Focal Length: Simplified Explanation [Con Ejemplos]

In this article we will talk about the focal length of a lens. A great unknown to most beginners, and a (fundamental) ally when you meet her.

I will explain what the focal length and the angle of view are, the differences between the different types of lenses according to their focal length and in what situations it is convenient to use one and the other so that you can get the most out of them.

You can not lose this! Get ready, it’s a long article. Of course, I am going to make it as simple as possible, but very complete, so that you have all the information in the same container. You’ll want to save it to come back to it more than once.


What is focal length?

The focal length is the distance between the optical center of the lens, and the sensor or focal plane on which the image is projected. It is also known as “focal length”, and is measured in millimeters.

In photography, the greater the focal length, the greater the “zoom” the lens will have, and the less part of the scene it will capture. The shorter the focal length, the farther things will look, say, but the framing or angle of view will be much wider instead.

Better to see it with the following examples, same scene, from the same position, with different focal lengths.

18mm 35mm 85mm 135mm

Focal Length Example

To further avoid getting into too many technicalities, the focal length is not measured from the sensor to the front lens of the lens, but is measured from the point where light rays intersect within it and are directed towards the sensor.

This point is very close to where the diaphragm is and is called the optical center.

Optical center of the objectives

How is focal length (or focal length) measured?

The focal length is measured in millimeters (mm), the longer the focal length (the higher the number), it is said to be a long focal length, it brings us closer to the subject and covers less of the scene. The shorter the focal length (smaller the number) the more scene it captures.

Just so we understand each other: a 70mm focal length means that the lens is capable of bringing us much closer to the photographed subject than an 18mm focal length lens and will show less of the scene.

And a 200mm will have even greater ability to approach the subject, reducing the part of the scene that will be photographed.

Effective focal length

The focal length of a lens, whatever its brand or model, uses as a reference the size of the sensor of a Full Frame camera (35mm).

Why is this important? Because depending on the size of your camera’s sensor (Full Frame, APS-C, 4/3, etc.), the effective focal length of the lens will vary.

If you have a camera with an APS-C sensor that is smaller than Full Frame, such as a Canon 700D or Nikon D3100, a 50mm lens on those cameras will be equivalent to a 75mm if you’re using Nikon or 80mm if you’re using Nikon. you use Canon.

Why? The focal length of a lens depends on the size of the sensor of the camera in which it is placed and as the APS-C sensor is smaller than the Full Frame standard, to know the effective focal length, you must multiply the original distance of this by a multiplication or trimming factor.

This factor depends on the brand (due to the size of the sensor), for example, for Nikon it is 1.5 and 1.6 for Canon.

How is the multiplication factor calculated?

Just as a fun fact, if you want to know how the multiplication factor (also known as conversion factor) is calculated, the following formula applies:

Sensor Width Full Frame (35mm) / Width of your camera sensor

If you do not know the width of the sensor of your camera, you can check it on the manufacturer’s website.

It is usually expressed in the following way: 23.1×15.4mm. Being the first part (23.1mm) the width of the sensor.

How does the crop factor affect your photos?

This interests you more. Imagine that you buy a lens with a focal length of 16mm because you intend to go on a trip and shoot some amazing landscapes, but your camera has an APS-C sensor: the distance effective of it will not be 16mm but 24mm (being a multiplication factor of 1.5).

Result: The lens is as wide as you might have imagined and the angle of view of the lens (now I’ll explain what this means) doesn’t allow you to capture the same portion of the scene as you would on a full sensor camera.

Let’s see an example that we have extracted from the Nikon simulator (we will talk about it below): same scene, from the same place with the same 24mm focal length. The difference, one is taken with a Full Frame sensor and the other with a DX sensor (this is what Nikon calls the sensor smaller than the Full Frame sensor).

24mm with DX sensor 24mm with full format sensor (Full Frame)

You may see it more clearly this way, I have marked, approximately, the portion of the scene that captures the cropped format (DX) compared to the full format (Full Frame):

Scene comparison with full sensor and cropped sensor

The effect, apparently, is as if you were zooming in, but it is not really so. Not that the lens gets closer on a smaller sensor camera. But it is rather a cropping effect, as if you had the original photo taken with a full sensor and in the edition you cut it along the dotted line.

Objective Types

Let’s see what types of goals there are.

Mainly we can make two classifications, depending on the focal length and if this is fixed or variable.

First we will delve into this last point, below, we will talk about the classification based on its focal length.

Typically, most of the lenses that come in kit cameras are lenses with a variable range of focal length.

That is, they are lenses that normally have a focal length that goes from one value to another.

For example, it is very common to find objectives of varifocal length, a 18mm to 200mm.

This means that the objective has all that range and that we can regulate it as we want, using from a very high value, such as 200mm, if we want to get closer to a relatively distant object; up to a much smaller value of 18mm, if what we want to capture is a complete frame of a panoramic scene, landscape, etc.

There are also lenses on the market with fixed Focal Length, namely, that you can’t zoom with them, they always work at the same focal length.

Those who read this blog regularly know how IN LOVE Mario is with the caller King of Lenses, the 50mm f/1.4. If you want to know the supernatural, almost magical, properties of this king of lenses, click here.

Fixed or varifocal lenses?

But… which one should be used, fixed or variable focal length?

Each type of goal, whether fixed or not, has advantages and disadvantages. Therefore, choosing the ideal will depend on your tastes and needs as a photographer.

Here I leave you a small analysis of advantages and disadvantages so that you can evaluate what type of objective is the one you prefer.

Fixed focal lenses

Those that do not vary their focal length. They can be, for example, a 35mm or a 50mm, like the king that I mentioned before.

Advantages of prime lenses

  1. Higher optical qualitywhich translates into greater sharpness in your photographs.
  2. They are built with fewer moving parts, so they are usually more resistant and robust.
  3. Being optimized for their focal length, they produce fewer aberrations.
  4. Greater luminosity that will allow you to take better shots in low light conditions and shallower depth of field when working with more open diaphragms (f/1.4, f/1.8, f/2.8). They also usually offer some bokeh very nice.
  5. Many of them are more economical than variable versions.


  1. Are less versatile since, as we have said, its focal length is fixed. However, this can be seen as an advantage as you will be forced to move around the entire scene to get the perfect frame. Although for certain types of photography, the lack of versatility can really be a problem.

Zoom or varifocal lenses

Advantages of zoom lenses

  1. They are very versatile, comfortable and will allow you to adjust the frame without having to move.
  2. They are essential for certain types of photographs where you require a greater focal range, such as nature or sports photography.
  3. you gather a large number of focal ranges in a single lens so you will avoid constantly exchanging lenses on your camera, gaining speed and avoiding bumps and dust on the lenses and on the sensor of your camera.


  1. They are usually quite more expensive that the objectives primeespecially those that cover a long focal length such as an 18-200mm.
  2. Having much more moving parts, they are more fragile before any blow or accident.
  3. Their weight and size is considerably larger.
  4. Are less bright, especially in the longer focal lengths.
Canon prime lenses

Mario’s experience with fixed and variable focal lengths

«My first camera was a Nikon D60it came standard with two lenses: one 18-55mm and one 55-200mm.

At first I was happy, I thought «this way I have an additional objective, two objectives will be better than one I say…», I still had that mentality of the more things the better, the «anxiety alive» as Mota would say.

But after a short time I realized that going on a photographic excursion with two lenses was not the most practical, since I had to change lenses every so often. I began to notice that there were people who had a single lens that ranged from 18 to 200mm, just one, so I thought «How practical! you don’t have to go around taking one off and putting another on every so often.”

I made up my mind to sell my two lenses and add a little money to buy a single lens that had the widest focal length range (18mm-200mm).

After some digging, I changed my mind again. Since then I continue to maintain the two different objectives (plus others that I bought). You will wonder why?

An objective, greatly simplified, is nothing more than a tube with a series of glass lenses inside. Those lenses are made in the best possible way to get the best pictures for the focal length that lens was designed for. The wider the range of the focal length, the more difficult it will be to get a picture. perfect. It is as if it were a question of specialization: an 18mm-55mm lens is specialized in that range and therefore will take better photos than an 18mm-200mm lens.«

Mario’s Golden Tip

Buying a single lens with a wide focal length range isn’t the end of the world either, you can still take decent photos with it (remember who takes the photos?).

In addition, it has the advantage of facilitating the task of having everything…