The camera is an optical lens system and one of its main characteristics is focal length. Actually, the focal length determines the scale of the image that you will see in the pictures - the larger the focal length of the lens, the visually closer the object being photographed will be in the photo.

The focal length of a lens is the distance from its optical center (the correct name is convergence point) to the camera matrix, that is, to the plane onto which the image is projected.

Rays of light are reflected from the object being photographed, pass through the lens (lenses), are refracted there and are reduced to optical center, after which they fall on the camera sensor. The plane passing through the optical center and perpendicular to the main optical axis of the lens is called focal plane. An image is formed on it, which is transferred to the sensor in an “inverted” form.

In fact, the whole principle of “transferring” a real image to a camera sensor can be represented like this:

At the same time, as the focal length increases, as the image is scaled and zoomed in, the angle of coverage seen by the lens will narrow. The figure shows why this happens.

Focal length is measured in millimeters and is usually marked on the lens barrel. There are lenses fixed focal length. They indicate one value in millimeters - for example 100mm.

If two values ​​are specified, for example 18 and 55 mm, then this is the minimum and maximum focal lengths that are available in this lens with variable focal length. Such lenses can change the focal length within these limits.

Generally, the longer the focal length of a lens, the longer it is...although there are exceptions.

Let's, purely practically, see how the frames made with one or another focal length differ. We shoot from one point and change focal lengths from 14 to 300 mm:

In addition, the focal length affects the perspective of the image. Large focal length makes the image flatter. Below is an example of what an image looks like taken with lenses with different focal lengths (for this example, the ruler was located at an angle of 45 degrees to the optical axis of the lens and the distance between the vases was 8 cm):

Have you ever wondered how the focal length of a lens affects the aesthetics of a photograph? Even when shooting the same scene, choosing a different lens can make a big difference in how the photo looks. The fact is that different focal lengths of the lens when shooting the same subject change the nature of the relationship between the subject and its background, and also affect the perception of the distance between them.

The illusion of reduced distance between the subject and the background is a property of telephoto lenses. They tend to flatten the image, while wide-angle lenses increase the effect of perspective. Do you know why 85mm lenses are so popular in portrait photography? These lenses have the effect of "flattening" the image plane, so that the nose and facial features do not appear larger in the picture than they really are.

While many people don't like using discrete lenses (fixed focal lengths), using this technique makes for great portraits. Personally, I shoot most portraits with 50mm or 85mm lenses. There are a number of reasons for this. First, such lenses flatten the image plane. With a telephoto lens, we can reduce or completely eliminate the geometric disproportions of facial features caused by the effect of perspective. The same applies to body parts of the subject.

The use of telephoto lenses also affects the depth of field. You may already know that depth of field is the range of distances from the camera where objects will be in sharp focus. Some people think that depth of field depends only on the aperture value of the lens, but the focal length of the lens has no less effect on it. A telephoto lens reduces the depth of field, helping to separate the subject from the background.

Often this is exactly what is required for portrait photography. By choosing a long lens, you take the background out of focus and draw the viewer's attention to the subject. And vice versa - the use of wide-angle lenses allows you to sharply convey not only the subject, but also its surroundings.

However, there is no perfect lens or focal length that fits all occasions. If you want to show how your subject relates to your surroundings, experiment with different focal lengths to see how they affect the relationship between subject and background.

To illustrate, I took a series of shots on the bridge near my house. Notice how the relationship between the bridge and the model changes in these shots.

I have used different focal lengths. The first lens was the Tokina 12-24mm f/4. The second is Nikon 35mm f/1.8. The last one is Nikon 80-200mm f/2.8 set to 100mm and 200mm. All shots were taken at f/2.8 to equalize the effect of depth of field (with the exception of Tokina set to f/4).

(Keep in mind that the pictures were taken on a Nikon D300 camera, so the focal length modifier must be taken into account since this is a DX format camera)

So let's take a look at the pictures. In each of them, I tried to keep the same composition, and the model occupied almost a full frame in height. Please note that the model takes up approximately the same space in the pictures, but the background is significantly different. Most striking is the difference in the size of the bridge in the background.

The first shot was taken at the widest angle (12mm focal length) with a Tokina 12-24mm lens. You can notice a strong perspective effect. The lines of the road lead the eye towards the bridge, which is barely visible in this picture. A wide angle also leads to a large depth of field - almost everything in the picture is in focus. As a result, everything is included in a single scene.

This shot was taken with a Nikon 35mm f/1.8 lens. 35mm is in the middle of the usual focal length range. The bridge now appears closer to us, and the depth of field is shallower compared to shots taken at 12mm. Although the angle is still quite wide, and we have just begun to separate the object from the background.

Here we are in the region of ideal focal lengths for portraiture. The picture was taken with a Nikon 80-200mm f/2.8 lens set to 100mm. Note that the model image has become more "flat". The bridge now appears much closer to the model, and we've removed the effect of the road lines leading the viewer's eye away. In addition, we began to get rid of the large depth of field by isolating the subject from the background. This focal length is well suited for shooting faces and waist-length portraits.

In the last shot, the lens was set to 200mm. The distance compression effect has reached its maximum, and the model appears to be standing very close to the bridge. We also got a very shallow depth of field, almost completely separating the model from the background. Even though we were shooting the same person standing in the same place, the different focal lengths resulted in completely different shots.

Conclusion

In this tutorial, I tried to show you the benefits of using different focal lengths. Test shots show that changing the focal length changes the scene.

Experimenting with focal length is a powerful creative tool. Choosing the right focal length is very important for getting the right composition for your shot. Wide-angle lenses allow you to include the background or create depth in your shot. Long lenses compress the distance between the subject and the background. In general, for each scene, you need to select the appropriate focal length of the lens.

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Translated from the site photo.tutsplus.com, the author of the translation is indicated at the beginning of the lesson.

The term focal length of a lens is familiar to many from physics lessons at school. The focal length of a lens is the distance from the lens itself to its focal plane, measured in millimeters. The focal plane and the plane of the lens are mutually parallel and the focal plane passes through the focus of the lens.

The focus is the point where all the rays that have passed through the lens converge. In a digital camera, a CCD is located in the focal plane. Thus, the camera lens collects the light flux and ensures its focusing on the photosensitive matrix. The degree of magnification of the lens directly depends on the focal length. As the focal length increases, the degree of magnification of the lens increases, but its angle of view narrows.

Figure 1. Focus and focal plane for a biconvex converging lens.

Depending on the focal length of the lens, lenses are divided into wide-angle and telephoto. Wide-angle lenses, they are often called simply "wide-angle", as if they move the object being shot away from the viewer, reducing it. The name just came from the fact that they have a very large (wide) viewing angle. Long-focus lenses allow you to enlarge (bring closer) the object being shot to the viewer, but they have a much smaller angle of coverage.

Figure 2. Types of lenses by focal length and angle of coverage.

What determines the focal length of an objective lens

Focusing on the subject depends on the size of the CCD - matrix. For film cameras, this size is the same as a 35mm frame width. films. However, in digital cameras, the dimensions of the matrices are much smaller and, moreover, differ significantly depending on the model of the camera and its manufacturer.

Therefore, it was decided to give the parameters of the focal length of the lens of the digital camera lens relative to the standard 35 mm. This made it possible to compare different types of lenses by the focal length of the lens, without taking into account the parameters of the matrices, and also to determine the following:

1. A lens with a focal length of 50 mm lens has a field of view corresponding to the field of view of the human eye and is mainly used for medium shots.
2. The lens focal length of 90 - 130 mm is ideal for portrait shooting. Such lenses have a shallow depth of field, which allows you to create beautiful bokeh.
3. Starting from 200 mm are telephoto lenses. They are ideal for shooting animals, birds or sports from long distances.
4. Lenses with a lens focal length of 28 - 35 mm are suitable for shooting indoors where there is not enough freedom of movement. Most often installed in low-cost entry-level cameras.
5. Lenses with a lens focal length of less than 20mm are called fisheye. The main application is the creation of artistic photographs.

Zoom lenses and digital zoom

In digital cameras, as a rule, lenses are installed that have a variable focal length of the lens. From what focal length is set, they can be both wide-angle and telephoto. The increase in focal length can be implemented through optics or software (digital).

The optical increase in the focal length of the lens is achieved by the optics of the lens, i.e., by changing the focal length. This technique is not image quality. Modern lenses allow you to get an image magnification of 12 times. The maximum magnification can be easily determined by the markings on the lens. Let's say the range is 5.4 - 16.2 mm. Then the maximum increase will be 16.2 / 5.4 = 3, i.e. a threefold increase.

Figure 3. Nikkor telephoto lens with a focal length of 80-400mm.

Digital zoom increases the magnification factor, but greatly degrades the image, so it should be used only in extreme cases, when the image quality is not so critical. A similar increase can be made on a computer during post-processing of the image.

The essence of digital zoom is quite simple. The processor of the camera or computer calculates what color pixels to add to the image and in what places when enlarged. The problem with image quality loss is that these new pixels were not accepted by the sensor because they were not present in the original image.

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Knowing what focal length is and what the features are is especially important when buying lenses. This lesson will give you information on how lenses with different focal lengths work, how to use them creatively and choose the ones that are right for you.

Step 1 - What does it really mean?

The focal length of your lens basically determines what zoom will be in your photos: the larger the number, the greater the effect of zooming in and out.

Focal length is often misunderstood, saying that it is measured from the front or back of the lens. It is really the distance from the convergence point to the sensor or film in the camera. Look at the diagram below where this is explained

Step 2 - Different focal lengths and how they are used

Ultra wide angle 12-24mm

These lenses are considered highly specialized and are not often included in the general photographer's lens kit. They create such a wide viewing angle that the image may look distorted because our eyes are not used to this kind of range. They are often used in event and architectural photography, for shooting in tight spaces. Wide-angle lenses, as it were, place the photographer in the center of events, making him no longer an observer, but a participant, creating the effect of presence. They are not very suitable for portrait photography, as they increase the perspective so much that facial features can be distorted and look unnatural.

Wide angle 24-35mm

Here you will find many kit lenses for full frame cameras, they start at 24mm, when the angle is wide, but the distortion is not yet so pronounced. These lenses are widely used for reportage photography, documentary photojournalists, because they have a wide enough angle to include a large number of objects, and at the same time, distortion is not so significant.

Standard 35-70mm

It is in this range of focal lengths of 45-50 mm that the angle of view of the lens will roughly correspond to how our eyes see (excluding peripheral vision). I personally would like to use this range when shooting outdoors or when meeting friends in a pub or at a dinner table. A standard lens such as the 50mm f/1.8 is a great value lens and gives excellent results. A fixed focal length lens will always give better image quality than a zoom lens. This is because it is built with a single purpose in mind. He does one job well and several jobs poorly.

Initial telephoto 70-105mm

This range is usually the extreme range for kit lenses. It starts with telephoto and prime lenses for portraiture (about 85 mm). This is a good choice for portraiture as it can capture close-up portraits without distortion, as well as getting subject-to-background separation.

Tele 105-300 mm

Lenses in this range are often used for distant scenes such as buildings and mountains. They are not suitable for landscapes, as they compress the perspective. Longer lenses are mainly used for sports or wildlife photography.

Step 3 - How does focal length affect perspective?

I already talked about this in the previous section, but to give you a better idea of ​​the effect of focal length on perspective, I took 4 photos of the same objects at different focal lengths and compared them. Three objects (soup cans) were in the same position 10 cm apart in each photo. It is worth noting that the pictures were taken on a crop camera, so the focal length will be slightly larger.

Now let's talk about what the crop factor is. In essence, this means that if any lens for a full frame (EF, FX, etc.) is put on a carcass with a crop factor, then part of the image will be cut off. The crop factor will be approximately 1.6. In real terms, this means that if you shoot with a 35mm lens, you will get the same result as if you were shooting with a 50mm lens.

How it works is shown in the pictures below. This is actually a zoomed image, narrowing the angle of view of the lens.

Even lenses that are designed for crop cameras (EF-S, DX) will experience a similar effect, as focal lengths are always specified for full frame. It's just that these lenses at full frame will give a strong vignetting effect, since the image is not projected over the entire frame area.

That's all! And two completely different shots taken at different focal lengths. The first is at 24mm, the second is at 300mm (both on a camera with a crop sensor).

When working with a camera, it is very important to understand what the focal length of a lens is and how to work with it. Every photographer should learn how to realize their creativity, using focal length as one of the tools in creating a picture. In addition, it is important for every person involved in photography to feel comfortable working with the camera, so you should decide which lenses are best for you.

1. What is focal length

Focal length (FR) is a complex mechanism that affects various characteristics of the future frame. Without going into details, the FR affects the scale of the image. The larger the focal length number, the larger the objects in the picture will be, and they will be located closer. Focal length is a numerical representation in millimeters of the distance between the camera's sensor and the optical center of the lens (convergence point). You can clearly see the principle of measuring the RF in the diagram:

2. Ranges of focal lengths. Their application

The focal length is divided by:

Ultra wide angle 12-24mm

These lenses are not used often. They capture a very large space and at the same time distort it greatly. Such a large girth is unusual for the human eye, so the pictures may seem strange to many. Such lenses are used when shooting large objects from a short distance or in architectural and interior photography in confined spaces. Such lenses are not suitable for shooting people, as they greatly distort the perspective and, as a result, the structure of the body and facial features.

Wide angle 24-35mm

These devices are called whale lenses. The 24mm FR produces virtually no distortion, although it is still visible to the naked eye. Such lenses are most often used by journalists in reportage and documentary shooting. Such a lens has a large frame capture angle, so it can accommodate a lot of objects in the scene. In this case, distortions practically do not appear.

Standard 35-70mm

These types of lenses at a FR of 45-50 mm have approximately the same coverage as the human eye. Peripheral vision is not taken into account. Standard lenses are the most common and are used for a wide variety of purposes.

“Fifty dollars” is the name of a lens with a fixed focal length of 50 mm. Its aperture is most often f1.8. Lenses with a fixed focal length give a better picture than zoom lenses due to the fact that they have a large aperture and are not sprayed over several focal lengths.

Telephoto lenses 70-105mm

After the value of 105 mm, long telephoto lenses begin, as well as fixes for portrait photography (approximately 85 mm). Telephoto lenses are great for portraiture. They do an excellent job of separating the foreground from the background, while not flattening or distorting the picture.

Super telephoto lenses 105-300+mm

Such lenses are suitable for photographing distant objects. It can be mountains, buildings, people in the distance and wild animals. For landscape photography, such lenses are not suitable, since at FRs over 300mm they flatten the perspective very much.

3. The perspective of the image and its FR

This section will describe the effect of focal length on perspective. In the picture below, three objects are photographed, which are located at a distance of 10 cm from each other.

4. Crop factor

If you have a camera with a crop sensor, then you should know what crop factor is. For example, if you take a full-frame lens and install it on a camera with a crop sensor, then the picture will be cut off at the edges. The crop factor is approximately 1.6. For a specific example, let's take a lens with a focal length of 35 mm. His shots on a crop sensor camera will look like photos taken on a full frame camera with a 50mm lens.

More details on how it works is shown in the diagram:

Buying a lens made specifically for crop cameras won't solve this problem, as it's common to list the focal length that would be relevant for a full frame camera.

For example, two more shots taken on a camera with a crop sensor. One shot was taken at 24mm and the other at 300mm.

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