Beginners guide to focal length

• The focal length of a lens is the measurement of the distance from the center of a lens to the point at which its image is focused. The longer the distance, the longer the lens. The longer the lens, the more telephoto capability it have. The shorter that distance, the wider the angle of view. 
• A full-frame digital sensor is equivalent in size to a 35mm film frame, making this the standard focal length baseline that today’s lenses are measured against. Smaller formats often have shorter focal lengths but in “equivalent” terms they’re much more akin to more familiar focal lengths.
• Lenses have various classifications based on focal length and the field of view they provide. A wide-angle lens provides a much greater field of view, and is generally considered to be any lens 40mm or shorter. 40mm-65mm lenses provide an angle of view that approximates that of the human eye. Telephoto lenses on full-frame cameras usually are lenses longer than 70mm, and they range upwards of 300, 600 and even 1000mm. The longer the telephoto, the narrower the angle of view and the greater the magnifying power it provides.
• The effect that a smaller sensor has on a lens of a given focal length is called a crop factor or magnification factor. This is because a smaller sensor produces a similar effect to cropping a larger sensor—effectively magnifying the image. If you photograph sports or wildlife, a 400mm lens placed on a camera with a 1.5 magnification factor would behave more like a 600mm lens.
• Prime lenses have a fixed focal length. Many lenses actually zoom from wide to normal, or normal to telephoto. Extreme zoom lenses actually encompass all these qualities in a single lens—say a wide-angle 30mm lens that can zoom all the way to a 300mm telephoto. These extreme zoom lenses are prized for their portability since they offer such a wide range of focal lengths in a single package. 
• Photographers shopping for point-and-shoot or compact cameras often encounter zoom lens descriptors such as 2X, 3X or 10X. This isn’t actually a representation of the precise focal length of a lens, but rather the zoom range that lens covers. A 2X lens, doubles its focal length from its widest to its longest setting—as in a 35-70mm lens. A 3X zoom triples the focal length (like 35-105), and a 10X zoom multiplies it as in a 35-350mm lens.
• The shorter a lens, the greater the depth of field will be even at wide apertures. In practice that means you have to be more precise when focusing a telephoto lens, whereas wide- angle lenses have such depth of field they can be very forgiving of improper focus. Many photojournalists for years have utilized this “benefit” of wide-angle lenses in difficult shooting environments, not only because they take in more of the scene and provide context, but because they have so much depth of field to provide focus from near to far.
• The longer the focal length of a lens, the more difficult that lens will be to handhold. This is true not only because longer lenses tend to be physically longer and heavier than wide-angle lenses, but also because subtle vibrations and camera shakes are amplified dramatically when using a telephoto lens. A good rule of thumb is to use a minimum shutter speed equivalent to the focal length. It is also advisable to use a tripod to minimize the shakes and vibrations to the lens.
• A macro lens, for instance, can focus extremely close, allowing for great magnification of small objects and fine details. One 100mm lens may be designated macro, while another is not. You’re bound to pay a premium for the added capabilities. 
• Many photographers utilize special devices to change the effective focal length, or at least the performance, of a lens. Teleconverters are popular among wildlife photographers and those who want to double or triple their lens’ focal length without carrying an additional, and often quite expensive, super-telephoto lens. Extension tubes are a similar device, but rather than changing the lens’ effective focal length they simply change the focusing range—making a lens focus much closer and behave more like a macro lens would. Like macro lenses, extension tubes are used to allow close focusing are ideal for flower photography and other close-up uses. The downside with both extension tubes and teleconverters is that each requires a sacrifice in available maximum aperture—often as much as two full stops that turn an ƒ/2 lens into an ƒ/5.6.