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ISO (Signal amplification)

The word ISO is derived from analog cameras and was used to measure the film’s sensitivity to light. In low light conditions, film or a roll of film with a high ISO was used. With the advent of electronic sensors on the cameras, the sensitivity is an inherent feature of the sensor that cannot be changed like a roll of film because it is determined by the sensor, while ISO can be changed by the amplifier.

The light passes through the lens and reaches the electronic sensor that transforms it into an electric signal. The signal is then processed by the amplifier and is amplified. To increase the ISO in digital cameras means to increase the power of the signal with the amplifier. It is just like increasing the volume of a stereo. If we’re listening to a song and cannot understand the words, we can raise the volume to hear better. If the light is low, we can increase the ISO so that we can “see” better.

With music, increasing the volume of a song also increases our distortion, generating a “rustling” noise. In digital photography we often talk about ISO, but it would be better to speak of ISO equivalent (equivalent to the sensitivity of old rolls of film) or of amplification. However, we still express the ISO on digital SLR cameras using the same values that were used with rolls of film. So if the amplification of the signal reaches a level similar to that of the ISO sensitivity of a roll of film, it is given the same value.  The ISO values are not continuous numbers but are quantized in steps. Depending on the quality and price of the camera, there will be a range of more or less high ISO values available along with a variable number of “steps”. For example, when two cameras have an ISO range from 200 to 3200, this does not mean that the intervals for both cameras are the same. The most economical one may have ISO values of 200, 400, 800, 1600, 3200 while the second model, the more expensive, may have 200, 250, 320, 400, 500, 640, 800, 1000, 1250, 1600, 2000, 2500, 3200. If the range is the same, having more steps is much better because, if it becomes necessary to increase the ISO, we could increase it little by little to avoid increasing the noise. In fact, if with both cameras we were at ISO 400 and we need to increase it, with the first, more economical, it would immediately at 800 while with the second, more expensive, it would be at 500, with all the benefits that we will see in detail later.

Besides having an ample subdivision of ISO values, it would also be advantageous to have a wider range of ISO values (such as 6400 or more). Having the ability to chose the amplification level of the signal is like having many different rolls of film with varying ISO’s at your disposal. This is very useful because with the simple push of a button, you can change the ISO at will, thus making the sensor more sensitive depending on the light conditions present.

With this in mind, what follows in the rest of this chapter applies only to the ISO of sensors in digital cameras, not for film.

Every reflex gives its best in terms of image sharpness to the lowest standard ISO. In order to understand this phenomenon, we have to introduce the concept of “electronic noise”. The electronic noise is generated by the sensor during the process of transforming the light to electricity.

This is a physical phenomenon that cannot be avoided. Sensors of various quality can have different signal/noise ratios, but every sensor will always have a certain electronic noise due to the various factors of how the sensor was constructed as well as the physics involved. The signal transformed into electricity produces noise. The signal amplification (using a higher ISO) also creates further noise. For example, let us use some fictitious numbers to illustrate this point. If we have an ISO value of 200, our signal would be 10 and our noise would have a value of 2 (a signal to noise ratio of 5). If we double our ISO to 400, our signal would be 20 with a noise of 4+1 (1 due to the amplification). We will have a total signal that is equal to 20 with a noise value that is equal to 5 but with a signal/noise ratio that is equal to 4. This is decidedly worse. If we further increase the amplification and bring our ISO value to 800, the signal will grow but so will the noise (see figure 01). While our image will be brighter, it will not be as “clean”. Returning to the stereo comparison, if we listen to the recording of a conference with noise in the background, we can raise the volume in order to try to better understand what is being said, but this will raise, at the same time, the background noise. In figure 02 we see a diagram of the percentages that shows in a more schematic and quicker way what is shown by the graphic in figure 01.