Lights and Shadow and Tone in Digital Photography

We have seen that an image with a complete tonal range represents the brightest and darkest zones of the scene with according tones. For example, and image has a poor range when the sun is represented by a dirty white or when the mouth of a railroad tunnel appears as a dark gray. In both cases the tonal range is compressed, since white or black isn't enough regardless that in the context of the scene these are the tones that correspond to there respective zones. In a digital image this can be corrected by setting the white point levels and black point .

The menu Image > Adjust offers different control tools with which we can expand the tonal range: Levels, Curves, Brightness/Contrast, Automatic levels and Automatic contrast.

The first two submenus, levels and curves are the most complete, since with them we can not only regulate the light levels, the shadows and the medium tones of the image, but also the values of different channels of a color image. The other three, Brightness/contrast, Automatic levels and Automatic contrast are quick tools, derived from the first two and that simultaneously affect all the tones of the image.

When we open the box Image > Adjust > Levels we will see a grain that represents the tones of the image ( histogram ). The ends of the entry and exit range correspond to the inferior values (0 = black) and superior (255 = white) of the 256 levels an 8 bit image provides. Underneath the histogram are the regulators corresponding to the shadows and light as for the gamma and characteristic contrast. These three regulators are which establish the correspondence between the (original) entrance and (modified) exit values. In a contrasted image, with light and dark tones, the ends of the histogram are close to the ends of the scale; nonetheless, when the image of the contrasted scene is reduced to a compressed tonal range, the histograms extremes are far from the ends: the brighter and darker zones of the scene aren't seen respectively as white and black. In these cases it is necessary to reassign the original levels to new exit ones. First we adjust the white spot by moving the white triangle to the right until the end of the

histogram. Next we darken the more somber tones of the scene by moving the black triangle from the left to the end of the histogram. We can see the changes in the image is the see box is full. Depending on the characteristics of the scene, the brightest point may be represented as a reflecting white which doesn't use ink to print (exit value of 255) if it is the sun, a point of white light or a flash, or, what is more frequent, with an exit value of around 245 if it is a white surface with detail. Reflecting white reaches the point in the histogram were we place the regulator; if we don't wish for such a high value for white, we can place the regulator a little to the right of the end of the histogram. Analogously, the darkest value of a scene (left end of the histogram) can be assigned an exit value equal to 0 if it is a black with no detail or a value around 10 if it is a deep shadow with detail. In this case, we will place the regulator a little to the left of the end of the histogram. Once the adjustments have been made we click OK and is we open the level box again we will see that the tonal range has expanded (the holes in the histogram normally don't imply problems with tone loss).

These adjustments of lights and shadows can be also made with the dialog box Image > Adjust > Curves clicking on the ends of the curve and dragging them horizontally. The effect is exactly the same as in the earlier example: we reassign black (0) and white (255) to be able to expand a compressed tonal range.

Medium Tones

For a general adjustment of the medium tones in an image we can also use the levels menu, altering the gamma or slope of the curve that represents the correspondence between the entrance and exit values. The gamma regulator is situated between the light and shadow regulator and affects above all, the medium tones. We can lighten or darken an image by moving the gamma regulator to the right or to the left, therefore altering the correspondence between the entrance and exit brightness levels. The same general effect, in the curves menu, is obtained by clicking a middle point in the line and dragging it upwards to lighten or downwards to darken the medium tones. The big advantage of the curves menu to adjust the medium tones is that while the levels menu only lets us adjust the general gamma, the curves box represents all the tones of the image. Depending on the characteristics of the image we can increase, maintain or reduce the gamma in one or various points of the line, which lets us regulate (directly or with an adjustment layer) up to 15 different brightness levels. This is specially useful when increasing the detail of a zone in the image without altering the general brightness: for example, in a contrasted image where the shadows are blended, an increase in the gamma in the levels menu will produce a greater detail in these zones but will also overexpose the lights. With the curves menu we can resolve the problem modifying only the brightness level of the shadows leaving the rest unaltered. When we move the cursor, which adopts the shape of a dropper, to the zone of the image whose brightness we want to alter and click on it, we can see it's position on the curve. First we will set the sections of the curve which we want to leave unaltered, clicking on them but not moving them. Next we will drag the segment to darken or lighten in the adequate measure.