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Tuesday, 19 April 2011

The Focal Length and Perspective Myth

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It's a common myth that changing the camera lens focal length, changes the perspective within an image. This is indeed a myth, as true perspective results from viewer's location and nothing else.

The location of the viewer dictates the spatial relationship between the viewer and their surrounding objects. This location also dictates the spatial relationship between those surrounding objects themselves "as viewed from the viewer's location". It is the combination of these spatial relationships that creates what we perceive as perspective.

This may sound a little confusing, so lets look at this diagram…

In this diagram, the camera ( A ) is focused on a single person ( B ), while a group of people ( C ) are in the background. The red lines indicate the Field Of View (FoV) of a wide angle lens, while the green lines indicate the FoV of a longer telephoto lens.

Accepting the principle that 'un-interfered with' light travels in straight lines, we can see that 'spatially' nothing has changed between the two lens focal lengths. The long lens with it's narrower FoV, simply sees a smaller section of the scene…. Person B is still the same distance from the camera, as are persons C. Plus, regardless of focal length used, the camera still sees the same relationship between person B and the partially covered person in C (as represented by the grey & magenta areas).

So there has been no "compression" of the image with the longer lens as often claimed. We are simply looking at a smaller section of the very same scene.

To demonstrate this, lets look at the following images that were taken from the same location, using different focal lengths.

Wide Angle Lens:

Telephoto Lens:

Now many people will claim that the wide angle image above demonstrates a different and much stronger perspective. However this is not the case as perspective in both images is identical.

In the following image, I have overlaid the telephoto view (lightened section) over the wide angle view and you can see that the images match perfectly.

To help see how well the images match, this is the very same image enlarged and cropped:

It must be noted that Depth Of Field (DOF) varies between the two images, but DOF is not what is in question here.

To truly change perspective and therefore the spatial relationship between ourselves and the objects around us, we must change our position. In this diagram the camera ( A ) has been moved closer to the single subject ( B).

Here you can see that the FoV for each focal length has not changed, but the viewer's relationship between ( B ) and ( C ) has changed (as represented by the grey & magenta areas) when compared to the original graphic. So by changing position, we have changed the spatial relationship or "perspective".

OK, so if the perspective did not change in the above images, why do I still think that the wide angle view shows more perspective?

Well we must remember that perspective is an illusion. It is created by the "seemingly" diminishing size of objects as they get further away and our brain interprets this diminishing size as distance and depth.

So by seeing more of a scene, we are therefore seeing more of the diminishing size relationships between the objects within that scene. This increases our spatial awareness and creates a much stronger "feeling" of perspective.

Conversely, by narrowing our FoV with a telephoto lens and seeing less of a scene, we reduce the amount of diminishing size relationships and thereby reduce our spatial awareness and our "feeling" of perspective.



Tags: perspective focal length diminishing size spatial awareness


  1. It's kind of a bit to do with semantics though, isn't it Russell?? What people mostly MEAN when they say "great perspective!" is that they like the way the camera _shows the relationship and distance between objects in the FOV_ and whilst the correct term for it might not be 'perspective', I'm not sure whether "great way of showing the relationship and distance between the objects in your field of view!" quite has the same ring to it... ;)

  2. I understand that it can be seen as a bit "You say tomAto, I say tomARto"... but it's important to understand the inter-relationship between field of view, focal length, sensor size, perspective, vanishing points etc, in order for us create the image that we aim to achieve.

    Probably not so much when just capturing an image on the fly, but very much so when tasked with bringing a designer's concept, or your own pre-conceived image idea to conclusion.

  3. It's also important to know that the wide angle or normal also has the compressed perspective from the same spot. You might not have your telephoto with you. With the huge resolution in today's cameras you can still get the shot with a wide/normal and crop.

    Great examples and writing. This is one of the biggest myths and even some teachers teach this wrong.