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What Is Focal Length?

Last Updated on May 24, 2022 by Devaun Lennox

Many factors combine to create the final image a camera captures, but easily one of the most important factors is the focal length of the lens used. And it’s often a photographer’s first decision when approaching a scene.

The problem is the focal length is quite a misunderstood concept amongst photographers. And many beginners inevitably believe it’s merely a descriptive term to describe how much of a scene a lens captures or how zoomed in it is. While this is true, there’s more to focal length than this alone, like its effects on depth of field and apparent perspective.

So understanding this concept in full is essential, as it’ll help you decide which lens will best capture your scene or subject properly. And that decision, in turn, will help give you the artistic freedom you likely desire. As a bonus, it’ll also clear up any potential confusion when you want to purchase a new lens for your current kit. Throughout this post, we’ll define the term, talk about its practical impacts, and cover some other helpful considerations.

Define Focal Length

Let’s first define “focal length.” The most agreed-upon definition is as follows: focal length is the measured distance, usually in millimeters (mm), between the optical center of a lens and the focal point of a recording medium (camera sensor or film plane). Note, we illustrate a camera’s focal point by engraving “Φ” on the top plate of a camera body. And focal length describes two specific optical properties of a lens, namely its Angle of View and Image Magnification.

More specifically, though, we measure the focal length of a lens by first focusing it to infinity. Then we measure the location where all incoming light rays converge after passing various internal lens elements and create a sharp image onto the recording medium. And the distance from the point of convergence, also known as the nodal point or optical center, to the recording medium is then stated as the focal length.

See this below illustration for how that looks with a 35mm prime lens.

Prime-Lens-Illustration-White

It’s important to highlight that the optical center is rarely the physical center of a lens. Instead, it’s the single point where the light focuses due to the combined lens elements and groups that help reduce distortions, aberrations, color shifts, and various other anomalies. So while the focal length provides a helpful visual, it doesn’t specifically address the physical size of a lens but its optical property. A great example is a standard 50mm lens compared to the 50mm pancake lens, same focal length, different sizes.

It’s also important to highlight that only the lens manufacturer can genuinely calculate the focal length of their designs. But, sadly, many lenses are different in focal length than advertised, take a 200mm lens that measures closer to 190mm. And this difference indeed changes their Angle of View and Image magnification. Even so, as photographers, we’re at the mercy of manufacturers’ predefined values. And we have to make do with whatever is available.

Thankfully, we can use the predefined value to understand the two most essential focal length characteristics: Angle of View and Image Magnification. The angle of view is how much a scene a lens captures, and lens magnification is how large individual elements appear in the frame.

Focal-Length-Illustration

In general, most cameras today offer a range from 10-200mm. However, many also offer specialty lenses that extend above and below this range. And nearly all lenses indicate their focal length through an engraving on the lens barrel to establish a consistent standard to gauge focal length.

Focal Length and Crop Factor

The crop factor is a term that accounts for the difference in Angle of View between sensors different in size than traditional 35mm film. And it’s an important concept to understand, as the focal length measurement itself now uses the sensor size of 35mm (36 x 24mm) as its standard. Meaning, all lens manufacturers use the 35mm negative size as the basis for designing their lenses. So when it comes to selecting the right lens, you’ll want to take into account your camera’s sensor size, as its crop factor will alter the advertised focal length and, thus, the Angle of View.

But, in short, you’ll want to multiply the advertised focal length by the camera’s crop factor to get its equivalent in full-frame. And that equivalent is what you’ll use to now determine various characteristics about the lens, like compression, Angle of View, and so on. It’ll also help you correctly identify which lenses are best for the type of image you are creating. Feel free to read our Full-Frame vs. Crop Sensor blog post for a detailed look at this subject.

The key takeaway for crop factor is this: focal length is solely a property of the lens. And the lens remains the same focal length regardless of the camera it’s used on, be it full-frame, APS-C, or medium format. Instead, the size of the sensor alters the lens’s apparent angle of view, making it different from the true angle of view. And that change is why we ultimately have a crop factor, so we can get an equivalent focal length that accounts for the new angle of view.

sensors-types-example-photo

** This is an image showing the entire lens projection of a full-frame lens and how each of the various sensor sizes crop that projection. The colors featured start with a full-frame camera and move down to a smartphone sensor. **

How Does Focal Length Affect an Image? And Why Is It Important? 

Focal length directly impacts the outcome of your image, so it’s important to know the practical effect it has. Below are the key areas it impacts.

Angle of View: 

The lens’s nodal point, which describes its focal length, determines how much of a scene that lens captures, a phenomenon called the Angle of View. Simply put, shorter focal lengths, like those below 35mm, produce a larger Angle of View. This larger Angle of View, in turn, captures more of the scene ahead, hence the name “wide-angle” lens. In contrast, a telephoto lens has the opposite effect and produces images with a more narrow Angle of View. If you want more details about how this occurs, see our Angle of View post. But, in short, the focal length you choose directly affects how much of a scene you can capture. 

Depth of Field: 

Focal length also affects apparent depth of field. A wide-angle lens has a larger apparent depth of field, producing an effect where much more of the scene ahead appears to be in focus. And this effect makes them ideal for capturing vast cities or landscapes, where you want more of the elements in the composition in acceptable focus. At the same time, a telephoto lens tends to have a more shallow depth of field, making them ideal for isolating smaller, more distant subjects.

Perspective and Subject Size:

The focal length you use will also change the apparent perspective in your images, a phenomenon that is a function of distance to subject and image magnification. Shorter focal lengths, like wide-angle or ultra-wide-angle lenses, have lower image magnification. This, in turn, forces you to get closer to the subject to fill the frame, expanding the perspective. And this makes it appear there’s more space or distance between the foreground and background elements to your subject. It also makes the subject appear smaller than in reality. While the telephoto lens has higher image magnification and requires you to be further away from the subject to fill the frame. And instead, this makes the foreground and background elements appear closer and more stacked together. This, in turn, compresses the perspective and reduces the relative distance of background elements. And it makes the subject appear larger than they are in reality.

The diagram below shows this apparent perspective change due to the image magnification of various lenses.

lens-size-distance-diagram

Stabilization:

Focal length also affects the stabilization of a lens. And without proper stabilization, you’ll see blurriness and a reduction in image sharpness caused by vibrations. Those vibrations can be from your hands moving while holding the camera. But, they can also be caused by depressing the shutter release itself. Either way, it’s important to highlight that telephoto lenses increase all camera shake appearance due to their tighter Angle of View and higher image magnification. And the more telephoto the lens, the worse the appearance of shake becomes. So to reduce any negative impacts on image quality, enable lens stabilization, if applicable, or use a tripod when working at slower shutter speeds, namely those below 1/100th second.

The Relationship Between Focal Length and Shutter Speed

Speaking of stabilization and handshake, let’s discuss the relationship between focal length and shutter speed. Understanding this relationship is essential when shooting handheld, as it directly affects your image quality.

As mentioned above, telephoto lenses exaggerate the appearance of camera shake, which, in turn, can cause unwanted motion blur in the photo and reduce its sharpness. They do this because their smaller Angle of View emphasizes the relative distance between the foreground, subject, and the background. And merely moving your arm an inch, up or down, can effectively translate into moving several feet over this large distance. So merely capturing a photo during this movement introduces motion blur into the picture.

So to combat this, it’s best to set your minimum shutter speed to the reciprocal of your current focal length as a general rule. Doing so will prevent most of the motion blur you’d otherwise see. An example, say you’re using an 85mm lens, you’d set your minimum shutter to 1/85. Or, in this case, the next closest third stop increment, which is 1/80th second.

Note: you can set the shutter speed to the next highest or lowest 1/3rd stop increment. But, the highest value will be safer and give you more of a buffer than the lower one.

Zooms vs Primes?

You can final focal length separated into two overarching categories: Prime or Zoom lenses. From there, they’re separated into sub-categories, which we’ll cover shortly below. But, let’s first talk about the difference between a Prime and a Zoom lens.

A prime lens is a lens with a fixed focal length. In contrast, a zoom lens has a variable focal length. Some popular prime lenses include 35mm, 50mm, and 85m. While, some popular zoom lenses include 16-35mm, 24-70mm, and 70-200mm.

Below are diagrams illustrating the difference between prime and zoom lenses. But, simply put, a zoom lens has a group of internal elements that are coupled to the zoom collar or ring. Rotating the zoom ring changes its position internally, altering the lens’s nodal point, and thus, equivalent focal length. You can see the area that varies in the second diagram, labeled “Variable Distance.”

camera-lens-35mm

camera-lens-70-300mm

Zoom Lens Benefits

A zoom lens offers several notable advantages given its variable focal length. Namely, it brings versatility and gives photographers the freedom to capture various subjects using a single lens. Take Tamron’s 18-300 F/3.5-6.3 all-in-one lens, for example. With this single lens, you can capture mediums from wide-angle landscapes or astrophotography, all the way to sports and wildlife. You can even photograph the moon. Having this flexibility otherwise requires nearly half a dozen prime lenses, which is very much not ideal if you’re carrying all of them around. And this type of flexibility and convenience is ultimately the key selling point for a zoom lens. It also makes them ideal for travelers looking for a one-stop solution. Plus, zoom lenses also reduce the number of times you’ll likely change a lens, saving time during quick shots and reducing the likelihood of accumulating sensor dust.

Prime Lens Benefits

While prime lenses don’t offer the flexibility in a variable focal length like a zoom lens, they make up for it with better optical sharpness and larger apertures. A fixed prime lens, having a set focal length, can maximize its optical construction to prioritize performance and image quality. And manufacturers don’t have to make compromises throughout the design merely to achieve a specific focal range. The result is a highly refined optical construction that removes distortion, chromatic aberration, and vignettes throughout most of the lens’ working range. We also often see more pronounced weather sealing and resistant coatings to help protect the lens. Plus, prime lenses of large working apertures, making them more versatile when working in low light and better at producing a soft background. Together, photographers get a more specialized lens that delivers the utmost performance without a zoom lens’s added bulk and weight.

Classifications of Focal Lengths

We classify camera lenses into three major categories and five subcategories based on their equivalent length in 35mm terms. Below are those categories and some information on each.

Note: there’s quite a bit of debate over which particular focal lengths belong to each category and how they separate. So use the information below as a general reference and just our perspective on the matter.

Short Focal Lengths (8-28mm)

Short focal lengths consist of wide-angle and ultra-wide-angle lenses. These lenses produce images with a large angle of view and low image magnification, letting them capture vast scenes by making subjects appear smaller than in reality and expanding the background. As such, they are best for architectural and landscape applications, where you want to capture a large environment.

Standard Focal Lengths (35mm-55mm) 

Standard focal lengths consist of mostly standard lenses, which closely match the angle of view of the human eye when we’re focused on a subject ahead. This focal length also closely matches the diagonal width of a 35mm full-frame camera, further creating a realistic perspective. As such, these lenses are some of the most flexible options in photography, from portraits to landscapes.

Long Focal Lengths (70mm-600mm)

Long focal lengths consist mainly of telephoto and super-telephoto lenses. And these lenses produce a higher image magnification and a narrow-angle of view, compressing the background and making the subject appear larger than in reality. As such, they’re best for shooting discreetly or shooting distant subjects like sports or wildlife.

Conclusion

In the end, it’s not that important photographers get bogged down by the definition and technicalities of focal length. What’s ultimately important is selecting the right lens that gives you the necessary framing, or more specifically, angle of view and image magnification. And focal length as a whole is merely a convenient means to describe how much a scene the lens captures, so you can better gauge framing.

But, truthfully, most photographers can do well by merely following in the steps of their predecessors in the mediums they enjoy. And if most photographers generally use a specific type of lens over another, then that’s a perfect means of selecting the appropriate focal length. However, having a better understanding of this concept will help make you into a more well-rounded photographer. And it will surely give you a level of understanding ideal for teaching.

Photo of author

Devaun Lennox

I'm a fashion, beauty, and commercial photographer turned impromptu photojournalist. Based in Las Vegas, my images are graphic, bold, and full-on contrast.