Many photographers misunderstand the concept of focal length. Contrary to what some people think, the focal length of a lens is not the lens’s physical dimension and it has little to do with its overall size. So, what is it then? In this article, I will answer that question and discuss how to decide which focal length is right for your style of photography.
Definition of Focal Length
Without getting into an in-depth physics discussion, the focal length of a lens is an optical property of the lens. It measures the distance, in millimetres, between the optical centre of the lens and the camera’s sensor (or film plane). It is determined with the camera focused to infinity. Lenses are named by their focal length, and you can find this information on the barrel of the lens. For example, a 50 mm lens has a focal length of 50 mm.
In the definition of focal length, I mentioned the “optical centre” of a lens. You may be wondering what this is. Well, a camera’s lens is not made from a single piece of glass. Instead, it is a combination of lens elements and groups of elements. These combinations help to focus the light and cut down on distortions. The location where all the light rays converge to form a sharp image is known as the optical centre of the lens.
Focal length is a property of the lens itself, not the camera. What I mean by this is that a 50 mm lens is a 50 mm lens, regardless of whether on a full frame, cropped sensor, or medium format camera. However, the size of the sensor does play a role in the lens/camera combination’s field of view, but more about this in a minute.
The Important Stuff
While the definition of focal length may be relevant to some people, as photographers, it is not something you need to remember. What is more important to understand is what focal length tells us. Focal length describes the angle of view of a lens. That is, how much of a scene before us the lens captures. And in addition, how large subjects within the frame appear. The longer the focal length of a lens, the narrower its angle of view. Subjects appear larger using long focal length lenses than they do viewing them with our eyes. On the other hand, lenses with short focal lengths take in a much wider angle of view. Thus, elements seem much smaller in the frame than they do to our eye.
Take a look at this illustrative sketch. According to Nikon, its 500 mm f/5.6 lens has an angle of view of 5o, while its 50 mm f/1.4 lens has an angle of view of 46o. And lastly, its 20 mm f/1.8 lens has an angle of view of 94o. As you can see, the longer 500 mm lens takes in a much thinner slice of the scene. As a result, only a portion of a single boat is captured in the shot. On the other hand, the 50 mm lens has a wider angle of view. Standing in the same location, you can capture a much broader section of the scene, including several boats and more of the distant rocks. However, with the 20 mm lens, you can capture the entire scene in a single frame.
On a recent trip to Lake Tahoe, I came across two fellows rock climbing while I was hiking. Although I did not have my camera on a tripod, I was sitting down on the rocks and captured all four shots from the same spot. Notice how my images narrow in on the scene as I zoom in on the climber. At a 25 mm focal length, you can barely spot the men. Alternatively, at 140 mm you can see the expression on the climber’s face. Incidentally, he managed to climb another 3 m up the face before he lost his grip, slipped off the rocks, and repelled safely back down to the ground!
Field of View and Equivalent Focal Length
The terms “angle of view” and “field of view” often get used interchangeably. However, as I said above, the angle of view is an optical property of the lens. It does not change regardless of what type of camera is being used. Field of view, on the other hand, is a result of the lens/camera combination. Field of view not only depends on the focal length of the lens but also the camera’s sensor size.
A full frame camera has a sensor that is the same size as a 35 mm film negative (36 mm x 24 mm). However, digital cameras today come with a wide variety of sensor sizes depending on the manufacturer and camera model. Sensors that are smaller than full frame are considered cropped sensors. This term comes about from the fact that these smaller sensors see less of a scene, in a similar manner to what cropping an image does.
The phrase “effective focal length” (also known as equivalent 35 mm focal length) is used to equate what a lens captures in 35 mm sensor terms. Because most people are used to working with 35 mm film cameras, at least those of us with a few grey hairs, the full frame format was adopted as the standard. Equivalent focal length describes the focal length of a lens you would need to put on a full frame camera to capture the same field of view as a given lens on a cropped sensor camera. This is where crop factors come into play. The equivalent focal length is found by multiply the focal length of the lens by the non-full frame camera’s crop factor. For Nikon, it’s DX cameras have a crop factor of 1.5. Canon’s EF-S cameras have crop factors of 1.6. Additionally, micro four-thirds cameras have a crop factor of 2.0, and Sony and Panasonic’s 1″ sensors have a crop factor of 2.7.
This image below was taken with a 24-70 mm f/2.8 lens at 44 mm on my full frame Nikon D800. If I had put this exact lens, at the same zoom, on my Nikon D500 (cropped sensor), it would still be a 24-70 mm lens at 44 mm. However, the reduced field of view – due to the cropped sensor camera – would not see the same area. In this situation, I would only capture what is outlined in red. My effective focal length on the D500 would be 44 mm x 1.5 or 66 mm. In other words, if I wanted to capture what is outlined in red on my D800, I would have to put on a lens with a 66 mm focal length. Of course, I could also zoom my 24-70 mm lens from 44 to 66 mm.
Nasim wrote an excellent article on Equivalent Focal Length and Field of View a few years back. For a much more detailed explanation, check his article out.
Classifications of Focal Lengths
Camera lenses are classified into five descriptive categories depending on their equivalent focal length. Ultra wide-angle lenses have a focal length of less than 24 mm in full frame terms. They capture incredibly broad views. However, because of this, they often present a distorted view of the world. They are fun lenses to use and have a very close minimum focusing distance and a large depth of field. If you photograph interiors, these lenses are well worth having in your bag.
Wide angle lenses have an equivalent focal length in the range of 24 mm to 35 mm. These lenses still take in a wide view and are often used by landscape and architectural photographers. When you use a wide lens, it is a good idea to try and include some foreground interest. This will give your photos a sense of scale and help to lead viewers into your image. Because these lenses have very large depths of field, it is easy to get both near and far objects in sharp focus.
Standard lenses have focal lengths between 35 mm and 70 mm. They capture the world in a way very similar to how our eyes see. They cause minimal distortion, so are a favourite of portrait photographers. Another feature of lenses in this focal range is their ability to isolate a subject from its background using much shallower depths of field than wide angle lenses.
The focal lengths between 70 mm and 300 mm are considered telephoto lenses. They are regularly used by wildlife photographers to get closer to their subjects without being seen. These lenses have shallow depths of field, even at small apertures, so acquiring sharp focus is critical.
Super telephoto lenses have focal lengths exceeding 300 mm. They are often used for photographing birds and other small distant subjects. These lenses can be very large and heavy and may require the use of a tripod to support them. They are also very expensive! Nikon’s AF-S NIKKOR 800mm f/5.6E FL ED VR lens is a whopping $16,300 at the time of writing this article! Luckily there are some much cheaper options available now that allow photographers with normal pocketbooks to get into bird photography!
Again, all the numbers I just mentioned are in full-frame terms. If you have a crop-sensor camera, you will need to find your equivalent focal lengths by dividing these numbers by 1.5, 2, or whatever your crop factor is.
Zooms vs Prime?
Prime lenses have a single, fixed focal length. A zoom lens, on the other hand, has a variable focal length. Some popular zoom lens ranges include 16-35 mm, 24-70 mm, and 70-200 mm. A great lens for travelling is one that covers both the wide and telephoto ranges, such as an 18-200 mm lens. The advantage here is that you will not have to carry a multitude of lenses with you, or change lenses to shoot wide vistas and close-ups of architectural details.
There is a downside to zooms, though: they are often not as optically sharp as primes. Although this gap is closing with newer and better technology, it still exists, especially when dealing with superzooms like 18-200 mm lenses. Another drawback is that they tend to have narrower maximum apertures than prime lenses. While a top of the line zoom might have a fixed aperture of f/2.8, primes in a similar focal length may open up much wider and often let in several more stops of light. This can make prime lenses more desirable in low light conditions. For more information on primes versus zooms, see our larger article on the subject.
Don’t get hung up on the definition of focal length, or even the difference between the angle of view, the field of view, and the equivalent focal length of a lens. What is important to remember is that lenses with long focal lengths bring objects closer, like a telescope. And, on the flip side, wide angle lenses are great for capturing large vistas. If you need to get closer to your subject than you can physically, opt for a telephoto lens. If landscape and architecture are genres you enjoy shooting, then make sure you carry a wide angle lens. For portraiture, and anything else in between, you can’t go wrong with a nifty-fifty in your bag.
If you have questions, please drop me a line in the comments below, and thanks for reading!