exposed image

In the beginning they divided the light from the dark, and then they divided the light into other parts and called them a lot of different things, stops, lumens, zones, tones, 18% gray, and they will forever be changing the names...

For many of us this history will start at the beginning of the time that Nikon introduced an affordable DLSR, the D70. Oh, there were others, but who could afford them? There was 50 years before the D70 came along and by then, some pretty good lenses were developed for the era of film. Digital cameras had an advantage over film which had to be manufactured to perform to specific speeds, colors and variations in acceptable balance of light color.
Many photo amateurs could not afford or gain skill in color photography and were satisfied to work only in black and white.
Ansel Adams
A very skilled artist, Ansel Adams shared his understanding of light and published books explaining the problems associated with producing quality images. His system of light management was called the Zone system. His system had about 9 stops depending on how an image was going to be reproduced. His zones followed the steps of exposure based on each step from white to dark being one half the light value of the previous step. Our current system of ISO speed has an arbitrary start at 100 and progresses geometrically to 200, 400, 800, 1600, etc. The speed values on our cameras have a similar relationship but with a different progression due to the math relationships of focal length compared to lens diameter. On aperture the steps look like: 2.8, 4, 5.6, 8, 11, 16 and 22. This geometric progression is something that will complicate how we control light values in our pictures. Because the changing of analog light to digital values the use of a binary based system works out perfectly in electronic design. JPEG the standard file and number system adopted for digital photography uses the numbers 0 to 255 to express the steps in light value. When we see the values of colored or monochrome light we see values for white as FF in hexadecimal, 255 in decimal and 11111111 in binary. This choice of values is adequate to drive a printer or computer monitor, but if you want to make adjustments to the image with a computer, you need a few more digits of color depth. It only takes five stops down from the brightest value until there are only 4 discrete steps in the number system. If you are trying to see more detail in the shadows of a picture the image is going to be very ragged and noisy in that area. Fortunately the number of steps in the raw or NEF image is 16 times more, somewhere around 4,000 steps in each color channel.

The human eye can distinguish 32 to 64 levels of gray, more levels when color is introduced. Therefore we can distinguish more steps in the brightest stops of the zone system. Although any photo editor program can tell exactly what the value of a tone is we as humans need a reference point to aid our analog eyes in judging color and tone. There are several test targets we could use as a measuring tool. I have chosen to go cheap and use the Kodak Q-13 target, available in most photography stores for about $25 US. It consists of two strips, one for color and the other gray. That is the header for this page.
The human eye is also non linear and can quickly adjust the pupil to be able to peer into shadows and see detail. Our brain automatically assembles a series of images to create an image of great tonal depth.

The D70 and many other digital cameras actually adjust the image linearity to approximate a curve we refer to as Gamma. A curve with a gamma greater than one has its numerical mid point adjusted to be brighter which also boosts the darker values to display more detail. Although the camera boosted the mid values some, it was possible to adjust contrast, saturation and hue by entering values from the menu. One of the things that made the D70 such an interesting camera is that Nikon provided a method to introduce a tonal curve that would multiply all the light values by a factor that the photographer chose. There was a great deal of excitement about curves that were named, "white wedding", "point & shoot", etc. These curves were created and distributed on the web and were a source of great discussion on the photo chat threads. Every new technology idea gets a new name every time it is improved. By the time it gravitated from the high end cameras to the D90 the name became "Picture Controls" and "Adaptive D-Lighting"

Continue to Chapter 2

White Balance
Focus Your Camera
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Back Home to Exposed

1  History of curves and programmable contrast and gamma

2  Picture Control and Picture Control Utility

3  Creating and installing Custom curves in the camera

4  How to design a curve for your needs

5  Active D-Lighting

6  D-Lighting applied after the shot

7  Where to go for more information

8  The DOWNLOAD page


© Leon Goodman 2009

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