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The missing variable here is how much of the scene projected by a lens are you able to capture? In order to know how wide or telephoto a lens is going to be, you need to know how big the sensor is that the camera is using. A 30mm lens (and a 100mm lens) will always magnify exactly the same amount, but the size of your camera’s sensor determines how much of this you will see. Which means that a 30mm lens isn’t always a wide angle. On a camera with a small sensor, it functions as a telephoto.

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One of the things that the focal length of a lens determines is the angle of view— which is how wide or telephoto a lens is.

When someone tells you “the focal length of this lens is 100 millimeters” it tells you something about the lens, but there is information missing which keeps you from knowing how useful that lens will be to you.

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The focal length of a lens is a number, measured in millimeters, which determines how wide angle or telephoto a lens is.

Now what about choosing between “prime” and “zoom” lenses. Prime lenses have a single focal length and zoom lenses can change their focal length. Why on earth would you choose a lens whose focal length can’t change if you can get one that does? (You might ask.) The answer is aperture. It’s much easier to design and manufacture a prime lens with a wide aperture for use in low light and to achieve shallow depth of field. There are zoom lenses with wide apertures but this pro glass is more expensive. If you can afford it, it’s a great thing to have.

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The wider a lens, the greater its depth of field (area that is in focus) at a given aperture. Extremely wide angle lenses can have everything from a few inches to infinity in focus at once and some telephoto lenses have a depth of field less than an inch.

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A larger angle of view means a lens captures more of the scene in front of you, the smaller the angle of view, the less it captures. This is measured in “degrees” and a distance; for example, a 300mm telephoto lens has a very narrow field of view at twenty feet, but step back far enough and you can fit the moon in it. A fisheye lens may capture 180 degrees—everything in front of the lens; top, bottom, left and right, while a telephoto lens shows many fewer degrees. Angle of View and Focal Length are inversely proportional, the longer the focal length, the narrower the angle of view. For more on wide and tight lenses, check out this Videomaker article.

2.8 is a pretty wide aperture, so a 28mm-120mm zoom lens with an aperture of 2.8 sounds like a pretty good thing to have. Be wary — most zoom lenses have a variable maximum aperture over the length of their zoom range. This means that it starts out at a wide 2.8 when it’s zoomed out all the way, but as you zoom in to the telephoto end of the zoom, the aperture closes up several stops. This will be represented on the lens as an aperture range, so the lens might say “28-120 f 2.8-5.6” which means the focal length range of the zoom is 28mm – 120mm and the lens’ maximum aperture at 28mm is 2.8 but when you zoom in to 120, that maximum aperture closes to f5.6.

The longer the focal length of a lens, the more magnified the image it is projecting will be. So, high number, telephoto lens, small number wide angle lens. A 30 millimeter lens is a wide angle then, right? This is where things start to get a little complex, because of the missing part of the equation.

There are three important things to look at when picking the right lens. One is its coverage: does it project an image large enough for your sensor? The second is focal length: how wide or tight an image will it throw onto your sensor? And the third is aperture: how much light do you need to get a useable image? (Other important factors about image quality we won’t talk about here.)

Because you can get away without having a scientific understanding of focal length, let’s put the conclusion at the beginning.

That said, different sensor sizes are here to stay, often based on the physical size of the device that’s holding them. The lens in an iPhone 5 has a focal length of 4.1mm on a sensor that’s only 4.54 x 3.42mm simply because of the tiny size requirements for the device. You’ll need to rely on your camera’s live view to really understand how the final product will look. In order to reduce the size and cost of lenses, some manufacturers made new lines of lenses around smaller sensor sizes. The DX line of lenses are made to cover APS-C sensors, while so-called “full frame” (35mm sensor size) lenses will work on APS-C sized sensors, the reverse isn’t true, a DX lens on a full frame sensor will show a circle around the image. When buying a lens, make sure it will cover your sensor size.

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Often now people will tell you what the lens would be in a 35mm equivalent by using a “crop factor” which is the size of your camera’s sensor in relation to a 35mm frame of film. APS-C sensors have a crop factor of about 1.5 so a 50mm lens on an APS-C camera would have similar coverage to a 75mm lens on a 35mm camera. Micro 4:3 cameras have a crop factor of 2. But contrary to popular belief, this doesn’t actually make your lens telephoto, it simply crops what a 35mm sensor would have shown you. (You can check this out in the live view.)

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Different lenses reveal different things. The amount of a scene that a lens will throw onto your sensor is controlled by its focal length. Lenses with a high (or “long”) focal length magnify the scene and show you a small amount of what’s in front of you. Lenses with a low (or “wide”) focal length show you a lot of what’s in front of you.

This means that if you’re in a situation where your subject may be moving quickly, getting closer and using a wider lens might be helpful, this is something that people doing street photography have been doing for years – wide lens get close, hope the focus is there.

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One of the most exciting innovations in video technology recently is that 35mm DSLR (digital single lens reflex) cameras have begun to shoot video and that varying number of sensor sizes seem to be settling down.

The concept of focal length used to make more sense when everyone used the same size sensor (35mm film). A director of photography on a film set would say “we’re going to use the 35mm lens here,” and everyone knew exactly what that result would be. Today’s vast number of sensor sizes make talk of focal lengths less important than knowing how many people you can fit in a shot with your zoom lens at its widest and how far away you can be from a person and still get a medium shot of them when it’s zoomed in all the way. You can use your camera’s live view to see this right away.

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At it’s very heart the “focal length” is the distance from the front element of the lens to the place where that lens will project a focused image. Usually this is measured in millimeters. If you take the simplest lens, a pinhole, which projects an always-in-focus image, and mount it on a tube 100 millimeters long, you will have a 100mm lens. If you mount that pinhole on a tube 30mm long, you will have a 30mm lens.

It also determines, in part, the depth of field (the area that’s in focus) and the relative size of objects in a frame (perspective). Wide angle lenses have a lower focal length, while telephoto lenses have a higher focal length number. Wide angle lenses make objects very close to the lens large and objects farther away very small with most things in focus. Telephoto lenses make things in the background larger and it’s more difficult to have objects in the foreground and the background in focus. Together these provide videographers and photographers with a number of stylistic and practical tools for capturing video in small places, across distances, making things look larger or smaller or isolating things from other things. You can use your camera’s live view to see what the difference is going to be as focal length changes.

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“Focal length” is a relatively straightforward scientific principle which seems a lot more confusing than it actually is because very often when it is used, an extremely important part of the equation is left out. When someone says “the speed of this is 90 miles” you immediately think “90 miles what? 90 miles an hour? A day? 90 miles a second?”

The focal length of a lens has a great impact on how objects appear in the lens. Wide angle lenses exaggerate the size of objects near to them and diminish the apparent size of objects further away from them. So if you’re taping a person with a wide angle lens and get them close to the lens, their nose will look larger and their ears smaller, not always the best way to make friends. But if you hold a fish up close to a wide angle lens and stand arms length back, your fish will look larger than it actually is. This can be a good way to make friends with a fisherman. (Homework assignment: using different focal lengths and your camera’s live view, try framing things very close to the lens and then farther back, making note of how the perspective changes as you do.)

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This didn’t really used to be much of a problem when everybody was using 35mm film—everybody had the same reference point; all the “sensors” were the same size. But over the last few years cameras have come out in a bewildering number of sizes—from DX and APSC to tiny sensors inside camera phones. Which means the actual focal length of a lens has become a more-or-less useless metric for determining how much you’re going to see if you look through it.