Cloudy Skies

Everyone says that cloudy skies create great lighting and "soft light," and it can. But it won't necessarily. First we have to understand that on a clear day "the sun" is not our only light source. The entire sky is.

To understand what happens to light as it passes thru the sky we first have to realize that the sun is not a point light source... because the sun is so far away minor changes in distance do not affect it and the light does not seem to spread out (i.e. sunlight is "hard" and does not follow the inverse square law) so it "acts like" a point light source *at your location.* But, it's actually sending light in all direction and it lights up entire continents at once.

The next thing we need to understand is that light is refracted as it passes thru the air. The light from the sun itself is white, and the sky is actually colorless. But that's not what we see. We see a yellow sun and blue skies becuase the light is being refracted.

As can bee seen in the image above shorter wavelengths of color (violet, blue, green) are scattered more easily than longer wavelengths of color are (yellow,orange,red). Because violet and blue are the short wavelengths of the visible spectrum they scatter first, which is why the sky is usually blue. But that blue light is not lost... we can see the blue sky which means the light is reaching you. This is why snow tends to record as blue. It is why the sky is a more saturated blue overhead (because blue light has a harder time traveling long distances). It is also why the sun appears "warmer" (more red) when it is lower on the horizon and much farther away; more of the shorter wavelength colors (greens and yellows now) are scattered over the longer distance. And it is why the entire sky is your light source, even on a clear day. 

In this drawing I have simplified things down to the "essence" of how light behaves. In reality the sunlight is radiating out to all points and the blue light is coming from all directions. But we are only concerned with things at one small point. And, since the blue wavelengths are of lower energy it is easily overpowered by the higher energy coming directly from the sun.
Regardless of the light source and direction, when light hits a surface it will either be absorbed or reflected (the colors we see). And it will reflect in a variety of directions depending upon the angle of incidence and the texture of the surface (again, very simplified in the drawing).


So what happens when we have a cloud blocking the sun? 

Firstly, the cloud causes additional scatter of the shorter wavelengths. And secondly it may be sufficient to more completely block the sun. The primary effect is a reduction of higher wavelengths (yellow) and an increase in the shorter wavelengths (blue). It is a reduction in contrast in the lighting ratio. And the light that is not being blocked by the cloud is still reflecting around off of all of the surfaces around you. It provides "fill light" at a greater ratio than it did before. This can be sufficient enough to cause "soft light."


But what happens if we have a cloud layer instead?

Again, the drawing is very simplified. In reality the cloud layer is reducing the amount of blue light we were previously getting from the blue sky, while simultaneously refracting/scattering *some of* the higher wavelength "sunlight" back towards you as blue from those same areas. But, it doesn't really change things a lot. We simply have more of the light scattering to blue and a reduction in the lighting ratio. The longer wavelengths of light that do not come thru are mostly reflected back into space and are mostly lost to us. This is why shade, shadows, and cloudy days record as blue. 


One thing that clouds do not do is make the light source "larger" and more "diffused." You can see this for yourself on a thinly clouded day... if you can see the sun thru the clouds at all it will not be significantly larger and the shadows will not be notably softer. Or you can simply hold up a layer of white fabric and look at the sun thru that.... it is even closer to you but the sun will still appear to be about the same size.
And having a cloudy sky or a clear sky doesn't change the fact that "the sky" is your light source; a cloud layer doesn't make the sky any larger. And if it did, the height of the cloud layer would matter, but it doesn't. The other thing that has not changed is the importance of the scattered light coming from all directions, and the importance of the light being reflected off of surrounding surfaces... this is what is truly causing the light to become "softer." And this is what you are looking for, not just cloudy skies or shade. What you usually want is an area of reduced lighting ratio (light shade) with a good amount of indirect light/bounced fill.


This all completely different from the common "softbox" analogy... if anything, a cloud layer would be closer to using a small light to light a large shoot thru umbrella. It *can* produce diffused light, but it takes a peculiar mix of events for it to be soft/even light. With a cloud/cloud layer it is simply equalizing the light ratio and blocking the direct light in favor of indirect light... but the result can still  be "softer" lighting.


And don't confuse the contrast/darkness of shadows with the "quality" of the light. When we speak about the quality of light we are generally referring to the hardness/softness of the shadows... how distinct or graduated the edges are. Another way of thinking of it would be in terms of sharp vs dull edged shadows.

A lack of contrast (shadows) just means flat lighting. Whether it is due to hard direct lighting filling in all of the shadows, or soft/indirect light not creating any shadows, it really doen't make a lot of difference. In fact, creating really "soft" lighting often creates higher contrast in an image due to the inverse square law... it just occurs in a different area.

If this raises any questions for you, let's discuss it in the forums.





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