Monday 1 December 2014

Birds and Light - Lighting and Perspective (Part 1)

Some limitations of human vision

We probably don't like to think of human eyesight as having analytical limitations.

Cayenne (Sandwich) Thalasseus sandvicensis eurygnatha and Snow-crowned (Trudeau's) Sterna trudeaui TernsArgentina.

When we are out birding, most of the time we are not trying to make subtle judgements about plumage colouration or tone.  But occasionally we are posed with a question like, "which species has a darker mantle in this flock of terns?"  Just how well equipped are we to make an accurate assessment?

Predatory animals and for example primates have forward facing eyes which provide a three-dimensional, Binocular view of the world.  Benefits include the ability to judge distances extremely accurately.  It takes a lot of skill and coordination for instance to hit a bulls eye on a dart board or score a 3 pointer in basketball.  From an evolutionary perspective this form of vision obviously helps with hunting, navigation, foraging etc.  But there are also some limitations introduced by this method of vision.

If you have looked at a 3D television without 3D glasses you will have noticed an annoyingly, blurred double image.  This is like the image that our eyes actually produce and can also be seen by intentionally crossing ones eyes.  Our left eye sees the world from a slightly different perspective to that of our right eye.  This is obvious by alternately opening one eye and closing the other.  Our brain takes these two perspectives and recombines them to create one sharp, 'properly aligned' image.  In doing so, the brain must actually distort the lines of the image to match up these two different perspectives.  This is a distortion of reality and therefore should be seen as a limitation, not an advantage in terms of forensic analysis.

By comparison, a camera has only one perspective.  The lines captured by the camera are therefore more accurate than those perceived by human eyes.  Normally we are totally unaware of this difference because straight lines are not very common in nature.  But when we photograph in the built environment for example we are immediately drawn to this perceptual differences between the human eye and the camera image.  Take this image below of Adare Manor in Ireland.

The lower left image accurately depicts the perspective and depth of the scene, given the position of both the observer and camera relative to this tall, impressive building.  Note how the left edge of the building is not vertical - this is correct in terms of normal perspective.  Human vision looks more like the image on the lower right.  Our vision tends to distort the environment around us, making lines straighter and more vertical in appearance.  This provides an advantage in terms of spatial awareness and navigation, but it is a distortion of reality.  Note a digital image may also suffer from lens distortion.  It is important to check for this anomaly rather than assume that the camera is always correctly recording angles and proportions (see HERE). 

This series of postings, Birds and Light, is all about light.  So what is the relationship between perspective and lighting one might ask?  Well, because our eyes distort perspective, we don't often appreciate the difference a subtle angle change might make to lighting in a scene.  And, because our eyes are effectively seeing two different perspectives, our eyes are seeing two subtly different lighting regimes.  We may sometimes be surprised or even confused by the lighting in a scene.  Consider the experiment below.

I have printed out a regular grid of rectangles.  Each column is identical.  Each row consists of rectangles offset by 15 degrees from the previous row. 

Having printed this grid I now look down the line of the page from right to left.  This is what my eye's see.

At the moment I am looking at a flat grid.  I need something three-dimensional in order to appreciate the effects of ambient lighting on what I am seeing.  On top of each of these rectangles I have glued a target.  Each target is identical, consisting of a disk with a hexagonal shape protruding from it.  Each target is centred and angled exactly in line with the grid box upon which it sits so that all the targets are angled to match the grid. 

I have taken a series of photographs of this target grid. Viewed from above, these targets have been laid out in the regular pattern shown above, and everything looks very uniform.  From the perspective of the camera looking down the line of targets from a slight elevation, these targets actually all have subtly different angles and and analysis of the tonal distribution on each of the targets shows they all have subtly different lighting signatures as a result.  No two targets look exactly the same.   

To the eye there may be just a subtle tonal gradient apparent between the front and back rows.  For a clearer picture of what is actually going on I have used a very handy software program called Color Quantizer to postarise and then recolour individual tonal levels.  This allows me to map the distribution of individual tonal levels across the whole image.  For simplicity I have greyed out the tones representing the background.

From the perspective of the camera, we can now verify that each of these targets is subtly different in appearance.  To the brain, which is combining two different perspectives on the same scene, this is an even more complex puzzle to deal with.

Mach Bands, Cornsweet, Checker Shadow and Chubb Illusions
I won't elaborate on these illusions here, but if you click on each of the links above they will take you to various web pages describing these individual optical illusions.  While each illusion may be explained by subtly different mechanisms (some possibly yet to be fully proven) they all impact on the same general aspect of vision, namely our interpretation of ambient lighting.  Are these phenomena all a direct consequence of the complex nature of light and perspective.  Are they compounded by our binocular vision?  Have some or all of these illusions arose for evolutionary benefit?  Is there any real benefit to these illusions at all?  I don't have these answers unfortunately.  All I can say is, the more I read about all of this the more uneasy I have become as regards the accuracy and reliability of human vision when it comes to interpretation of subtle lighting and colour.

Take the example below.  In the field we might often be presented with a scene where, while scoping a large flock of gulls or terns we are presented with a bird in the background which appears interesting.  We decide to compare it's mantle tone with a bird in the foreground.  They seem very similar.  Are they the same or is one darker than the other?  

In the field, this is not an easy distinction to make.  However armed with a digital camera we can make a more helpful comparison.

So, as it turns out, the targets in the background are all a couple of shades darker than the ones in the front row.  When objects we wish to compare are separated our eyes cannot make an accurate distinction (global analysis).  When close together (local analysis) we can do a much better job.

Bins or Scope?
Here is an interesting question.  If our eyes and visual system are adapted for binocular vision then presumably our vision, including our interpretation of perspective and lighting is much the same when we use a pair of binoculars.  But what happens if we use a monocular or a scope instead?  Is perspective and lighting more accurately gauged through a scope than through bins?  Or, are our eyes simply not equipped to be able to take advantage of monocular vision when an opportunity presents itself?

And what of the other, 'lazy' eye?  Most experienced birders tend to use their dominant eye for the scope and have somehow 'trained' their brain to ignore the image coming from the other open eye, which is usually staring at the ground or off into space.  Those inexperienced in using a scope will usually close one eye, or place a hand over it, because they find it distracting and they are unable to give full attention to their scope eye.  So, at some point our incredible brain has learnt to decouple the images from our two eyes, ignore one and focus our attention almost exclusively on the signal coming from the other eye.  This is no doubt amazing, but how can we be sure that the other eye is still not influencing our vision and judgement, including our global and local assessment of subtle tones?  Should a critical observer carry an eye patch with them just in case? ;)  A camera might be more useful!

See also Lighting and Perspective (Part 2).

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