Friday, July 31, 2009

Perspective

So I thought I'd start off with what should be a fairly straightforward (but often turns out to be extremely confusing!) topic, namely: perspective. I should point out at this stage that my explanation only deals with the parameter(s) that can affect perspective - that is the observer's position in relation to the subject. All other possible parameters such as aperture, shutter speed, ISO and even focal length (more on this in a future blog) are assumed constant.

The Wikipedia entry is worth a read in so far as explaining what linear perspective is. In simple terms, a given object appears larger or smaller depending on how far we (the observer) are from the subject. The closer we are, the larger the subject. The further away we are, the smaller the subject. Simple enough so far...

We can generalise the above to say that the observer's location in 3D relative to the subject's location in 3D is what determines perspective. For example, if taking a picture of a flower, the perspective changes considerably if we shoot the picture looking down on the flower or if we lie on the ground and shoot the picture looking up at the flower! It's therefore important to think of perspective in terms or 3D points (x, y, z). We can change our:
  • Distance (y) from the subject.
  • We can change the angle (by changing x) to the subject (e.g. having the subject exactly in front vs. having the subject to the left or right/at an angle).
  • We can change our elevation (z) with respect to the subject (e.g. looking up at a flower vs. looking down at a flower).
...these three parameters (your spatial coordinates) quite simply are the parameters that affect perspective.


Relative Perspective
A more subtle (but possibly more important!) point about perspective is what I'd call relative perspective. Simply put, that is how multiple subjects (in a photo) appear in relation to one another. For example, take a look at this image:

It's a photo taken underneath a bridge with major support beams running along the length of the photo (i.e. from top to bottom) and minor beams/spacers running along the width of the photo. As intelligent human beings we intuitively know that all the beams must be of the same length & width and equally spaced between one another. The photo however does not show this! Take the major support beam in the centre, for example. We notice that at the top of the photo (closer to the observer/photographer), the beam looks wider/thicker than at the bottom of the photo (further away from the observer/photographer). This is the effect of perspective! The same can be said for the spacing between two given beams - it seems to get smaller as we go from top to bottom of the photo. Theoretically, if the photo extended to infinity, then one could imagine the two beams meeting. This point is known as the 'vanishing point' and is entirely the result of perspective.

Now consider all the major support beams. Concentrating on the bottom of the picture, we can notice that going from the centre of the picture to the left, the distance between two successive beams seems to get smaller the further left (i.e. further away from the observer/photographer) we go. This is the effect of relative perspective. The same thing can be said about the minor beams/spacers. The distance between successive spacers going from top to bottom of the picture (i.e. further away from the photographer) seems to get smaller. Generally speaking, one can say that the closer the observer/photographer is to the closest/primary subject (say the major beam in the centre of the picture in this case), then the larger that subject will appear in relation to the other (more distant) subjects (i.e. all the other major beams in this case).


Lens Focal Length
I didn't touch upon focal length in this blog as I will treat it's relation to perspective in another blog.


References
  1. Wikipedia - Perspective.
  2. Klaus Schroiff - Perspective.
  3. Basics Photography: Composition - David Prakel; AVA Publishing 2006.
  4. Wikipedia - Perspective Distortion.
  5. Cambridge in Colour - Camera Lenses.