Non Linearity Zones and Human Vision

Let's consider how this corresponds to the familiar Zone Scale and, more important, to the ways that we perceive light.

What the characteristic curve shows is that there is less contrast in the darkest and lightest values than there is in the middle tones.

This corresponds to the fact that when the light in a room is very dim it is hard for us to see the difference between various objects on the walls and floors. This is also true if the light is blindingly bright. It is only when a moderate or normal amount of light is available that we can see clear distinctions between different surfaces.

If you look closely at the Zone Scale you can see this same relationship between the various Zones. There is more difference between Zones V and VI than there is between Zones I and II.

FIGURE 145 The Zone Scale — A non-linear representation.

FIGURE 145 The Zone Scale — A non-linear representation.

The fact that human vision responds to light in a non-linear way means that we are able to see over a much greater range of brightness levels than would otherwise be true. Some of this non-linear response is due to the fact that our pupils open wider when it's dark and close down when it's very bright.

But in a deeper sense, the biochemistry of the light-sensitive cells in our retinas also contributes to this effect in ways that are very similar to the effects of light on film. This is known as the Adaptation Effect of human vision.

In the Zone Scale representation of non-linear gradations, there is a natural amount of compression of the tonal values at the high and low ends of the scale, which allows for a very even distribution of the tones on the scale.

This is why the middle gray tonal value (Zone V) falls in the middle of the Zone Scale in a way that feels "natural" to our eyes.

0 0

Post a comment