Saturday, June 4, 2011

Atmospheric perspective

There is a remarkable colour difference in objects photographed at different distances. Not just the object itself, but also its background have very different colours. Of course Marcel Minnaert has something to say about that.
Ouderkerk aan den IJssel - Dorpskerk - Nederlands Hervormd - 1200-1600
Seen from a distance of approximately 5 km.

The change of colour of the object is explained by atmospheric perspective:

173. Atmospheric perspective
A forest in the distance forms an excellent dark background to observe the scattering of light by the atmosphere. The further away we are, the more it looks hazy and bluish.

The deep column of air between us and the forest, illuminated from the side by sunbeams, scatters light in our direction and this light is superimposed on the dark background, just like the light scattered by a veil covers the objects behind it. In front of this background the contrast between light and dark areas is weakened, the colour is smoother and more blue.

Involuntarily we estimate the distance of trees by the amount of atmospheric perspective. A tree at a distance of 100 meters  already has a more bluish tint than a tree next to us. The green of meadows in the distance turns blue-green surprisingly quickly and then blue. Check this in the landscape!

And the tint of the low sky is explained in this manner:

178. When is the distant sky orange? When green?
We have seen, that in cloudless air the sky at the horizon should exhibit the same color as a sheet of white paper, illuminated directly by the sun (chiaroscuro). It is also clear that at sunset, when the sun colours everything with a warm orange glow, we will notice this same tint all along the horizon.

Waddinxveen - Sint Victorkerk - Katholiek - 1880
Seen from a distance of approximately 1.5 km.

And also the change of colour of the background is explained:

There is also another sky colour effect ... the light intensity increases from the zenith towards the horizon, and at the same time the colour changes from blue to white ...

... layers close to the ground contain more suspended dust particles floating above the ground. These increase the scattering of light and make the color whiter. Where the sky is the darkest, it is also always the bluest and its color is most saturated.

A somewhat controlled experiment

I wanted to experiment with this atmospheric effect and during a trip to Leeuwarden I found a suitable place for this.  The Tesselschadestraat is a 600 meter long street with two black buildings at either end. I walked from one building to the other and made photographs in both directions. Here are the pictures taken at both ends of the walk.
Top left is start of the walk - bottom right is end of the walk.
Top right and bottom left is the view of the opposite building, at the start and end of the walk.
Even at this short distance the color and hue difference is visble. When viewed from a distance the building looks lighter and bluer than when viewed from close by. The contrast is also quite different. But the experiment is not entirely objective - I forgot to use a fixed aperture and fixed shutter speed. So the results are distorted by the automatic adjustments of the camera.

I tried to focus on the atmospheric perspective by selecting the color of the building at the same spot. This does only work in one direction. The lower series of colors corresponds with the building on the left - the upper series of colors with the building on the right. Only one of two color series shows a clear atmospheric effect. I don't know what causes this - it's not caused by the direction of the sunlight, because the sunlight came from a direction perpendicular to the line of sight. 

1 comment:

  1. The atmospheric perspective picture got invited into a Flickr group. It seems that there exists a Flickr group devoted to atmospheric perspective - but they call it aerial perspective. Which is also correct of course: