The Quality of Digital Images

It's always a little ironic when someone looks at a fine digital print and says that it looks just like a "real" photograph. But of course what this really means is that the technology is doing exactly what it hopes to do; that is, translate a purely electronic digital file into what appears to be a full-fledged, continuous tone photographic image.

In purely technical, and very general terms, most digital photographers would agree that Figure 71 is an example of a "high quality" image and Figure 72 is an exaggerated "low quality" version of the same photograph.

FIGURE 71 High quality digital image.
FIGURE 72 Low quality digital image.

To better understand the differences between these two images, let's translate them into aesthetically neutral gradations that represent the tonal values in each photograph.

FIGURE 73 High quality digital gradation.

FIGURE 74 Low quality digital gradation.

Using these two gradations as examples, we can now say that what digital photographers mean when they speak of "image quality" is the goal of reproducing smooth, gradually continuous gradations that are typical of analog imaging media like film.

For reasons you will soon understand, Figure 74 has noticeable steps in the transitions from one tone to the next.

The more smooth and seamless the transitions are from one tonal value to the next, the higher the image quality is said to be.

With these ideas in mind, the following example will illustrate how both resolution and bit depth determine the quality or smoothness of the above gradations.

Imagine that each pixel in the above gradations is a step in a flight of stairs. The "resolution" of the staircase would be a measure of the actual size of each individual step (pixel); the smaller the step the more smooth the gradation.

But keep in mind that although the size of each pixel (step) can change, it can still only be one tonal value. In this case, each pixel is limited to one of only ten different tonal values.

This can be illustrated like this:

a. Low Resolution/Low bit depth b. High Resolution/low bit depth

FIGURE 75 Low and High Resolution Illustration.

Notice that by greatly increasing the resolution of Figure 75(b) its slope is much smoother. It has much smaller pixels (steps) but the gradation itself is still not continuous, because the number of individual tonal values that each step is allowed to be in this illustration is still limited to 10.

Remember that we defined bit depth as the number of different tonal values (also known as "levels") that each pixel is allowed to be in a given image.

With this in mind we could say that Figure 75(b) would be an example of a high resolution, but low bit depth image.

Increasing an image's bit depth allows each pixel to be more levels of tonality.

If we were to optimize both the resolution and the bit depth of this illustration the result would look like this:

FIGURE 76 High resolution, high bit depth illustration.

What this illustration makes clear is that learning how to optimize both a digital image's resolution and its bit depth are important for achieving the best quality in your work.

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