Focal length and perspective

This is a repost from my old blog. It was one of the most visited page and since I keep seeing people asking the same question over and over again, I think it would be a good idea to forward them here. I have a few more of these posts that I will file under ‘learning the basics’.

Using tools in new ways is an essential skill for the photographer who wants to develop his creativity. In this post, I am going to do a quick exploration of the link between focal length and perspective.

To better appreciate the differences between wide and long lenses, it is important to understand two key principles:

  • The focal length of a lens determines the field of view. This number, expressed as an angle, determines the amount of information that is available in the viewfinder / sensor.
  • Sensor/negative size is constant. This means that if you get more information in the frame (using a wider lens), all of these elements will take less space on the sensor to make them fit.

Knowing that, if you take two shots of a subject at different lens range (let say 24mm and 70mm) and you want the subject to be the same size in both picture, you will have to be closer to the subject when using the short lens.

Now lets put this into practice! The shot bellow is done using a 24-70f2.8L at its shortest focal length (efl 24mm). Notice how much of the background is visible and the distance between the two pots.

24mm

Now lets zoom in (efl 70mm) and move away from the subject so the orange pot stays the same size in the frame. We quickly notice a few things have changed. The subject has a more regular shape, we see a smaller portion of the background and the green pot is now much bigger and closer to the subject.

70mm

This effect is called perspective compression. It is why it is better to use a long lens to do a portrait than a wide angle: the features of the model seem flatter (very important for the nose!).

If you can’t change significantly the subject to camera distance (as in landscape photography) using a long lens will create a similar effect. Since a small part of the subject will be in the frame, and an even smaller part of the background, it will give the impression that the two elements are closer than they really are. But this is only going to be an impression since they will have both been enlarged by the same proportion.  (the above paragraph was edited post publishing for clarity)

That being said, it can be interesting to use a wide angle to do a portrait when you have an interesting background and you want to do a trompe l’oeuil, as in this picture (of me).

portrait

You have to look at the size of the door in the background to appreciate the effect a wide angle can produce!

So next time you go shooting, bring both a super short and long lens (or a zoom) and experiment by taking the same shot but at opposite focal length!

About Tommy

Photography allows me to be what I want to be, to be where I want to be, and to do what I want to do ... I'm not professional photographer and I don't need a title, I love to take photographs and that is what I do, I love to learn and I always try to do it better ...
  • http://www.mattneedham.com Matt Needham

    “It is the same principle that is used in landscape photography: instead of shooting with a wide angle, using a telephoto compress the horizon and makes all the mountains seem closer to each others…”

    Not unless distance to subject also changes. Perspective is controlled by where the camera is in relation to the subject/scene; focal length controls magnification. If shooting the same scene from the same place it doesn’t matter if using wide angle or telephoto perspective remains the same.

    • admin

      Hi Matt, while the situation you describe is not what I tried to illustrate in my example (because I change the distance to subject between shots to make the subject size constant), you are correct. I will update the article to make this clearer, especially in the section you outlined. Thanks.

      While mathematically it is true that perspective does not change at constant subject to camera distance, the fact that only a portion of the subject can be framed (and an even smaller portion of the background) gives the effect of ‘perspective compression’. And in the end, it is the result that counts.

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