    
Walter Susor
White Crane
Username: wsus
Post Number: 1814
Registered: 08-2003
Posted From: adsl-75-6-241-208.dsl.pltn13.sbcglobal.net
| | Posted on Sunday, August 22, 2010 - 01:15 am: | 
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http://www.dpfwiw.com/exposure.htm
This is an excellent treatise on digital photography with emphasis on obtaining well exposed, color corrected photos. The article is comprehensive and readable. The information presented should be especially useful to someone who is making the transition from film to digital. Many of the remarks are referenced to the author's 2 megapixel Olympus camera but also apply to todays 10 MP and higher cameras (my camera is 5 MP and I found this article very informative) The author has also attached links to additional reading.
Here is a sample from the section on outdoor light variations.
Sunlight Variations
Most of the variations observed in the color of sunlight at the Earth's surface stem from spectral variations in atmospheric scattering. In clear air, short (UV-A and blue) wavelengths suffer up to 16 times more scattering than longer (red and near IR) wavelengths because scattering efficiency by air molecules (N2, O2, CO2, etc.) varies inversely with the fourth power of wavelength.
Direct sunlight here on Earth is yellower (redder and greener) than the light leaving the sun because the atmosphere scatters a good bit of the blue away before the light ever reaches you and your subject. (Yes, some of the scattered blue will eventually scatter back to the surface, but there's still a net loss of blue.)
Overcast light is bluer than direct sunlight because visible wavelengths are all scattered equally well by particles the size of condensed water droplets in clouds - hence clouds in shades of gray at midday. Scattered blue light falling onto the cloud tops from above gets tossed back in the mix, now with an even chance of getting to the ground. There's still a net loss of blue, but under overcast skies, it's smaller relative to losses at other wavelengths.
Outdoors, shaded areas are illuminated predominantly by skylight, which is quite blue on sunny days and closer to neutral on overcast days. Shaded spots under leafy trees can also pick up extra green light reflected from or transmitted through overhead leaves.
In the morning and evening, sunlight takes a much longer path through the atmosphere, losing even more blue and some green to scattering along the way. That's why early and late day sun is redder, and why sunrises and sunsets feature the warm long-wave (red, orange and yellow) colors we so admire.
Walter |