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Perspective = Object Distance: The Internet Myth

Discussion in 'Open Discussion' started by Hikari, Apr 26, 2011.

  1. Hikari

    Hikari Mu-43 All-Pro

    Nov 26, 2010
    There is an internet myth circulating that the only factor in perspective is object distance. Pictures taken at the same object distance display objects in them at the same relative size. So, the hypothesis goes, these picture have the same perspective. Perspective is simply relative object size under this hypothesis.

    So, you get your identical twin friends and you go to the gym to take some pictures. One twin stands 10ft from the camera and the other 20ft from the camera. You take the picture and lo and behold, the relative size of those two people are 2:1. You change lenses, and the same difference in relative image size. It must be true, except...

    Now I ask one twin to stand 20ft from the camera and the other twin 40ft from the camera. The difference in image size? The exact same, 2:1. Object distance has changed and we still have the same perspective because the object distance is not important, but the ratio between the different object distances. So if it is a problem of ratio of distance to the camera, then any two objects can be placed in the frame at any distance as long as the ratio between remains constant. Not quite the hard and fast rule that is claimed.

    And what does this distance hypothesis mean? Nothing. For example, would a landscape taken with a wide and telephoto lens have the same perspective? Since you cannot get closer to the horizon, I guess it would. But which two objects in the image do we use to prove the same perspective? This hypothesis requires an absolute frame, but none exist. How do we compare different photographs of different subject matter? How do we chose reference points?

    It is true the ratio of the camera-to-object distance is directly related to the ratio of the image size of the two objects, but as a description of perspective alone, it is wanting.

    For centuries, the study of perspective, as we use it in imaging, has been how the appearance of depth or three dimensions is rendered in a two-dimentional plane--it is our perception of space, rather than an absolute description of it. These factors help artist understand the perception of depth as well as how to accentuate or eliminate that quality. Here are two images that show the problem. If only object distance were the only factor describing perspective, then these two images would have the same perspective. But they certainly don't look the same. Why? The only change is focal length.

    First, when we look at an image, we chose relative features to give depth clues. The most common is foreground and background. The steam winding through the forest floor starts close to the bottom of the image, leading up through the forest to a far point where the water enters. The small waterfall does not have a great deal of depth. Water seems to flow over it from the background almost immediately to the foreground.

    The second point has to do with two-point perspective, a very old concept. With a change in focal length or cropping, we are not just cutting into an image, but we are also magnifying it. This magnification is the key factor. In two-point perspective, two vanishing points are placed the horizon and are used as linear perspective anchors. How far apart these points are determines the perspective; closer together, the stronger perspective, the further apart, the weaker. When you magnify an image, you are displacing those vanishing points further away weakening the sense of depth or perspective. So even while the relative image size of objects do not change when changing focal length or cropping, the vanishing point which are used for spacial information are being displaced.

    There are many image qualities that relate to simply how we perceive them, perspective is one, sharpness and depth of field are others. Perspective has many factors influencing it--object distance is only one and not that significant.

    BTW, the waterfall was cropped from the larger image and flipped.

    Attached Files:

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  2. sprinke

    sprinke Mu-43 All-Pro

    Apr 5, 2011
    Pasadena, CA
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  3. Streetshooter

    Streetshooter Administrator Emeritus

    Dec 15, 2009
    Phila, Pa USA
    Very interesting indeed. You are looking at perspective as a photographer.

    True perspective is not controlled by lens Focal Length. It is a constant that changes as YOU move. Light travels in a straight line. Take those lines to a point at infinity and no matter what lens you use, perspective remains the same. Painters have fought this for many many years. Fight all you want, it's a fact of nature and can't be changed....Look at the cubist. Picasso in particular, played with perspective. DuChamp was another trickster with perspective. Did you know that he even played with film. He took sheet film and made it concave, then exposed it to change perspective. Of course it didn't work but he had a good time trying.
    ....along comes the photographer and figures, I don't want to be stuck in the box that a painter is. I will change image size and perspective by changing lenses.

    The photographer thinks that by changing lenses, perspective changes. It doesn't. What changes is the Field Of View. The distortion imposed by the lens, is still the same perspective and try as you may, you can't change that.

    In your post, you explain how subjects move to change the image ratio size. True. You are right on. That is not perspective. That is image ratio as you well know.

    As you move, you change perspective AND object size. If the subject moves and you stay still, the only change is location of the subject and the image size.

    It's ironic that you posted this as I am in the middle of writing a lecture on this exact subject.

    Good job my friend....
    • Like Like x 1
  4. flash

    flash Mu-43 Hall of Famer

    Apr 29, 2010
    1 hour from Sydney Australia.
    Probably best that I stay out of this one.:biggrin:

  5. Djarum

    Djarum Super Moderator Subscribing Member

    Dec 15, 2009
    Huntsville, AL, USA
    Perspective is about the relationship between the photographer and the subject, ie distance between them. If I stand in one spot, take a picture with a wide angle lens, and stay in that spot, and use a tele, I do not change perspective. Its like cropping an image.

    Now, lets say I have a wide angle lens, take a picture, and produce a scene. If I want to get that same scene with a tele lens, I have to back up. By backing up and taking the same scene, I have in fact compressed the image. While I may have the same scene in terms of length and width, the relationship between the objects appears to change. A tele lens compresses an image. The relationship between objects in the forground and background seems closer. The relationship between objects in the forground and back ground using a wideangle lens to produce the same scene seem farther apart.

    I find the example in the middle of this page very useful:

    Understanding Camera Lenses


    In your example, the two images in fact do have the same perspective. The left image is just a crop of the right image. If you would have moved farther or closer to the image, then the perspective would have changed. All you have done is changed the FOV. When cropping or using a telephoto lens, clues about depth perception do change. Depth perception is related to some degree to perspective, as I mentioned about compressing or stretching an image. Just because clues about depth are not as evident in a cropped or magnified image as they are with a full image or larger FOV of the image, the perspective still remains the same.
  6. Pan Korop

    Pan Korop Mu-43 Veteran

    Mar 31, 2011
    Phare Ouest
    Funny the pretended "Internet myth" has endured since the Renaissance.

    From this time, perspective was, without getting into details:

    The representation of a three-dimensional scene on a flat surface, taking in account the relative distance of the objects between themselves and respective to the position of the eye of the observer.

    As some pre-Internet pre-Kodak guy illustrated it :
    Subscribe to see EXIF info for this image (if available)

    (Albrecht Dürer [Public domain], via Wikimedia Commons)
  7. Hikari

    Hikari Mu-43 All-Pro

    Nov 26, 2010
    Djarum, so if I present two different images, can you tell by looking at them if they has the same or different perspective?
  8. Hikari

    Hikari Mu-43 All-Pro

    Nov 26, 2010
    Please show there is only one attribute to perspective. Please show two-point perspective is a myth. The five-page of so entry in the Encyclopedia of Photography does not agree with that.
  9. Djarum

    Djarum Super Moderator Subscribing Member

    Dec 15, 2009
    Huntsville, AL, USA
    You posted two images that have the same perspective. If there are two seperate images of two seperate scenes, they are going to have different perspective since they are two seperate images.

    I think one of the issues here is that in art, in general, there are different types of perspective. Aeriel perspective and Height perspective, in art, not just photography, are other types of perspective. When perspective is generally discussed in photography, it is discussed in the context of linear perspective or perspective distortion as a tool that a photographer can use.

    I believe what you are trying to say, is that perspective does change. Linear perspective does not change. Other types of perspective might change. Our own sense of perception might change. But linear perspective doesn't change.
  10. SRHEdD

    SRHEdD Mu-43 Top Veteran

    Feb 24, 2011
    Viera, Florida USA
    Hikari, I couldn't quite get the first part of your post...

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  11. Hikari

    Hikari Mu-43 All-Pro

    Nov 26, 2010
    By your definition of perspective, it will not matter what I shot as simply the object distance controls perspective. Can't you tell the difference in perspective from one picture to another?

    Photographic Materials and Processes states under linear perspective that object distance and focal length are controls. Two-point perspective is just a form of linear perspective.
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