There are many kinds of f laws that make a photo scream “amateur,” but poor tone and contrast characteristics are among the most common. Amateur photographs often have a lot of midrange contrast, with tones in the highlights and the shadows pushed far towards the pure black and white. Faces suffer especially. They acquire hard lines and sunken eyes, aging a person 10 years, and excessive contrast can make a placid expression seem glowering.
Fortunately, problems like these can be easily fixed in the computer (unlike, say, awful focus or composition).
The Shadow/Highlight adjustment
Difficulties arise when you want to improve both shadows and highlights at the same time. You could do that with a curve shaped like the one in Figure 1. This kind of curve expands the contrast in the highlights and shadows but compresses tones in the midrange. Subtle adjustments can work well, but strong curve changes like Figure 1 don’t usually produce such attractive results. The midtones become so compressed and low in contrast that the print looks flat and lifeless.
By way of example, I applied this Curves adjustment to Figure 2, left. That photo preserves the original quality of an on-camera flash from the 1950s, but toning the harsh highlights down and bringing out the details that were lost in the shadows would give the photograph a more professional look. The Figure 1 Curves can do that (Figure 2, middle), but look at what happens to the midtones, most obviously in the tablecloth and faces. They look flat and artificial, as if someone just painted the tones and colors onto the photograph.