What is focal length in photography?

Focal length is one of the most essential concepts in photography and yet another addition to our “Terms and Definitions” series. It determines not just the magnification of your subjects but also how much of the scene your camera captures. Whether you’re shooting landscapes, dramatic portraits, or wildlife in its natural habitat, understanding this concept gives you greater control over your choice of lenses and your photography overall.

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In technical terms, the focal length is the distance, measured in millimeters (mm), between the optical center (or nodal point) of a lens and the focal plane (where your camera’s sensor is located) when the lens is focused at infinity. Simply put, it’s a measure of how strongly the lens converges or diverges light rays to form an image. This measurement profoundly impacts the field of view, magnification, and perspective of your photographs.

In film photography, the concept is the same – only it’s the film you think of instead of the camera sensor.

How it works?

A lens’s focal length affects the angle of view, which is the portion of the scene captured by the camera.

• Shorter focal lengths (e.g., 14mm, 24mm) offer a wider field of view. This makes them ideal for capturing vast landscapes or architectural scenes.
• Longer focal lengths (e.g., 10mm, 200mm, 400mm) have a narrower field of view and bring distant subjects closer. They’re often used for portrait, wildlife photography, and sports photography.

Of course, these examples are just the most common uses – and there are always exceptions. There are brilliant landscape photos shot with longer lenses and gorgeous, unusual wide-angle portraits. While the focal length controls how much of a scene you include in the image, it also influences the depth and compression of the scene. A wide-angle lens exaggerates the sense of space, while a telephoto lens compresses perspective, making objects appear closer together.

Types of lenses by focal length

As I mentioned above, different lenses serve different photographic purposes based on their focal length. Here are the main types of lenses by this criteria:

Wide-angle lenses

• They typically range from 14mm to 35mm.
• Offer a wider field of view, making them perfect for landscape photography and architectural photography.
• Can distort objects at the edges, but this distortion can be creatively used.

Standard lenses

• They usually have a focal length of around 50mm.
• Closely mimic the natural perspective of the human eye (on full-frame cameras, but we’ll get there).
• A popular choice for portraiture, street photography, and everyday use.

Telephoto and super-telephoto lenses

• They start at 70mm and can extend beyond 600mm.
• Excellent for sports photography, wildlife photography, and isolating distant subjects.
• Super-telephoto lenses (300mm+) are often used in astrophotography, and nature photography, much like a telescope.

We can also differentiate between prime and zoom lenses.

Prime lenses feature a single, unchanging focal length (50mm or “nifty-fifty” being a very popular choice, and my personal favorite). They’re praised and loved for their sharpness, lightweight design, and shallow depth of field. Since they usually have a wider maximum aperture (you can find a super-affordable f/1.8 prime lens), they’re great in low-light conditions.

Zoom lenses cover a range of focal lengths (e.g., 24-70mm, 70-200mm, or the “kit lens” many of us got with our first camera, 18-55mm). They are versatile and convenient for photographers who need to adapt quickly and cover a broad range of scenarios. While prime lenses are known for exceptional sharpness and large apertures, modern zoom lenses also excel in these departments. Although, if you want a large-aperture zoom lens, it could be bulkier and more expensive than a prime. Still, it can be a great choice and have you covered in many situations.

How camera sensor size affects focal length

Okay, now that we’ve covered the basics, let’s take it a step further. The next thing to know is that the camera’s sensor size also plays a role in determining the effective focal length. There’s a difference between full frame, APS-C, and MFT (Micro Four-Thirds) sensor sizes and the equivalent focal length of a lens we use on these.

• Full-frame cameras have a sensor equivalent to 35mm film. In other words, they provide the true focal length as stated on the lens. On these cameras, a 50mm lens closely replicates the perspective of the human eye. It offers a natural field of view, making it feel like what we perceive with our eyes. This makes it a popular choice among street photographers, making their subjects “closer” and more familiar to the viewer.
• APS-C cameras feature smaller sensors with a crop factor, typically 1.5x or 1.6x (for Canon) of the full-frame sensor. So, if you take a 50mm lens on an APS-C camera, it will behave like a 75mm lens on a full-frame camera.
• Micro Four Thirds (MFT) cameras feature an even smaller sensor with a crop factor of 2x. For comparison, a 50mm lens on an MFT camera provides the equivalent field of view of a 100mm lens, making it highly effective for portrait photography and distant subjects while maintaining portability.

The relationship between focal length and depth of field

Depth of field is the portion of an image in sharp focus, and focal length is one of the major factor in determining it.

• Shorter focal lengths provide a larger depth of field, making more of the scene appear in focus. So, with wide-angle lenses, you’ll have more of the scene in focus (this is just one of the perks of these lenses for landscape photography)
• Longer focal lengths create a shallow depth of field, making less of the scene in focus. This makes them ideal for portraits where you want to isolate your subject from the background.

Keep in mind, though, that focal length is just one part of the equation. The aperture and the distance to the subject also affect the depth of field, so all three factors work together in the optical system.

Focal length formulas and technical details

If you’re interested in the technical side and a bit of math, we got you covered. You can calculate the focal length using the formula:

f = 1 / (1/v + 1/u)

Where:

• f: Focal length of the lens.
• v: Distance from the lens to the image plane (sensor or film).
• u: Distance from the lens to the object being photographed.

This formula, derived from the thin lens equation, helps explain how focal length relates to the lens’s focusing capabilities. When focusing on infinity, u approaches infinity, and the equation simplifies to f = v, making the focal length equal to the distance between the lens and the sensor. This mathematical formula helps lens designers optimize the optical system to minimize distortions and maximize performance.

Choosing the right focal length for your photography

So, with all of this in mind, how do you choose the perfect focal length for your project? When picking out the best lens for your needs, consider these things:

• Subject and genre: As mentioned, landscapes often benefit from wide-angle lenses, while portraits and wildlife usually require longer focal lengths. This is a rule of thumb and one of those rules that’s meant to be broken, at least on occasion. But if you’re still looking for your unique style and genre(s), this is a good starting point. Also, I’d suggest starting with a zoom lens to have you covered in various situations.
• Portability: speaking of zoom lenses, they are great for travel. They’re portable and versatile, and they’ll spare you from overpacking and carrying tons of heavy gear.
• Creative intent: On the other hand, a prime lens with a specific focal length can push you to compose more thoughtfully and take you out of your comfort zone. I sometimes play with this concept and I’ve never regretted it – no matter how challenging it can be.

Final thoughts

Remember that focal length is a fundamental aspect of photography that directly impacts which lens you choice and how you capture a scene. By understanding the characteristics of shorter and longer focal lengths (and everything in between), you can make informed decisions to match your artistic goals. Whether you’re using a prime lens, a zoom lens, or you switch between them, mastering focal length expands your knowledge immensely and opens a world of creative possibilities.

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