Factors we must take into account to meet requirement 0: Factor 3 – Image quality

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Which of the two images in figure 26 do you think is better? – I bet you also vote for the right one, and I think this is because the right image appears sharper than the left one. This judgment is normal, because if we disregard the characteristics of the subject, all viewers assign the greatest importance to the impression of sharpness for the perceived image quality. But what is visual sharpness?

We can measure the brightness of a light stimulus in cd/m² and determine its chromaticity by its wavelength structure, but sharpness is a purely perceived property of a visual scene that we cannot determine directly. It is only in the eye of the beholder. Generally, we characterize a visual impression as sharp when we clearly distinguish the objects. Thus, in contrast to tasted or smelled sharpness, visual sharpness is the impression of sharpness that we perceive at the edges and borders between objects. The impression of sharpness is greater the more of these edges we perceive and the clearer they are, i.e., the greater the contrast between their two sides.

Desert landscape with rolling sand dunes under a clear blue sky. In the foreground, green shrubs with small yellow flowers add contrast. In the background, a range of dark, rocky hills stretches across the horizon. — Figure 26: Image quality 1

Desert landscape with rolling sand dunes and sparse vegetation under a clear blue sky. Yellow flowers and green shrubs are in the foreground, with distant mountains visible on the horizon. The edges of the objects have more contrast and thus appear sharper than in figure 26. — Figure 27: Image quality 2

A gray diagram with four vertical elongated rectangles of varying lengths. The rectangles, lighter gray than the background, are positioned at different heights and distances from each other, creating an asymmetrical pattern. — Figure 28: Increased contrast and impression of sharpness. On the right side, where the contrast of the gray stripe is increased by the added white and black stripes, we perceive a sharper edge.

This gives us a quick way to influence the image quality: we only need to increase the contrast. That’s precisely what the major film manufacturers do, and they’ve been successful in boosting the contrast in prints by 15%. For slides, a value of 10% has proven to be sufficient. These values guarantee an appropriate increase in color saturation, a crucial factor in perceived image quality. This is because, as a rule, colors appear brighter to us in memory than they were in reality.

The resulting characteristic

All of the above factors boil down to the fact that the photographic process must first distort the contrast so that we perceive it as required or as expected. Each criterion on its own requires a greater or lesser amount of contrast enhancement. In total, the following gamma values result for the various output media:

Color print: 1.3-1.4
Slide: 1.8-2.0

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Factors we must take into account to meet requirement 0: Factor 3 – Image quality

The resulting characteristic

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