aware that a lens in macro mode usually has a very narrow depth of field -- so getting your focus right is crucial. Increase your Lens IQ By understanding how your lens works, you can make smart decisions when it comes to taking pictures. It also can help you when selecting a digital camera or decide what you'd like to have for your next upgrade. With the rapid increase of quality and sophistication in digital camera lenses, having a solid understanding of how they work will ensure that you get the most from your camera's eye.

This story is taken from "Exploring Digital Photography" (Element K Journals). Creativepro.com readers can subscribe to Element K Journals at a discount. Click here to learn more.

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