As we saw in the post on the triangle of the exposure, the ISO is one of the factors that determine the amount of light captured by the sensor of a digital camera, or more precisely, in digital photography bold ISO indicates the sensitivity of the sensor to light. The higher the value of the ISO, the higher the sensitivity to light, in analog photography (remember when we brought the films develop, to change ISO it was necessary to change film, today just press a couple of buttons.
As aperture and exposure time, also the ISO proceeds to stop, where each value of the scale of the ISO is equal to twice the its predecessor, the minimum value of ISO as possible to the day d’ today is 50 and the scale of the ISO is as follows: 50 100 200 400 800 1600 3200 6400, and those just listed, are the stop integers, the digital cameras allow you to set third also stop.
What is the most important utility of the ISO?
The situations in which we no longer need to act on the ISO are those in which the available light is poor, for instance to closed environments or to photo evening outdoors.
Because of the interdependence between the elements of a triangle of exposure, maintaining the ISO, to obtain a correct exposure in these situations, we should:
•Raise the exposure time, risking to obtain a photo moved or wakes moving objects,
To avoid these two solutions, we can then raise the ISO, up to which the values of openness and exposure time are similar to those that we want.
But then, it is always possible to eliminate a low light shooting up to the ISO?
ISO and noise
Unfortunately, increase the ISO, and then the light sensitivity of the sensor, amplifies the signal received from the sensor, and then also the noise that it is normally present; therefore, photos with values of ISO high may have high levels of noise, but what is the noise in an image?
The noise is manifested in the photos mainly as a loss of sharpness and the appearance of colored spots, especially visible in correspondence with the dark areas. High noise levels can make a photo totally unusable, fortunately, in some cases, the noise can be exploited our advantage, in fact, especially in black and white noise may seem a photo aged or as if it had been captured on film.
Values Acceptable of ISO
The ratio between noise and ISO varies, depending on the model of camera to the present day are on the market camera which allow to obtain acceptable noise levels with values of ISO very high (3200 for the Nikon D3s, one of the most advanced models). This is, however, of cameras extremely expensive, the cameras digital SLR of low level easily produce excessive amounts of noise already above value of ISO 800, and then, if you do not possess a camera semi-professional or professional, we are careful to avoid the high values of ISO or if appropriate, we refer to the reviews on the Internet to decide which values will produce acceptable noise levels, and in every case, accumulating experience with our camera, we will be able to identify the correct values of ISO applicable to the situation in which we find ourselves, for this purpose, an exercise useful can be precisely that of taking pictures of objects of different colors trying gradually to increase the ISO. Looking at these photos on the computer screen we can become aware of how much noise has been added to each picture by a particular value of ISO.