The exposure in photography
In this article, we will focus on the essence of photography: light. Light is electromagnetic radiation which may or may not be perceptible to the human eye. The way in which this light is captured when taking photographs will necessarily have an effect on the final result. To act on the capture of light, a set of photography techniques has evolved over the years.
Light illuminates elements to make them visible to the human eye. Without light, there is no photography. And even with the latest camera, without analysis and interpretation of light, it is almost impossible to take good photos. Knowing how to manage light helps improve the quality of photography. In this article we will discuss the essential treatments to “shape” this light.
The histogram of this photo indicates very dark pixels, therefore an underexposed photo (here deliberately in order to bring out the details of the lens).
One of the most important things in photography is to check the exposure of an image and correct it if necessary. Exposure represents the amount of light that will reach the camera sensor; it is therefore an important parameter in taking photos. Exposure can be expressed in the form of the aperture of the diaphragm and the time during which the sensor is exposed to light (exposure time). An overexposed photo is a photo where too much light has been captured: this photo is called “bright” (highlights). Conversely, an underexposed photo has only captured a little light: it is therefore “dark”.
By taking a look at the histogram of the image, we can see the good or bad exposure of our photograph. But what is the histogram? Present in current digital cameras as well as in editing software, it graphically represents the distribution between light and dark tones. This histogram must be well centered in order to have a well exposed photo.
However, you should know that the histogram strongly depends on the type of scene photographed. The perfect histogram does not exist. Indeed there is no good or bad histogram, it is up to you to make good use of it for a better rendering according to your wishes.
To correct a photo that is too dark, we can modify the exposure by taking a larger focal aperture for example. Smaller focal apertures let in less light and larger focal apertures let in more light. The aperture will correspond to the opening diameter of the diaphragm. This diaphragm will be used to regulate the light which passes through the lens and will also influence the rendering of the image.
Adjusting the shutter speed is an essential parameter in photography since it determines, with that of the aperture, the correct exposure of your photo, its overall sharpness in certain cases and of course the sharpness of the subject. This is the speed at which the camera shutter closes. For a good exposure, a slower shutter speed is recommended.
ISO sensitivity can be adjusted in addition to exposure settings to modify the latter. This ISO sensitivity represents the sensitivity of the sensor to light, so the higher it is, the more it will increase the exposure of the shot; the photo will therefore be brighter. However, this parameter may cause digital noise to appear (image degradation phenomenon with a “grain” effect).
ISO sensitivity influences noise (grain) in your photos. The lowest ISO sensitivity will provide a relatively normal exposure. Low sensitivity can be used when sufficient light is available.
To conclude, the different parameters that we have just seen allow you to play on the exposure. The parameters to choose depending on what you want to do are as follows: the aperture of the diaphragm, the shutter speed or even the ISO sensitivity. Nowadays, some of these settings are managed automatically in many existing photographic systems, notably mobile phones but also with editing software.
These image treatments, carried out in real time, are part of the library of functionalities thatDeverne offers in its embedded vision system design services.
