Focal length of the lens

Focal length of the lens Is one of the most important lens parameters. The focal length of a lens indicates how far or close (wide) the lens can 'see'.

Focal length of the lens

Focal length of lenses - article from Radozhiva

Focal length is measured in millimeters, centimeters and meters. For example, the designation of the fixed lens Nikon 85mm F / 1.8D AF Nikkor says that his focal length is fixed and is 85 millimeters. A designation Tamron 28-200mm 1: 3.8-5.6 Macro AF Aspherical XR (IF) A03 suggests that the focal length of the lens can vary from 28 millimeters to 200 millimeters. Lenses in which the focal length can vary are called zoom lens (zoom lens, zoom lens). Zoom ratio is calculatedI am dividing a larger number by a smaller one, in this example 200mm \ 28mm = 7 times.

Usually, the larger the focal length of the lens, the larger the size of the lens itself, especially its length.

Focal length Is the first thing to look for when choosing a lens. It shows what angle of view the camera will capture when working with one or another lens.

Attention: the focal length of the lens is the physical size of the lens itself, it does not change and does not depend on the type of cameraon which the lens is used. But for cropped cameras and for cameras with different physical matrix sizes, the parameter EGF (Equivalent Focal Length), it shows the real viewing angle for a 35mm film, which is obtained when using one lens or another on cameras with different matrix sizes. More details in the section crop factor.

Here is an example of how the space that the camera can cover when using lenses with different focal lengths changes.

17mm

Focal Length 17mm

Focal Length 17mm

24mm

Focal Length 24mm

Focal Length 24mm

35mm

Focal Length 35mm

Focal Length 35mm

50mm

Focal Length 50mm

Focal Length 50mm

70mm

Focal Length 70mm

Focal Length 70mm

85mm

Focal Length 85mm

Focal Length 85mm

100mm

Focal Length 100mm

Focal Length 100mm

135mm

Focal Length 135mm

Focal Length 135mm

200mm

Focal Length 200mm

Focal Length 200mm

300mm

Focal Length 300mm

Focal Length 300mm

For examples, I used a camera Nikon D200mounted on a tripod. All photos were taken at F / 5.6, the following lenses were used:

It is often said that the photographer should have a set of lenses that covers the desired range of focal lengths and thus covers all possible situations in the work of the photographer. One of the most classic sets for full-frame cameras can be considered: 14-24mm, 24-70mm, 70-200mm, 200-400mm. For cropped cameras, usually a good set consists of 11-16mm, 16-50mm, 50-135mm lenses. Covering the entire range of focal lengths is not worth chasing; you can safely do without a single fixed lens. The division into different types of lenses can be found here.

In addition to the angle (field) of view, the focal length affects the perspective of the image (due to the difference in distance to the subject with equal proportions of the subject). At different focal lengths, the sizes of objects are correlated differently in photographs. Long-focus lenses bring closer not only the subject itself, but also the entire space behind the subject and in front of the focus subject. And wide-angle lenses average the sizes of all elements in photographs. Also, different focal lengths affect to blur the background.

17 mm

Aspect ratio at 17mm

Aspect ratio at 17mm

24 mm

Aspect ratio at 24mm

Aspect ratio at 24mm

35 mm

Aspect ratio at 35mm

Aspect ratio at 35mm

50 mm

Aspect ratio at 50mm

Aspect ratio at 50mm

70 mm

Aspect ratio at 70 mm

Aspect ratio at 70 mm

85 mm

Aspect ratio at 85 mm

Aspect ratio at 85 mm

100 mm

Aspect ratio at 100 mm

Aspect ratio at 100 mm

135 mm

Aspect ratio at 135 mm

Aspect ratio at 135 mm

200mm (195)

Aspect ratio at 195 mm

Aspect ratio at 195 mm

300 mm

Aspect ratio at 300 mm

Aspect ratio at 300 mm

Personal experience:

I do not like to convert the focal length from millimeters to degrees of field of view, who is interested, then there is an article on Radozhiv 'Relationship between focal length, viewing angle, and focus distance'. In practice, it is enough to understand what the approximate angles are given by a lens with a certain focal length in order to intuitively know how another longer or wider lens will behave on the camera. Advise experiment with the lens simulator. There is still an interesting article about dependence of the focal length of the lens on the focusing distance.

Conclusions:

The choice of a lens, first of all, entails determining the desired focal length range. Focal length indicates how wide or narrow the lens 'sees'. Focal length is very important to the blur zone of the blur zone and perspective images.

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The material was prepared by Arkady Shapoval. My Youtube channeland Radozhiva's group on Facebook и VK.

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Comments: 120, on the topic: Focal length of the lens

  • Kate

    Good afternoon, question d5100 with a dx af-s 18-55 lens, the focal length is not set to more than 24 (the ring does not spin), what can I do?

    • Igor

      Good day. Perhaps something fell into the zoom mechanism. It can be like a small fragment of some internal part of the lens, for example, a plastic sleeve, or a foreign object. Take it to repair, there they will solve a similar problem quickly enough.

  • Dmitriy

    so and I did not find the distance between what and what.

    • anonym

      Between a rabbit and a magician, hence the name "focal"

  • Vitaly N

    Arkady, I suggest the text "Attention: the focal length of the lens is the physical value of the lens itself, it does not change and does not depend on the type of camera on which the lens is used." make 3, no 5 times more and allocate fatter. Maybe they will see after that. Although this will not help some.

    • Vitaly N

      Arkady, onanimy do not go into this topic and do not read it. What to discuss then?

      • Alexander

        Arkady, can I somehow ban the trolls and remove their obscene language?
        Well, it is impossible to read your forum without stumbling upon rudeness like the next Vitaly N with his "onanim".

        • Arkady Shapoval

          slowly clean

  • Ira

    Very interesting. It explains well what the focal length is and show how it affects distortion when shooting portraits: http://linkphoto.ru/fokusnoe-rasstoyanie-obektiva/

    • Vitaly N

      How much did you pay for advertising? Here is an order of magnitude more information about the RF and EGF.

    • Sergei

      Irina, thank you. And then the article does not answer this question, although it is called “12. What is the focal length of lenses ” https://radojuva.com/o-fotografii/
      There is only: "The focal length of the lens is one of the most important parameters of the lens." And then it went on what it affects, etc. etc. And the concept itself is not disclosed. By the way, in the Internet there are very few sites that really explain what a focal length is.

      • Vitaly N

        It would be better if they did not write there ... And you did not read ... Wikipedia in this case is more useful if physics was not studied at school.

        • Sergei

          This article discusses what affects the focal length, but there is no concept of focal length.
          Here you can say what is the focal length of the lens?

          • Novel

            Sergey, Ira recommended a good article, everything is well-written there.

          • Vitaly N

            Distance from the optical center of a lens or lens group to the focal point. But not to the matrix or film. These distances will only match when focusing at infinity. Otherwise, it turns out that by refocusing the lens, attaching it to another camera or placing spacers, we change the focal length. This is from the same opera that the focal length depends on the size of the matrix. This is how delusions are born. About EGF there is generally crumpled and smeared, which is also very bad. The article is purely advertising with information ripped from different places. Plus - the influence of the viewing angle on the portrait, but this is also rip-off from another site. It also shows the effect of the viewing angle on perspective, which is much more important. And without copy-paste.

            • Alexander

              Sergey, we read the primary source - Nikon's opinion (manufacturer!):
              The focal length of the lens is the distance between the lens and the image sensor when the subject is in focus, usually stated in millimeters (eg, 28 mm, 50 mm, or 100 mm). In the case of zoom lenses, both the minimum and maximum focal lengths are stated, for example 18–55 mm.
              https://imaging.nikon.com/lineup/dslr/basics/19/01.htm

              The focal length of the lens is the distance between the lens and the frame sensor (!!!) when the subject is in focus, usually indicated in mm (28mm, 50mm, 100mm). In the case of zoom, the minimum and maximum distances are indicated, for example 18-55mm.
              And then the direct relationship between the focal length, frame format (sensor size) and the angle of view is indicated.

              • Vitaly N

                English is very sparse and the meaning of the words differs from the sentence. They call a lens a lens. Now turn on the logic, remember the flange of the SLR and how the lens should be located so that it is 18 mm or less from it to the matrix. Where did the mirror go? Or 300 mm - I have a telephoto lens with this focal length of 170 mm in total. Nikon's website is also written for those who did not study physics, if only there was something. Whoever needs it knows himself.

              • Vitaly N

                And by the way, since when did Nikon become the primary source? In the Middle Ages did he do optics and do physics? Or all the same, only uses the knowledge of those years?

              • Arkady Shapoval

                In this case, Nikon's site is not the primary source and, purely physically, the definition of the focal point for Nikon is incorrect. The focal length is not the distance between the lens and the sensor. Depending on the type of lens (retrofocus, telephoto or shirik), these are physical quantities between certain points in space, while these points can be both along the length of the lens and beyond. The lens is not just one converging lens, but a complex.

              • Michael

                Well it started. FR is the distance from the optical center of the lens to the focal point of the beam of parallel rays. Vitaly said correctly. FR is a property of the lens, whether there is a film matrix behind it or not. It is better to teach physics from a physics textbook, and not Nikon's website

            • Alexander

              The English term FOCAL LENGTH is translated into Russian as FOCUS DISTANCE, like it or not. In Arkady's article, it is specifically about the focal length of the lens, it is this value in mm that is indicated on the lenses.
              clearly from the Nikon website
              https://imaging.nikon.com/lineup/dslr/basics/19/img/1902_07.jpg
              Example description from Kenon to lens 16-35 / 4
              Covering a useful wide-angle focal length range, the EF 16-35mm f / 4L IS USM is a Canon L-series lens
              The format of the frame is indicated - Format Compatibility Full-Frame and the angle of view at a given focal point on a full frame Angle of View 108 ° 10 ′ to 63 °

              what else to argue about?
              Arkady, can I somehow post information from Nikon’s website to an article to stop the endless flood about anything?

              • Vitaly N

                That's where the focal length is shown correctly. https://www.exler.ru/likbez/fotografiya-dlya-chajnikov-chto-takoe-fokusnoe-rasstoyanie-obektiva.htm

              • Arkady Shapoval

                Yes, I’ll ban you right now for the flood. Because you yourself do not understand and have set a flame here.
                As a counterexample: the Nikon’s 45mm working distance is precisely the distance from the sensor to the lens (to its body or back lily). At the same time, with the 18-55 lens that you indicated, 18 mm of distance from the sensor to the lens does not work, since there is another 45 mm of working distance between them.

              • Alexander

                So, dealt with the service of a famous brand.
                I was wrong, I believed Nikon - it is wrong there, and very wrong.
                the canon indicated more accurately, not without details. Vitaly N also has an incorrect definition.
                of the most accurate definitions:
                This is the distance between the secondary main point to the back focal point when the focus is set to infinity. The secondary main point is one of 6 main points that are used as reference points in optical lenses: front and back points, secondary (2) and primary points (2). Since there is no predefined position for the secondary principal points in a complex lens, they can be anywhere, depending on the design. And from here a simple conclusion - there is no way to independently measure the focal length of the lens.

            • Roman

              Well, in fairness, most lenses change the focal length when refocusing (the notorious focus-breathing). In film optics, the construct is usually more complicated and there this effect is eliminated.

              • Vitaly N

                In reality, not really. The viewing angle changes, while it seems that the focal length is changing.

              • Alexander

                focus-breathing - changes in the distance between the secondary main point to the back focal point.

              • Alexander

                Vitaly N, in this case Roman is absolutely right. It is the focal length that changes slightly - between the secondary main point to the back focal point. The angle of view changes as a CONSEQUENCE of changing the focal length.

              • Vitaly N

                Alexander, take a simple system with focusing by moving the entire lens block. Well, focal cannot change by definition, but focus-breathing is very strong. The difference is that the focal point itself determines the focus point of parallel rays, which is not the case when focusing at a distance less than infinity. Therefore, when focusing at shorter distances, the viewing angle decreases. The focal change in this case is imaginary, but it really is. Especially on simple telezoom.

              • Vitaly N

                Oops, the angle should increase. Foam ...

              • Alexander

                Vitaly N,
                once again see the exact definition of the focal above.
                focus-breathing - changes in the distance between the secondary primary point and the back focal point.
                and this distance MAY change in a complex lens, thereby changing the viewing angle.
                and in film optics it is precisely this problem that has been eliminated, where the focal point is rigidly fixed.

              • Vitaly N

                And, of course, there are lenses that also change the real FR when focusing - the optical points in modern lenses are not static.

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