In the good old days there wasn’t much to know about lens focal lengths. A 100mm lens was a 100mm lens; the only thing you needed to know was whether that lens was a telephoto (as it would be on a 35mm film camera), a normal lens (like on a medium-format camera) or a wide angle (as it would be on a 4×5 view camera). Most people quickly learned what the focal lengths represented for their particular camera format. But these days, with so many digital camera sensor sizes and other lens peculiarities, the millimeter measurement of a lens’ focal length tells only part of the story. Many other factors go into determining the effects a particular lens will produce—from magnification factor to zoom range and much more. What follows is a lens focal-length primer, with several key points that will help you understand focal lengths as they relate to your camera and to the pictures you use them to make.

1. 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 it’s considered. The shorter that distance, the wider the angle of view. The most common measurement of lens focal lengths is in millimeters, although some old-school photographers still refer to large format lenses in inches. (If you’re interested, you roughly can convert inches to millimeters by using a 1:25 ratio. An eight-inch lens approximates a 200mm lens.)

2. 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 (say, a 10mm wide angle that seems unbelievably short) but in “equivalent” terms they’re much more akin to more familiar focal lengths (say, a 17mm lens that is the equivalent to a 28mm lens in 35mm equivalent terms).

3. 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 (again, in full frame equivalent terms). A normal lens—on a full frame DSLR—is the distinction given to any lens that ranges roughly from 40mm to 65mm or so. These lenses are “normal” because they 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. That’s why wildlife and sports photographers so often use 600mm and longer telephotos. Most amateur users, though, tend to top out around 300mm lenses for most uses.

4. 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. Some photographers object to this narrowing of the angle of view because they’re used to a lens of a certain focal length producing a certain corresponding angle of view. Other photographers actually prefer a crop factor because it has the effect of making a long telephoto lens behave like an even longer telephoto lens. 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. That’s a heck of a telephoto bonus.

5. Some lenses are called prime lenses, which means they have a fixed focal length. Other lenses are zoom lenses, so they can be adjusted across a range of focal lengths. Some zooms fit within a particular classification, such as wide-angle zoom, normal zoom or telephoto zoom. 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. The downside is that some extreme zooms are more prone to vignetting and chromatic aberrations when used with wide apertures and zoomed to the extremes.

6. 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, for example, 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 by a whopping factor of ten (as in a 35-350mm lens). The bigger the X factor, the larger the range of focal lengths covered by a lens. Remember though, just because two lenses offer 2X zooms doesn’t mean the lenses have the same focal length. For that, you’ll have to compare actual millimeter measurements in 35mm equivalent terms.

7. The longer the focal length of a lens, the shallower the inherent depth of field that lens will produce. 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.

8. 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—for example, when handholding a 500mm telephoto lens, be sure to set the shutter speed no slower than 1/500th of a second.

9. Some lens designations mean that even though the focal length may be the same, the lens won’t perform the same. 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, but if making big photos of little objects is important to you, it’s well worth the investment.

10. 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 (with a 2X or 3X teleconverter) without carrying an additional, and often quite expensive, supertelephoto 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. Worth it, though, if you’re working at smaller apertures, with flash or if you simply need the close focusing or telephoto extension effect.