Wednesday 12 March 2014

Gestalt - Size Matters

Judging Size and Proportions

Size and proportion is difficult to judge accurately in the field.  We have all misjudged the size of a bird flying above us.  Birds rarely stand still for long and it can be hard to make a useful size comparison against another bird or object in the environment. 

One might expect that it should be far easier to judge size and proportions accurately from a good photograph.  Well, this is not necessarily the case.  Below are a number of examples of how a bird’s apparent size and proportions can easily be altered due to camera optics, perspective, and even the mind’s eye. 

This is caused by variations in magnification over the field of a lens.  It results in a deviation from the rectilinear projection of the image in 2D and may appear as barrelling, pincushion or the combined moustache distortion arrangements.  Focus is unaffected.  The easiest way to check the extent to which your own camera lens suffers from lens distortion is to take a picture of a grid of squares and check for bowing effects at the sides of the image.  Make sure the camera lens is parallel to the grid when taking the photograph so as not to introduce 3D or perspective distortion (see below). 

This effect is mostly associated with zoom and fish-eye lenses.  If you have a zoom lens it is worth repeating the above experiment at the lowest and highest magnifications of the lens and comparing the results.  Fixed lenses can also exhibit lens distortion, eg. wide-angle or fish-eye lenses, as well as other lenses when photographing objects at short focal distances.  Lower quality telephoto lenses may also exhibit lens distortion.

Lens distortion will of course affect every photograph you take but will be less easily detected in images which don’t have straight lines in them.  For images of birds the effect is usually not obvious to the eye, particularly if the image has been cropped.

3D Distortion - Perspective and foreshortening
Architectural photographs of buildings often look unnaturally distorted due to perspective, i.e. the distance and angle of the camera relative to various points in the scene.  What we often forget is that our own visual perception of the same scene is based on the brain’s interpretation of the image obtained from both our eyes (refer to CAMBRIDGEINCOLOUR).  Our brain, and how we subconsciously interpret the scene will often differ from reality.  In our mind’s eye, lines will tend to appear straighter and more parallel.  When we look at a photograph of the scene our brain it seems does not adapt as quickly and something which we may otherwise subconsciously ignore or be less aware of in life suddenly becomes noticeable.  The image of the building suddenly looks distorted.

For birders the situation is somewhat more straightforward.  Birds are usually too small and too far away from us for perspective to be a significant issue.  However when faced with a bird at extremely close range, such as a bird coming to a feeder or a bird in the hand, perspective does start to play a role in terms of our perception of size and proportion and of course in terms of photography.  If the observer is standing close, over the bird, looking down the head will obviously appear larger relative to the feet than it would if the bird is being viewed in side profile.  This type of distortion may appear similar to but should not be confused with lens distortion.  A key distinction between lens distortion and perspective distortion is that lens distortion can be easily corrected with 2D software whereas correcting for perspective distortion requires a more complex, 3D correction.  Another distinction is that whereas lines will appear bowed due to lens distortion, lines will remain straight when viewed in perspective.

Another very important consideration in terms of the relative size of objects in perspective is the focal length of the lens.  For more information please read  PERSPECTIVE DISTORTION.

Size Illusion
As stated above, the brain is capable of “correcting for” perspective to present the world to us in a slightly more two-dimensional looking space.  No doubt there are some evolutionary advantages to this but from a scientific and investigative perspective this adaptation is not always helpful.  If you search the internet for optical illusions you will quickly discover that there are many types of optical effects that appear to fool our mind’s eye.  Some size illusions are related to perspective (eg. the PONZO ILLUSION) and may be invoking by the same innate brain response referred to above.  Another interesting illusion is the MOON ILLUSION and the related EBBINGHAUS and DELBOEUF illusions.  Most of these illusions seem to work best with simple shapes like discs and squares but I was able to recreate the effect to some extent with a silhouette of a gull below.  

The mid-sized gulls appear to differ in size though they are in fact identical.  The reason the gull, surrounded by smaller gulls appears somewhat larger to the eye than the bird standing out on its own but surrounded by even larger gulls obviously has something to do with the wiring of the brain.  From my limited research into the Ebbinghaus and Delboeuf illusions it seems that there are two key factors involved.  Firstly, and perhaps most importantly , the proximity of the subject to other objects affects it’s apparent size.  This is perhaps also key to the Moon illusion where the moon appears larger when it is closer to the horizon.  The second factor is an annulus or a circular pattern around the object.  This factor may have something to do with the design of the eye. 

This illusion might manifest at a roost where various species congregate in close association at high tide.

These are just a few examples of factors that can determine how we perceive size and relative proportion in birds, including in bird images.  When it comes to bird identification one must therefore bare in mind that the lens may distort the bird and its surroundings to varying degrees.   The brain also plays a major role in how we perceive size and proportion. 

If we want to minimize all of these impacts it is advisable to obtain a range of images of a bird from varying distances and perspectives, to carefully take measurements and to compare and average the results from different photographs.  It is also worth investigating the accuracy of the lenses used.

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