A good way to analyze an image is to measure different locations (densities or intensities) and then draw these measurement points using graph paper. When done, the diagram resembles a half bell curve. We divide this curve into regions. The lower part is called "toe". This graph graphically indicates that the image is starting to form slowly. Next, the area of the straight line will be referred to. This part of the graph graphically shows a proportional response to the exposure light. We can measure the angle of the straight line with a protractor. When the image contrast is low, the angle of the straight line is pressed. If the image has a high contrast, the straight line shows that fact. Its angle will be 45 ° or larger.
Historically, we have transformed this angle into a tangent (TAN) using trigonometry. Most images with sufficient contrast have a straight line with an angle of about 40 °. The tan of 40 = about 0.8. This is traditionally the target contrast for images that show a respectable contrast. We're talking about the science of sensitometry – taking test shots and the science of densitometry – measuring photographic images. It is common to call the TAN of the angle of the straight line "gamma".
The following graphic refers to the film – digital images are a subset of the film when it comes to the determination. we also draw these.
35 = gamma 0.7 = flat 45 = gamma 1.0 = contrast