Stop will help the photographer to achieve a correct exposure of the image, even in more complex lighting conditions. To understand better what the stop represents, it is necessary to understand first the elements that intervene in the realization of the photographic image. Simplifying to the extreme, the camera is not other than a tool that allows the photographer to measure out the right quantity of light and send it off the photosensitive support (silver film or digital sensor).
The quantity of light has to be perfect; if there is too much, the photograph will come out too clear (overexposed) or, viceversa, if it is too little the photo will come out too dark (underexposed). Explained this elementary concept but of obvious importance, we analyze what are the elements that influence the brightness of the image. Practically, we try to understand what the photographer can do to control the exposure of their image.
The photographic medium is that device, chemical or electronic, that has the purpose to record the bright information. Inside the camera, it is positioned right behind the lens. Interposed between the support and the lenses, a device, called shutter, blocks the light and to maintain the support in the dark until the photographer does not decide take a photo.
The picture is taken at the moment the shutter is opened for a specific time allowing the light to hit the support which can record the image projected by the lens. The time of the opening of the shutter (called also shutter speed) is the necessary time to the support to store the image. If the time is too short, the image will not succeed to form itself and therefore the final photograph will result dark (underexposed).
On the other hand if the shutter speed is too long, the image will be formed with too much lighting and it will become too clear (overexposed).
For how much said, it is included that the exposure depends from 2 factors:
- The speed of the support to store the projected image
- The quantity of light that falls on the photographic medium
The speed of the photographic medium (called also sensitivity to light or ISO speed) it is measured in ISO. In the the digital cameras, the photographer can set ISO speed as he wants, as the ISO speed is variable in a range of values more or less extended according to the quality of the camera. In the obsolete photography chemistry, the film speed was a standard value and depended from the film that we put inside of our analog camera. Increasing the speed it is necessary a shorter exposure to light to succeed in saving an image. Or, if we maintain unchanged the shutter speed, we require less light to store the same image.
The range of the ISO speed adopts a special numerical characterization: every value doubles with respect to its precedent. This is used to simplify understanding. Every double of the value of sensibility,in fact, the speed of record of the support doubles as well. The variation between a value and its successive (or preceding) is called STOP. For example, passing from 200 to 400 ISO we can affirm that we increased the ISO speed by 1 STOP. Or, passing from 800 to 400 ISO we can say we decreased the ISO speed by 1 STOP. For all, it would seem suitable to use a photographic medium with a very high ISO speed so that it would be possible to take photographs in every situation. However, it is not quite true since to use a high ISO speed will make it impossible to photograph in conditions of strong light (for example a sunny day). In addition an high ISO speed determines a deterioration of the image quality.
Using a digital medium, we notice that the problem of the digital noise is the appearance of unaesthetic colored dots due to an electric amplification of the signal. Using instead analog supports (exempt from digital noise!) the silver grains, due to their greater dimension, become visible producing an unaesthetic grain. In both cases it is shown a considerable deterioration of the comprehensive definition of the image.
The shutter, like said before, is the device between the lens and the photographic medium that allows the physical barrier of the light to maintain the support in the dark until it decides to take a photograph. Lets leave the particularity of the structure out and lets stop, instead, on the functional peculiarity. When the photographer takes a photograph, the shutter is opened to allow the light to hit the support. The time of the opening (or shutter speed) has to be such to guarantee the correct record of the image on the support. In function of the quality of the camera, they may have also times of exhibition of 1/2000 sec. Wanting, instead, to realize superior times of exhibition to 30 seconds, some cameras have the pose B (bulb) where the shutter remains open until the photographer holds pressed down the button of release. Looking at the drawing itself one conduces that, going from right hand towards the left, the exposure extends.