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What is the focal length of a lens?


Focal length, a whole program… When one starts in photography, as in many other fields, one does not escape this step: learning, reading and writing about the subject. Before jumping into a new passion, each aspiring enthusiast must familiarize himself with a specific vocabulary and the matching concepts. The future photographer is not spared and he will be obliged to integrate some basic notions that will allow him to better understand photography in his practice and help him to obtain better results. I am not a supporter of the application of technique in its purest definition but I must recognize that some principles are elementary if one wants to progress. This is the case of the one we will discuss today: the focal length of a lens.

What is the focal length?


The focal length of a lens can be defined as the distance in mm that exists between the focal plane of the sensor and the optical center of the lens. That was a bit stodgy, wasn’t it? More simply, the optical center is like a point inside a lens through which light rays pass, intersect and are directed to the image sensor of your camera to record the photograph and allow it to appear. This optical center is always very close to the diaphragm of the lens.

For this reason, this millimeter value is the first characteristic used by photographers to define the different lenses. We speak of a 50 mm or an 8 mm, for example.

So what effect do different focal lengths have on the same image? The smaller the focal length, the larger the angle of view of the lens, thus increasing the size of the scene we see. The longer the focal length, the smaller the angle of view, thus reducing the size of our photograph. For this reason, as you can see below, the use of a telephoto lens can create a feeling of compression of the scene or frame (very interesting for the foreground), while using a wide angle lens gives the feeling that the scene is enlarged, allowing more elements to enter the frame.

This is why we will see later that using telephoto lenses, we have the impression that the scene and the frame are compressed (they are the most suitable for successful first shots). On the contrary, using a wide angle lens gives us the feeling that the scene is expanding with more elements inside.

Angle of view


The angle of view of a lens is a technical characteristic that manufacturers provide for each model of lens, but what does it really mean?

The angle allows us to determine the field of view of the lens and thus determine the number of elements that can enter our frame. This concept allows us to classify lenses according to different categories, which you probably know:

Wide angle


Let’s start with the wide angle lenses, which include focal lengths smaller than 35 mm and capable of capturing images with an angle of vision greater than the human eye (without taking into account the peripheral vision), ie, more than 63º of the visual field.

If we want to go a little deeper, we could also consider wide angle lenses whose focal length is between 10 mm and 14 mm and fisheye lenses (whose name refers to the type of vision characteristic of fish) and those whose focal length is below 9 mm.

Normal


In photography, we usually define a lens as “normal” when its field of view and its characteristics are close to human vision. The definition problem for a normal lens arises when it has the option of peripheral vision. Authors who include peripheral vision consider normal lenses to be 35 mm while authors who do not include it opt for 50 mm, sometimes even 70 mm.

Telephoto lens


A telephoto lens (whether short or long) has focal lengths greater than normal, 50 mm or 70 mm according to the authors for full frame sensors or 35 mm when it is the film of our retro photographers. In other words, it is a lens whose focal length is greater than the diagonal of the sensitive surface of the sensor.

The concept of perspective distortion


Before you let yourself be seduced by one of these lenses, I think it’s a good idea to discuss the concept of optical distortion. Sometimes this concept is misused, used in a situation that does not correspond to it. Often you will read that the best lenses for portraits or fashion shots are telephoto lenses (because we often advise those above 70mm). A little explanation is in order… from the point of view of classical photography, the belief that these lenses “compress” the perspective has become widespread, when in fact it is the frame that is compressed and the depth of field that is reduced.

Let’s repeat: the perspective of a photograph does not depend on the focal length of the lens used but on the distance of the objects we photograph. As a general rule, we will remember that: when we increase the focal length, we create a compression of the frame of our image, causing the sensation that the background is behind the subject, while when we decrease the focal length, our sensation of space is enlarged and many more elements can enter the frame.

Not all lenses have the same purpose and one lens will not be able to save you in all situations. Next, we will tell you the uses that are attributed to each focal length. We do not claim to have the truth, so please take this guide as a series of suggestions to test your creativity.

Sports


If you love the explosive side of sports, the action, the gestures and that wonderful moment of surpassing the athlete, your favorite focal length might be a telephoto lens that could reach 300mm. If you don’t feel like investing in a fixed lens of this type, you might want to consider a more versatile zoom lens, the classic 70-200mm.

Some high-end lenses of this type, such as the Canon EF 200-400 MM F/4L IS USM Extender 1.4x, have a multiplier that allows us to get longer focal lengths (in the case of the Canon 200-400, we could multiply the focal length for 1.4).

If your lens does not include this possibility, do not worry, you can find a wide variety of multipliers on the market that you can attach to the base of your lens to increase the focal length and thus achieve the same results as those described above. Normally, these multipliers (also called teleconverters) offer a multiplication factor of 1.4x or 2.0, which we can achieve with a 70-200 lens:


  • With a 1.4x multiplier = 98 – 280 mm

  • With a 2.0x multiplier = 140 – 400 mm


Be careful because this type of accessory does not apply to any lens. They are usually brand specific like DZO. The best solution would be to check the manufacturer’s web page to make sure that the accessory will be compatible with your lens. Indeed, an unsuitable multiplier could create aperture problems with the autofocus, not to mention that the image quality could slightly decrease.

Landscape


Landscape photographers generally use the smaller focal lengths for their wider angle of view, so that we can place all the elements we have in front of our eyes in our frame.

For this reason, lenses with focal lengths 10-20 mm, 12-24 mm are commonly used although it must be admitted that the majority of professionals prefer to use fixed focal lengths that offer a higher quality rendering. Therefore, it is not uncommon to find 12mm, 14mm or 15mm lenses in landscape photography.

Portraits


It depends on the brand, but there are two fixed focal lengths that are the reference for this type of photography: 85 mm and 135 mm. It is difficult to decide on one of them and this question could create an endless debate among you photographers because the appreciation of the results is above all a question of taste and sensations. If you don’t have a fixed focal length, play with your zoom until you reach these values.

If you had to choose only one fixed focal length without spending a fortune, I recommend the 50mm lens. A great classic that has proven itself and that will not ruin you. It’s a good solution for your portraits because it offers an angle of view close to that of the human eye.

It all depends on the type of shot you want to make. To guide you and help you choose the right focal length some classic values :


  • General shot and American shot : from 35mm to 50mm

  • Medium shot: from 50mm to 85mm

  • Foreground and close-up: from 85mm to 105mm

  • Detail shot: 105mm and above

Mode


Many fashion professionals use a classic 70 – 200mm telephoto lens to get close to the models. Indeed, the photographer will often be placed at a certain distance from the subject and the telephoto lens is the best solution to capture a model at a distance and to focus quickly and efficiently. To shoot fashion portraits, you can rely on the previous point about portraiture. Once again, all these rules are not to be taken literally, in this sector more than elsewhere perhaps, it is important to innovate every day.

Social and street photography


Urban or social photography has always been influenced by the photographs of Robert Capa and Cartier-Bresson, who used a 50mm as their main lens, or Winogrand, another of the great masters of urban photography, known for using a 28mm. I’m willing to bet you’ll be seduced by this focal length after taking a look at his photos.

A big trend for this type of photography is to use an intermediate focal length: 35mm. This is a focal length also used by Capa and Cartier-Bresson, but especially by the American photographer Don McCullin (born in London in 1935). A photographer who has inspired many generations: his immense artistic career ranging from the end of the 40-45 war to the Vietnam war, through the legendary reports on the Beatles

Fixed or zoom lens ? Which lens to choose?


A permanent discussion on all photographers’ forums: fixed or zoom lens? As a general rule, professionals will always choose the fixed focal length, except in certain occasions where they want to privilege speed. Why this craze? Because the fixed focal length simply offers superior images, it has been optimized for a specific focal length.

Another important detail: the maximum aperture of zooms does not often exceed the values of f/2.8 or f/4 and most offer values between f/3.5 and 5.6. On the other hand, most fixed focal lengths offer apertures between f/1.4, f/1.8, f/2, f/2.8. This means that fixed focal lengths are much brighter than zooms and thanks to this, you will be able to obtain blurred backgrounds more easily, among other things.

Another point in favor of fixed focal lengths is without doubt the sharpness. In general, and many tests will show this, fixed lenses offer sharper results than zooms set at the same focal length (if we select these two lenses in the same price range).

However, we must admit that zoom lenses are more versatile, they allow to cover a larger number of focal lengths, which makes the zoom solution more comfortable (one lens instead of three, yes, we can feel it), but we must not forget that they also weigh more because of their optical structure, which implies a larger number of lenses.

A fixed lens will force you to build your picture better: you have to think about the position, you move much more than with a zoom. When you have to cover an event like a wedding, you will certainly have to use a zoom lens to privilege the speed but even in this case, no question of standing still, you have to transform yourself into an invisible dancer who twirls around the participants.

Conclusions


Here is a series of tips that should help you find the right focal length more easily depending on the type of photography you want to do. But the rules are also there to be broken, use a classic base but innovate to give a really personal touch to your shots.