how a photograph is made:

∑ exposing the image: the meter of the camera measures the light reflecting off the objects in the scene and suggests an exposure setting (shutter speed and aperture setting/f stop that will balance the highlights in the scene with the shadow detail. The exposure setting gives you an accurate rendering of the ‘middle gray’ areas of a scene that is composed of equal parts highlight and shadow information. The recommended exposure is determined by the light sensitivity of the film (the speed at which it responds to light measured by ISO) and the available light.

∑ the light reflecting off the subject is focused through the lenses onto the light sensitive surface of the film. the light sensitive emulsion is chemically altered by exposure to light and will create a latent image corresponding to that which has been focused through the lens.

∑ light areas in the scene will reflect more light causing a greater chemical change and a greater density in the negative. The darker areas in the scene will not cause the same reaction and so will be less dense, more transparent.

∑ in development, the unexposed light sensitive material is washed away leaving only areas exposed to the sun. the highlights in the scene will appear darker on the negative than the shadow areas, which will be clearer, relative to the exposure. Once printed, these values will be reversed and the scene will look like it did when you composed it in the viewfinder.

∑ If the scene has more highlight areas in it, the recommended exposure will likely be too low to record the highlight accurately because your camera “thinks” all photographs are made up of equal parts highlight and shadow and the recommended setting will always provide you with an exact rendering of the middle gray values in a scene. Similarly, if the scene has few or no highlight areas, the recommended setting will give you an overexposed negative.

∑ If your original scene has more highlights than shadows or vice versa, more shadow than highlight areas, you will need to alter your exposure setting accordingly—increasing or decreasing the recommended exposure by f stops corresponding to zones 0 – 10 of density (middle gray is zone 5, white with no detail is zone 10, black with no detail is 0).