ISO sensitivity is, along with diaphragm aperture and exposure time, one of the three elements that define exposure in your photos, known as the famous exposure triangle.
ISO sensitivity marks the amount of light our camera needs to take a picture. This concept, which comes from conventional photography, is maintained in digital photography, although its fundamentals are somewhat different. In the following article, I will explain them to you. Do you want to know more?
The Origins of ISO Sensitivity in Photography
Although it is used less and less, ISO sensitivity is a concept we have inherited from analog photography. Photographic films consist of silver halides, millions of light-sensitive transparent crystals, grouped together. We could say that they would be roughly equivalent to the pixels of the sensor of our digital camera. The size of these crystals is what marks the sensitivity of the film and the grain that is appreciated in obtaining the developed copies.
With the move to digital photography, the concept of ISO sensitivity was retained, although the operation of the sensor does not have much to do with this aspect, nor does the result.
ISO in Digital Photography: What Is It and How Does It Influence Our Photographs?
As we have told you on other occasions, light is the most important element when taking photographs. To control the light that enters our camera, we have 3 fundamental parameters, known as the “triangle of exposure:
- Shutter speed
The main difference between the ISO and the other two parameters is that both the shutter speed and the diaphragm naturally let more or less light into the camera’s sensor, while ISO digitally amplifies the signal, making us gain more light but at the cost of losing image quality.
Therefore, whenever possible, it is advisable to leave the ISO as low as possible (100 or 200 depending on your camera model), and modify only the shutter speed and aperture parameters. However, this will not always be an option, as we may find ourselves in various situations where we will be forced to increase the ISO if we want to take a well-exposed photograph:
- When there’s too much darkness.
- When we want to freeze a movement and we can no longer open the diaphragm.
- When we want to close the diaphragm enough to gain more depth of field.
- When we want to get more stars in our night photography.
Noise when increasing the ISO: what is it and why is it produced?
As I told you, by increasing the ISO to gain more light, we are going to generate noise in our photographs. The noise is that kind of grain that appears especially in the darkest areas of the photo. To understand the concept of noise and how it is generated, we must first understand how the capture of images in our camera works.
All digital cameras have a sensor, usually CMOS or CCD type. The sensor of our camera is the chip in charge of capturing the image. It is made up of a mesh of thousands of photosensitive cells in which the image formed by the lens is received.
Each of these cells generates an electric current in the presence of light. This electrical current will then be converted into numerical data that will be stored in binary digital form in the camera’s memory giving rise to a pixel. Thanks to the sum of all these pixels we will obtain our final photograph.
In addition to being activated by electric current, each of these cells generates a more or less fixed amount of electric current (and therefore data) at random, even in the absence of light and in relation to temperature (the higher the temperature, the more it will generate). These random and content-free data are the much-feared noise.
So, as I said before when we raise the ISO what we do is not to increase the sensitivity of these sensor elements, but by a later amplification of the signal, they emit. As these elements have a more or less fixed base signal emission, by capturing a weak light signal and amplifying it, we are also amplifying a good portion of the random data emission of the chip, which will mix a quantity of random signal without content to the signal corresponding to the image. And that’s why when we increase the ISO we also increase the noise in our photos.
ISO and Noise in the Digital Edition
You might think then, after knowing how the noise is generated in our camera, that it would be worthwhile to underexpose a photograph rather than increase the ISO, so as not to generate noise. Well, you’re wrong. A badly exposed photograph will always have a lower quality than a well-exposed photograph. And trying to expose it later in programs like Photoshop or Lightroom will always generate more noise than if we had exposed it correctly when shooting, even if it had been increasing the ISO. So, raising the ISO is not bad, you just have to control it and know how far your camera can go.
Each camera has a different noise treatment, and it’s up to you to consider which ISO you can upload to your camera and take pictures with acceptable noise. We can always reduce the noise a little in editing programs, but we will lose definition and if we go too far we can blur the picture so much that it ends up looking like an oil painting. So, unfortunately, noise is an enemy that we will always have to deal with.
However, we will not always consider him “our enemy”. Today’s vintage nostalgia has led many photographers to add noise to their photographs with digital editing software. By adding noise to your photo, you can create an antique look, emulating the grain so characteristic of analog photographs. So, you’ll be the one to judge how much noise suits your photos best.
- The highest image quality with a digital camera will be obtained by using it at its lowest equivalent ISO sensitivity.
- The use of higher ISO sensitivities will result in an increase in randomly distributed pixels, mainly in the shaded areas of the image. Noise, unlike grain, will not be proportional throughout the image but will be more evident in dark areas.
- Noise manifests itself more in some channels than in others. Normally the blue channel is usually the one with the most noise. This channel can be edited later with an editing program to reduce the noise by applying filters.
- But be careful: it is fundamental to make a correct exposure in our photo regardless of the ISO we have selected because using low ISOs at the cost of under-exposing a photo in excess can result in worse quality photos than the equivalents well exposed with higher ISO.