Lenses: How does focus work?

Simply defined, a lens is something that bends (or refracts) the light that passes through it. As you can imagine, lenses are everywhere in our world — from a drop of water on a car’s windshield, to your eyeglasses, right down to your actual eyeballs.

Thus, a single object that bends light is called a simple lens. Similarly, a set of two or more lenses working together is called a compound lens.

In general, camera lenses are almost always compound lenses. To reduce confusion, when we talk about the simple lenses inside a compound lens, we refer to them as elements

Your lens will arguably have the most important impact on your final image — far beyond the rest of the camera hardware you’ll ever own. Still, how does it actually work to produce a focused image? Plus, what do all the numbers and letters in a lens name mean? Let’s take a look.

Focus, please!

To start, imagine a magnifying glass, which is a simple lens. If your magnifying glass is the right distance from whatever you’re looking at, the refraction will make your subject look bigger. This is because the lens gathers light from a wide area, bends it at the right angle, then shoots it into your eye. This functions similarly to the way a funnel might gather and redirect rainwater.

Comparatively, if you’ve ever tried to light a leaf on fire with a magnifying glass on a sunny day, you understand exactly how bending light works. You collect and bend the light from the sun onto a specific point, which is the point of focus on the leaf.

A camera lens works the same way, collecting and bending the light onto your digital sensor (or film, if you’re a time traveler from 1995). This is  why you never want to point your camera directly into the sun. It can fry your sensor — and possibly your eye, depending on the camera.

An object viewed through a magnifying glass looks bigger, but it’s usually distorted and fuzzy as well. How do you reduce the distortion of the image and make it sharper?

First, you use another lens–one that straightens the light a little. In fact, that’s how simple telescopes work —  they have one lens to bend the light to make things look larger, then another to bend it again to make it look clearer. In fact, this is how two simple lenses become elements of a compound lens.

By varying the distance between the two lenses, you can change how much the light gets straightened. This changes the focus of the light and changes how blurry the picture is. That’s why when something is blurry, we call it “out of focus”.

Likewise, to get even clearer and less distorted images, we add even more elements to unbend and rebend the light exactly right. Plus, we add a mechanism to move those elements around to adjust the bending, and therefore your focus and/or magnification.

The math necessary to figure out how many elements you need, and the precise shape is extremely complicated.

An average lens might have as many as six or seven elements in it. Thus, more expensive and complicated lenses may have as many as 15 to 20. Different-shaped elements can make for different effects, which is how you get specialized glass like macro, wide-angle, fisheye, apochromatic (“APO”) or telephoto lenses.

Technical details

Now that we have a basic idea of how a lens works, let’s get into some of the technical nitty-gritty. When you buy a lens, it usually comes with a bunch of numbers and letters in the name, like “Nikkor 20mm f/1.8 AF”. The first part of that name is simple: Nikkor is the brand name.

Focal length & distance

The first number after the brand name (measured in millimeters) is the focal length. This is not to be confused with focal distance. The focal length refers to the distance between your camera sensor (or film) and the “rear nodal point” of your lens. Defining a “rear nodal point” usually takes a master’s degree in optics and a couple of textbooks, so for our purposes, just think of it as your lens’ sweet spot.

In short, the farther away you need to be from your subject, the higher the focal length should be. A 300mm lens is great for shooting wildlife, because you can be far away from your subject. It takes a narrow shot so your subject takes up more of the frame.

Comparatively, if you want to get up close to your subject, a 20mm lens might be what you need. In brief, this will take a wide shot–so your subject takes up less space in the frame. Zoom lenses, which have a movable sweet spot, will have two focal lengths listed. They’ll have minimum and a maximum like “80mm-200mm”. Furthermore, lenses that can’t zoom are called prime lenses.

Focal distance is the closest you can be to your subject and still have it in focus. In other words, focal distance is your minimum focus distance.

Focal distance is the closest you can be to your subject and still have it in focus.

In general, it’s important to note that focal length and focal distance are not always related! Two 20mm lenses from different manufacturers may have very different focal distances. Focal distance is not usually in the name of the lens, so you will have to check the tech specs to find it.

Focal plane & depth of field

Additionally, you might hear about focal plane. Focal plane (also called depth of field) is how deep your area of focus is. It can be hard to explain, but imagine you’re taking a portrait of a person. If you have a very narrow focal plane, if your subject’s eyes are in focus but their nose won’t be, and neither will their ears.

Similarly, if you have a wide focal plane, their entire face — nose, eyes, ears — is in focus at the same time. In general, you don’t usually want to shoot with a very narrow focal plane. That said, sometimes it’s necessary because of lighting conditions or because you want a specific artistic effect.

The focal plane of a lens is controlled by the size of your aperture. The bigger your aperture, the narrower your focal plane. Most modern lenses have an adjustable aperture. Aperture is usually measured in f-numbers or f-stops, though on some video-specific lenses they’re called T-stops instead.

The smaller the number, the bigger the aperture. Yes, this seems backward. It isn’t really, but that’s a different article. Aperture is usually in the lens name, and typically indicates the maximum f-stop for that lens. It’s the “f/1.8” in “Nikkor 20mm f/1.8 AF”.

Autofocus and manual focus

The letters at the end of a lens name can stand for a bunch of different things, which vary from manufacturer to manufacturer. While “AF” almost always means autofocus, there is no standard guide for other letters.

Autofocus means that the lens has tiny, extremely precise motors in it that, in concert with your camera sensor, will adjust the distance between elements and focus your picture automatically. However, some lenses are manual focus only, which means you have to adjust the lens by hand to focus it.  

On the whole, very few professional shooters use manual-only lenses anymore. In fact, autofocus has been standard on most lenses for 40 years. However, most lenses or cameras will have the option to turn autofocus off.


Optics is a very complex field of physics and engineering, but camera lenses don’t have to be complicated to use and understand. The best way to learn is through experience. Here’s your homework: buy (or borrow) a short-length lens, somewhere between 16 and 25mm.

Shoot a subject with it at a low aperture, then a high aperture. Then, get a longer lens (at least 80mm) and shoot the same subject again at a low and high aperture. Lastly, compare the images. Let us know how it turns out on the Videomaker forums.

My cat Egon shot with a 16mm lens at f/22. Note how his entire head and body is in focus. Copyright Mike VanHelder 2014
The same cat, Egon, on the same day but shot with an 85mm manual focus lens at f/1.4. Note how his eyes are in focus but his nose and ears aren’t, and his body is just a blur. Also his face looks much wider than it does in the last picture. Copyright Mike VanHelder 2014
Mike VanHelder
Mike VanHelder
Mike VanHelder is a working writer, photographer and videographer in Philadelphia, PA.

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