Understanding Focal Length And How To Use It Update 10/2021


Focal length is a confusing concept for many photographers. Some people believe that the focal length of a lens determines its physical size, but this is not true. So, what exactly are we dealing with here? In this article, I’ll explain how to choose the right focal length for your particular photographic style.

1. Definition of Focal Length

Lenses have optical properties such as focal length that can be discussed without going into too much detail about the physics. It measures the lens’s optical center’s distance from the camera’s sensor in millimeters (or film plane). With the camera set to infinity, the results are captured. The focal length is used to name lenses, and this information can be found on the barrel of the lens. The focal length of a 50 mm lens, for example, is 50 mm.


The “optical center” of a lens was mentioned in the definition of focal length. You may be puzzling over what’s going on here. However, the lens of a camera is not constructed of a single piece of glass.. Lens elements and groups of elements are combined to make up the entire system, as opposed to individual lens elements. These set-ups aid in the concentration of light and the reduction of chromatic aberrations. The optical center of a lens is where all of the light rays converge to form a crisp image.

The focal length of a lens is determined by its optical design, not by the camera. A 50 mm lens will always function as a 50 mm lens, regardless of whether it’s mounted on a full-frame, cropped sensor, or medium format camera. Lens/camera combination field of view is influenced by sensor size, but more on that in a moment.


2. The Important Stuff

Photographers don’t have to remember the definition of focal length, even if it’s important to some of them. Focus length tells us a lot about how a camera works. The focal length of a lens determines its field of view. In other words, how much of what’s going on in front of us does the camera capture? Also, take note of how large the objects in the shot appear to be. As a lens’s focal length increases, it narrows in perspective. Long focal length lenses make objects appear larger than they actually are when viewed through the human eye. Conversely, short-focal-length lenses capture a much wider field of view. As a result, objects in the frame appear much smaller than they actually are to the human eye.

Take a look at this rough illustration to get an idea of what I mean. While the 50 mm lens has an angle of view of 46 degrees, Nikon’s 500 mm f/5.6 lens has an apparent field of view of 5 degrees. The camera’s 20 mm f/1.8 lens, on the other hand, has a field of view of 94 degrees. It’s easy to see that the 500 mm lens captures a significantly smaller portion of the scene. As a result, the photograph only includes a small portion of a single boat. The 50 mm lens, on the other hand, provides a greater field of view. Keeping your eyes fixed on the same spot, you’ll notice that a wider view of the scene is available, revealing several boats as well as further away rocks in the distance In contrast, the 20 mm lens allows you to take a single picture of the entire scene.


3. Field of View and Equivalent Focal Length

The terms “field of view” and “angle of view” are frequently used as synonyms. In any case, as previously stated, the angle of view is an optical characteristic of the lens. It’s the same no matter what kind of camera you’re using. The lens/camera combination determines the angle of view. The size of the camera sensor has an impact on the field of view as well as the focal length of the lens.

The sensor size of a full frame camera is the same as that of a 35 mm film negative (36 mm x 24 mm). Sensor sizes vary widely from one manufacturer and camera model to the next in today’s digital cameras. Cropped sensors are those that are smaller than a full frame sensor. As with cropping an image, this term refers to how much less of a scene these smaller sensors see.

Effective focal length, or equivalent 35 mm focal length, refers to the amount of light that falls on the 35 mm sensor when shooting with a given lens. The full frame format became the standard because most people, including those of us with a few grey hairs, are used to working with 35 mm film cameras. Full-frame-equivalent-lens describes the focal length of lens needed to capture the same field of view as an equivalent cropped-sensor-lens lens. Crop factors enter the equation here. The non-full frame camera’s crop factor multiplies the lens’s focal length to arrive at the equivalent focal length. The crop factor for Nikon’s DX cameras is 1.5. There are crop factors of 1.6 on Canon’s EF-S DSLR cameras. The crop factor for micro four-thirds cameras is 2.0, while the crop factor for 1′′ sensors from Sony and Panasonic is 2.7.

This picture was taken with my full-frame Nikon D800 and a 24-70 mm f/2.8 lens at a focal length of 44 millimeters. It would still be a 24-70 mm lens at 44 mm if I had used the same zoom on my Nikon D500 (cropped sensor). The cropped sensor camera’s reduced field of view, on the other hand, will not capture the same amount of detail. In this case, I’d only take pictures of the parts that are highlighted in red. With the D500, my effective focal length would be 44 mm x 1.5, which equates to 66 mm. To put it another way, if I wanted to capture the area highlighted in red on my D800, I’d have to use a lens with a 66 mm focal length. My 24-70 mm lens had a zoom range of 44-66 mm, so that was an option as well.


4. Zooms vs Prime ?

The focal length of a prime lens is always the same. The focal length of a zoom lens, on the other hand, can be adjusted. Zoom lenses with focal lengths ranging from 16 to 35 mm, 24 to 70 mm, and 70 to 200 mm are common. An 18-200 mm lens, which covers the wide-angle and telephoto ends of the spectrum, is a great travel lens. The benefit of this is that you won’t have to carry around a bunch of different lenses or switch lenses to capture landscapes and architectural details at the same time.

Zooms, on the other hand, have a drawback: they are less sharp optically than prime lenses. A widening gap still exists, even with advances in technology like 18-200 mm lenses, but it is closing. Another problem with zoom lenses is that their maximum apertures are typically smaller than those of prime lenses. High-end lenses with fixed f/2.8 apertures are rare, but prime lenses with similar focal lengths can open up much wider and often let in several more stops of light. In low-light situations, prime lenses may be preferable.

5. Lens Focal Length Comparison

The equivalent focal length of camera lenses is used to classify them into five descriptive categories.

5.1 – Ultra Wide-angle Lenses

Full-frame focal lengths for ultra wide-angle lenses are less than 24 mm. They have a wide field of view, which they capture admirably. However, as a result of this, their perceptions of the world are often skewed. When used properly, they’re a lot of fun to work with because of their short, minimum focusing distance and large field of view. These lenses are a must-have if you photograph interiors.


Focal Length 18mm – Aperture ƒ/3.5 – Shutter Speed 1/640s – ISO 500

5.2 – Wide Angle Lenses

In the 24 to 35 mm range, wide-angle lenses have an equivalent focal length. Landscape and architectural photographers still use these lenses because of their wide field of view. When photographing with a wide-angle lens, make an effort to capture some foreground interest. Adding a sense of scale to your photos will entice viewers to stay on your page longer. Lenses with a large depth of field make it simple to get both close-up and far-away subjects clearly in focus.


Focal Length 28mm – Aperture ƒ/3.5 – Shutter Speed 1/800s – ISO 100

5.3 – Standard Lenses

The focal lengths of standard lenses typically range from 35 mm to 70 mm. In many ways, they mimic our eyes in how they see the world. Because of their low chromatic aberration, these lenses are popular among portrait photographers. The ability to isolate a subject from its background with much shallower depths of field than wide angle lenses is another feature of lenses in this focal range.


Focal Length 70mm – Aperture ƒ/7.1 – Shutter Speed 1/320s – ISO 100

5.4 – Tele Lenses

Telephoto lenses have a focal length between 70 mm and 300 mm and are referred to as such. Photographers who want to get up close and personal with their subjects often use these devices. Small apertures on these lenses produce shallow depths of field, making it imperative to achieve a razor-sharp focus.


Focal Length 100mm – Aperture ƒ/2.8 – Shutter Speed 1/160s – ISO 500

5.5 – Super Telephoto Lenses

The focal length of a super telephoto lens is greater than 300 millimeters. Birds and other small faraway subjects are frequently photographed with them. These large and heavy lenses may necessitate the use of a tripod in order to be stable. They’re also incredibly pricey! With that said, the current price for this lens is an eye-watering $16,300. Fortunately, there are now some significantly less expensive options available, making bird photography accessible to photographers of all budgets!


Focal Length 500mm – Aperture ƒ/4 – Shutter Speed 1/500s – ISO 12800

All of those figures are, once again, in full-frame terms. If you’re using a camera with a crop sensor, you’ll need to multiply these focal lengths by 1.5, 2, or whatever your crop factor is to get your equivalent focal lengths.


Don’t obsess over the term “focal length” or the distinction between “angle of view,” “field of view,” and the equivalent focal length of a lens. Long focal length lenses, such as a telescope, bring distant objects closer. Wide-angle lenses, on the other hand, are excellent for capturing expansive landscapes. Telephoto lenses allow you to get much closer to your subject than a normal lens would allow. If you enjoy shooting landscapes and architecture, you should always have a wide-angle lens on hand. Whether you’re shooting portraits or anything else, having a nifty-fifty on hand is always a good idea.