The Digital Linear Effect

The reason why all of this is important is because digital camera chips respond to light in a very simple and straightforward way that has important implications for digital photographers.

We can say that the response of digital chips to light is "linear" and the following example illustrates what this means:

Imagine that Figure 146 is a digital light sensor that responds to light by filling up with photons until it reaches its capacity. As of this writing, digital sensors are able to record approximately

FIGURE 146 Empty digital sensor.

6 stops worth of contrast, and, as we already know, as you decrease exposure, each photographic f/stop lets in one-half the amount of light as the next one.

When photographers shoot in the camera's raw format, the sensor is able to capture 12 bits of visual information. This translates into a total of 4,096 pixel levels.

The question then is: How are these 4,096 levels of tone recorded (or in this example we could say "collected")?

The brightest light values of your subject of course make the strongest impact on your sensor, so they are the ones collected first.

• Since the sensor can record a total of 4,096 levels of tone, that means that the first and brightest stop of exposure will fill up with 1/2 of these or 2,048 pixel levels.

1st STOP -





2nd STOP -

1st STOP -

1 st Stop - 2048 Pixel levels

2nd Stop - 1024 Pixel levels

3rd Stop-1024 Pixel levels

FIGURE 147 Digital sensors exposed to 1,2, and 3 stops of exposure.

The second stop, (tonal values that are one stop darker than the first), will fill up with 1/2 of what remains or, 1,024 pixel levels.

The third stop fills with 1/2 of what remains or, 512 pixel levels.



2nd STOP-


4th Stop - 256 Pixel levels



2nd STOP-















4th Stop - 256 Pixel levels

5th Stop - 128 Pixel levels

6th Stop - 64 Pixel levels

5th Stop - 128 Pixel levels

6th Stop - 64 Pixel levels

FIGURE 148 Digital sensors exposed to 4, 5, and 6 stops of exposure.

• The fourth stop has 256 levels.

• The fifth stop will have 128 levels.

• And finally, the sixth stop, the darkest, shadow values of your subject are only left with 64 levels to record them.

This Digital Linear Effect has two important implications.

First, what we have learned from this exercise is that digital chips have a natural tendency to under represent the shadow values that are so important to the quality of our images.

Because of the digital linear effect, digital cameras don't produce a tonal gradation that looks like Figure 149 with middle gray in the middle where it feels natural to our eyes.

FIGURE 149 Tone-mapped, non-linear gradation.

FIGURE 149 Tone-mapped, non-linear gradation.

Instead, digital cameras produce a gradation like Figure 150 where most of the scale is devoted to the lightest tonal values and very little is left for the darkest tones.

FIGURE 150 Camera Raw linear gradation.

The difference between these two ways of measuring and rendering light values has one other extremely important implication.

In Appendix C we learned that the more pixel levels a digital image devotes to rendering a tonal gradation, the more smooth, detailed, and photo-realistic that gradation will be.

Because of the Digital Linear Effect, the number of pixel levels that digital cameras assign to the light, middle, and dark values of the spectrum aren't equal!

An equal distribution of levels across a "normal" non-linear gradient would look like this:

FIGURE 151 Pixel levels of a tone-mapped, non-linear gradation.

In this example the highlight, middle, and shadow values each have an equal number of pixel levels defining them.

Instead, digital camera sensors distribute pixel levels across the linear spectrum in a way that can be diagrammed like this:

FIGURE 152 Pixel levels of a Camera Raw linear gradation.

Notice that the highlights have many more pixel levels defining them than do the middle or shadow values. This is like Figure 148(C) turned on its side.

One important function of raw conversion software is to remap the linear tonal gradation produced by digital camera to a gradation of tones that looks more natural to our eyes. This is called "Tone Mapping." This is a function that has dramatic implications for the way we should expose and process digital images.

The fact that so few levels of tone are devoted to the shadows explains why banding and pos-terization is such a problem when the contrast of underexposed digital images is expanded (see Figure 92).

If you properly expose digital images as explained in the section The Zone System of Digital Exposure on page 138, then your shadows will fall on exposure stops as near to the middle of the scale as is possible given the dynamic range of the image that you're shooting. This way the images will be recorded with more levels for you to work with when you edit the images in Photoshop.

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