The eye is an amazing organ. It’s made up of tens of millions of rods and a few million cones. Rods are luminance measuring cell, while cones are a color measuring cell. Somewhere between 75 and 150 million rods are distributed throughout the entire retina, with a concentration around the fovea decreasing in density as one moves away. That being said, there are very few found actually in the fovea. Cones, on the other hand are found mainly in the fovea, very few are scattered outside of it. There are also only 6 – 7 million found in the average eye. Thus, your brain is much more sensitive to luminance values than color.
The fovea is a round indention about one and a half millimeters in diameter. It could be thought of as a 1.5mm x 1.5mm sensor of film, but with a resolution of 337,000 cones. Each cone has it’s own nerve ending, and so each individual value is recorded. In contrast, rods are grouped together by proximity and return an averaged value. This makes the human eye great at picking out detail, but terrible at discerning gradations. The eye produces the illusion of continuous tonality. The eye is limited to about 650 distinct luminance values or a range of 30+ stops. An tonal change of 2% is needed before the individual can discern a difference. Because of this, a photograph that looked “normal” indoors when taken outside, even though our eyes would render the fine tonal values, the whites would appear lighter and the blacks darker. Even though we may be able to reproduce an image with thousands of grey levels over say a 10 stop range, our eyes wouldn’t discern that much of a difference, thus, no matter how “good” a reproduction is, there are always limits to what’s useful.
Light is never perfectly white, it’s typically tinted somewhere between red and blue, this is called color temperature. Temperature, because it’s literally measured by how hot the light must be to produce that tint. For instance, a photoflood lamp, which is a warmish light, is about 3200K, daylight is about 5500K and skylight is about 12,000K. The term Color Constancy refers to the ability to accurately perceive colors, even though the light temperature changes. Even though, in different lighting temperatures, an object may reflect more blue or red light, we relate the objects innate color, and that’s how we “see” it. The human eye can see about 650 luminance values of 30 stops, although only about 16 stops at a time, and it’s adjusts on the fly to the various levels of luminance. Film and digital sensors are constant, they are limited and understanding how they are limited is essential to taking a successful photograph. Using light meters and adjusting the exposure helps us measure light and determine how much is actually hitting the film/sensor. Because of the constant nature of the camera, it is possible to take a long exposure, and see a “normal” tonality, that would be impossible with the naked eye.
Journaled Time: 4 hour – reading, research and taking notes
Total Class Time: 18 hours
