Focal length in photography: what is it?

Since I started writing on this blog about travel and photography, I realize that I didn’t yet take the time to talk about the basics of photography, the theoretical aspects. I’m one of those who sincerely believe that you can’t be a good photographer or learn photography until you master all (or most) of the technical aspects. So, I decided to start by explaining the focal length in photography.

Other technical articles will then follow, to help you understand the basics of photography, including the concepts of aperture, depth of field, ISO sensitivity, shutter speed, hyperfocal, stabilization, bokeh (background blur), histogram and much more! We’ll slowly get a foot on the ladder, little by little, so that you can learn about the more technical aspects.

The purpose is obviously not to make you disgusted with photography, that’s why I’ll try to keep it simple and clear. Whether you buy a Canon, Nikon, Sigma, Panasonic, Leica, Zeiss, or any other brand, all lenses have the same concept and notion of focal length. Moreover, between us, understanding focal length will for example allow you to choose more precisely your camera accessories such as your camera bag, a tripod and others.

Understanding the exact definition of the word “focal length” will help you answering one of the first questions many people ask: how do you choose a camera lens? Because buying a DSLR, bridge or mirrorless camera is good, but having an idea of which lenses you’ll choose is even better! Why am I talking about lens, and not focal length then?

Purely and simply because the focal length is one of the main characteristics of a lens. It is defined in millimeters and represents very roughly the distance between the sensor of your camera and the optical center of your lens. For now, you don’t need to know more about it to become a photographer. That’s what I retained overall, and that was enough for me to understand the definition of the focal length.

In my opinion, it is the most important element to choose a lens, along with the maximum aperture.

More generally, the majority of people associate focal length with the “magnification level” of their lens. This is quite simplified, but indeed, it is the main influence of the focal length on a picture. Simply put, it changes the angle of view and the way we will see the subject we shoot. However, it is important to know that the choice of focal length influences many parameters, whether on your image or on your lens itself. I’ll talk about it in the two paragraphs below.

To fully understand this notion of focal length of a lens, it is very important to know how to differentiate APS-C and Full-Frame sensors.

The majority of beginners and amateurs (but not only) use a small sensor (APS-C), which have a magnification factor of 1.5, or even 1.6 (Canon). I see it coming, you’re going to tell me that it’s only a detail of numbers, right? And in reality, not at all! To make it simple, the viewing angle with an 85 mm focal length on a Full Frame camera is more or less the equivalent of a focal length of 50 mm on an APS-C camera. Don’t believe me? Do the calculation 50mm x 1.6 and you’ll see. We talk about “equivalent focal length” to give the equivalence of the focal length on a full frame sensor.

This is a very important element to understand, and it will be the subject of a more detailed article later on. You just have to keep in mind that for an equal focal length, the APS-C sensor “zooms more” than a Full Frame sensor. Conversely, always at the same focal length, the Full Frame camera will have a wider field of view than an APS-C sensor. Knowing that Full Frame sensors are the reference when speaking of focal length.

This is therefore quite a decisive factor when purchasing your camera (and your lens). There are several main categories of focal length in photography:

• Wide-angle focal lengths: we generally consider a focal length as “wide angle” when it’s below 28mm to 35mm. You will sometimes also find the term short focal length, or wide angle, or ultra-wide angle (UWA). We find lenses called “Fisheye” too, which are lenses where distortion has not been corrected (which gives quite a nice effect sometimes!), offering a very large field of view,
• Standard focal lengths: this is often the first lens you have with your camera (Ex: 18-55 mm). It is generally a focal length between 40 and 60 mm, depending on the type of camera body used,
• Telephoto lenses: we usually classify lenses this way when they have a focal length of 85 mm.

Once you understand the different focal lengths in photography, let’s see how they influence your image as well as the lens you are about to buy. This is what interests us the most.

As mentioned in the paragraph above, choosing a focal length is not that simple and our choice does matter for the final result of our image. I present below the main influences, in my opinion:

It is the most visible element when changing the focal length. You should just experience it by being in the same place and shoot an exact same scene with variable focal lengths (e.g., 18mm, 55mm, 200mm, 500mm, etc.). The longer the focal length will be, the more our viewing angle will be reduced and, therefore, small. This is quite simply what a zoom lens does in reality: reducing our viewing angle and focusing on a detail.

Another notable influence in the choice of the focal length in photography is just what we could call the magnification of the object. Indeed, with a picture shot at the same place, the longer your focal length will be, the bigger the subject will appear and the more it will be “zoomed” in reality.

This is especially the reason why wildlife or sports photographers use very long focal lengths (e.g., 300mm or 500mm), as they seek to enlarge their subject. Very often, the reason is simple: the subject is too far away, and you can’t get closer. For example, I took this picture below in the Lake Manyara National Park, in the North of Tanzania. You should know that it was taken with a telephoto lens with a long focal length of 300 mm. So, try to imagine how would be the same photo shot at 11 mm. The giraffe would have been only a point on this image, barely perceptible.

Usually, a long focal length will be used, since the subject cannot be approached closely. I voluntarily exclude macro photography, as the subject is very close to the lens in this discipline.

Another point to understand about the focal length of a lens is its impact on the perspectives and distortions of the image. When I talk about perspective, I can summarize it the following way. Small focal lengths (e.g., wide angle) tend to “exaggerate” (roughly) perspectives and give more space between different subjects on a picture. Concretely, it will feel like we have a greater depth of field. In reality, perspectives with a wide angle are “increased”. If you get closer to a subject with a wide-angle lens, it will appear “larger”, like very much present in your image. In addition, since the wide-angle has a wider vision, the converging lines are more present in the image which leads to a deeper effect.

Conversely, long (telephoto) lenses will tend to “flatten” subjects, so that you sometimes feel like all the subjects are on the same level in the picture (whereas they may be several kilometers away in reality).

These precise points will obviously depend on your distance from the subject. If the subject is far away, these rules are 100% true. If the subject is close, things will be different. I will present the notion of depth of field in another article.

I also talk about distortion in the sense that ultra-wide-angle lenses will tend to distort some subjects. You will see them especially on the edges of your image. For fun, try shooting a portrait of someone up close with a wide-angle lens! Guaranteed laughs with this kind of test.

To get a deeper understanding of the notion of focal length, another aspect you should know is the appearance of vignetting on the edges of your image. This is simply an effect appearing mainly on images shot with wide-angle and ultra-wide-angle lenses (less than 24mm).

Generally, the smaller the focal length, the more vignetting effects on your images is possible. This will obviously be related to the aperture chosen for the scene. If you want to check the vignetting effects of a lens with a particular camera, you can have a look at the dxomark website, the gold standard for this subject.

The image below shows you as an example the vignetting that appears on a Canon 14mm f/2.8 L II wide-angle lens on a 6D body (which has a fixed maximum aperture of f/2.8).

To summarize this image, you will notice that with a small focal length, such as this 14mm from Canon, a large vignetting appears at large aperture (between f/2.8 and f/4). The aperture is then wide open and, with this type of focal length on this camera (full frame), black borders appear on the edges of the image. That’s why it’s such an important factor to know when buying your lens and choosing your focal length.

Before buying and choosing your focal length, last point to consider is the focusing distance of your lens. I voluntarily exclude lenses for macro photography since they were specifically designed to have the smallest possible focusing distance (to take pictures of small insects for example).

The rule is generally simple: the longer your focal length, the longer the minimum focusing distance. Concretely, this means that when you use a telephoto lens type Canon 70-200, you have a minimum focusing distance of 1.20m. So, you won’t be able to shoot a subject if it is closer than that. This is then another point to note when choosing your focal length.

Want to know more about focusing in photography and how does it work? I’ve written a full article.

Here, I’ll explain how the choice of your focal length influences the lens you are going to buy. I see three main points:

The diameter of the lens: most of the time, the longer your focal length, the larger the diameter of the lens. This will especially influence the purchase of the filters you will place on it (polarizing filter or neutral density (ND) filter),

The length and weight of the lens: here again, the longer the focal length, the longer and heavier the lens. To give you an idea, my Canon 70-300 L IS lens weighs 1kg. Next to it, my 55 mm f/1.8 weighs only 160g.

The price: it is usually always true, the bigger the focal length, the more expensive the lens. Note that fixed focal length lenses are generally cheaper than zooms and offers larger maximum aperture.

To quickly answer this question, it will mainly be about knowing how you will use the lens. Specifically, what kind of image do you want to shoot? If you’re more into landscape photography, you will most likely choose a short focal length, and even a very short one for a better angle of view (less than 30mm).

For portrait photography, it is generally recommended to have a focal length of 50mm in APS-C, and 85 or 100mm on a Full-Frame camera. For wildlife photography, a minimum focal length of 300mm will be necessary.

In short, you should have understood it by now: think about what you want to shoot before buying anything!

Small conclusion on the subject, to summarize everything we’ve just seen.

For short focal lengths:

• Perspectives will be increased,
• Subjects in the image will appear far away,
• The viewing angle will be larger,
• Vignetting and deformations may appear,
• The focusing distance will be smaller,
• They are adapted to subjects such as landscape, street photography, architecture.

For long focal lengths:

• Elements in the image appear close (notion of magnification of objects in the scene),
• The viewing angle will be small,
• The depth of field is shorter
• The focusing distance will be longer,
• They are suitable for portraits, sports and wildlife photography,
• The length, weight and price will all be bigger.

That’s it, I hope this article on focal length in photography have allowed you to understand all the nuances this notion can have. Now, you should normally be able to master the choice of the focal length when buying your next travel lens.

I hope this article has not been too stodgy to read if you are new to photography, and I just ended a full article on photography terms including all the necessary notions to get started. Soon, autofocus, framing, white balance, post-processing, sharpness and diffraction will no longer hold any secrets for you! What did you think of the article? Is it too difficult to read? Too detailed, too complex? Your opinion is important for us, as it can permits me to adjust for the other following articles. To keep learning, I invite you to come and discover an essential concept: the exposure in photography.

See you soon for a new article on the basics of photography.

Sylvain