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The Lens

June 20, 2008 - Dave Hecei
The lens is what makes photography possible. While you can take a photo with just a small pinhole, no glass elements at all, the lens is what shapes and controls the light. To choose the right lens you first must know how the lens choice affects the image.

Lenses are available in many focal lengths. The focal length, usually measured in millimeters, determines what and how you see a scene. The ‘what’ is the field of view, while the ‘how’ is the perspective. The field of view is how much area the lens can cover. The perspective is how the lens sees objects in the foreground and background. Remember that a photo is a 2D rendition of a 3D world. Back in the 35mm SLR days, most cameras came with a ‘normal’ lens. This was usually a 50mm lens, or something close to it. They called this a ‘normal’ lens because it gave you about the same magnification and perspective that the human eye sees. Anything lower than 40mm was called a wide-angle lens, while anything above 60mm was called a telephoto lens.

A wide-angle lens has a large angle of view. Wide-angle lenses are well suited for landscapes where you want to show the wide-open area. To go along with the ability to capture a wide vista, the wide-angle lens also changes the perspective. With a wide-angle lens, the apparent distance between objects is expanded. If you have something close to you in the foreground, objects in the background appear further away than they actually are. Sometimes, wide-angle lenses are used for special effects where you want to distort the perspective.

A typical wide-angle lens is somewhere around 18mm for DSLRs (equivalent to a 28mm for film SLRs). Ultra-wide is 10 to 12mm. Unfortunately, most DSLRs have a built in multiplier for lenses, 1.5x for Nikons and 1.6x for Canons and others (Olympus’ Four-thirds system is actually 2x). This happens because the digital sensor is much smaller than 35mm film is, thus the multiplication necessary when figuring out the effective focal length of the lens you are using. This makes it harder to do wide-angle photography (not actually harder, just more expensive).

A telephoto lens is just the opposite. A telephoto lens is used to bring a distant subject closer, or to magnify it. Telephoto lenses have a much narrower field of view and will also compress the perspective, making the distance from a foreground object and the background look closer. When shooting with a telephoto lens, you don’t have to be as close to the subject to fill the frame. This is great when you either can’t get closer to the subject, or if you want to be further away from the subject so you don’t disturb it or its natural behavior.

One of the great advantages of a DSLR is that same multiplier that hinders wide-angle shooting. Because of the 1.5 or 1.6x multiplier due to the sensor size, you get more bang for your buck in telephoto lenses. A typical 70-200mm zoom on 35mm gives you 4x power at the 200 setting. On a Canon, that 200mm is actually 320mm giving you about 6.4x power. If you put a 400mm lens on a digital camera, you get a 640mm.

Another type of lens is the macro lens, or close-up lens. This is usually a single focal length lens that has the ability to focus very close. Moving the lens further away from the camera sensor allows for very close focusing. Macro lenses have extra long focusing threads that allow it to move out, much more than a normal lens. Macro lenses are also designed to have a very flat focusing plane to assure that the outer edges of the photo are just as sharp as in the center. Macro lenses are available in many focal lengths like 50, 60, 90, 100, 105, 150, 180, and 200mm. Remember that a 50mm macro will have a wider field of view than a 200mm, and the 200mm will also compress the perspective.

 
 

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