If you look at the main aspects that govern exposure, you would obviously realize that there are three. Three pillars unto which all exposure constructions are resting. The number three is auspicious, in religion as in photography. It is also pertinent to mention here that the number three is a recurring phenomenon in photography. Today, however, we are concerned with one of these pillars that are fundamental to all photography – ISO.
What is ISO?
ISO is an acronym. It stands for International Organization of Standardization or I.S.O. It is an international body that standardizes a lot of things. Before digital sensors this would have been ASA or American Standard Association. That was back in the days of film. What ISO does, basically, is it tells how sensitive the image medium is. During the days of film, obviously, it was the sensitivity of the film and now it is the digital sensor.
So how is ISO measured?
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ISO is measured in numbers. In the days of film, ASA 50, 100, 200, 400 and even 800 would have been general placed; standard film sensitivity. The same numbering system has been retained when the standard moved from ASA to ISO. Higher numbers mean higher sensitivity to light and lower numbers mean the exact opposite. In the older days there were special purpose films which could go even further. The usability of images shot using such special purpose films, for general photography that is, was not that much.
Cut to 2015 and there are cameras that has a native five digit ISO sensitivity that can be further pushed using software. The Canon 5D Mark III, e.g., can shoot at a native ISO of 25600. At such high ISO, low light is no longer a problem. With film camera, professionals such as wedding photographers per se, were stuck with a small elbow room. You would shudder looking at an older edition of Martha Stewart’s Wedding magazine. Red images which appear to have been doused in red wine were commonplace; and this is the most definitive wedding magazine of the time. It’s incredible what modern technology can do.
Interchangeable ISO
In as much as image medium sensitivity is concerned, digital technology has greatly improved that aspect of photography. It is now possible to shoot 10 – 20 or more images, with each one having a different ISO. Back in the days of film you could only do this if you were prepared to waste a lot of film. Hypothetically, assuming that there were usable films of 20 different levels of sensitivity.
What is native and expanded ISO?
A digital camera has a native range of ISO and beyond that camera manufacturers boost sensitivity using software. The sensor’s defined range of sensitivity is denoted as native ISO and anything beyond that is termed as expanded ISO. Though camera manufacturers leave no stone unturned trying to drum up the incredible ISO range of their cameras, it is a completely different thing whether images shot at ISO 25600 ISO or even ISO 51200 are at all usable.
Higher ISO increases noise
What does happen when you increase the ISO of your camera? The sensor increases the gain. In other words it tries to boost the light information that it sees. Now, each camera sensor is designed keeping in mind the kind of signal that it can actually handle. You cannot expect to push a 1600 max native ISO camera to shoot beyond a five digit ISO. It is not designed for that. Also, at that ISO the amount of noise (digital) would be too much making an image unusable.
Additionally, smaller sensors suffer more from noise related issues than their larger brethren. Larger sensors have more space between each pixel. It allows for smoother capture of light. Smaller sensors have the same pixels more tightly packed leading to a significant amount of loss in light.
Up-side to higher ISO
Higher ISO is not always a bad thing. At higher ISO you literally increase your chances of making a sharp image with minimum blur, even in low light. Imagine what you can do with an ISO of 51200!
Measures to counter noise in digital photography
To avoid the problem of noise most photographers opt for a smaller ISO number. Noise generally happens when you try to increase the exposure for dark / shadow areas in an image. Image information is overshadowed by noise. Another method to avoid noise is to use a long exposure, with Long Exposure Noise Reduction (LENR) turned on.
The other method involves post-processing and include both tweaking the luminance and the color sliders under ‘Noise Reduction’ to remove color and luminance noise. Not dragging the sharpness mask can also help to avoid noise but more than that artefacts. Artefacts is another side effect of higher ISO.
What is the relationship between ISO and the rest of the exposure parameters?
ISO, by now you must have realized, has a bearing on the exposure settings. When you switch from ISO 100 to ISO 200 you are basically doubling the light sensitivity of the sensor. That allows you to increase the shutter speed from say, 1/100 to 1/200 of a second. If you can move all the way to ISO 1600 and still your camera produces usable photos, you can shoot at a shutter speed of 1/1600 of a second! In a low light situation that is an incredible advantage.
ISO has a similar relationship with aperture. Increasing the ISO has the same effect as using a smaller f-stop, minus the shallow depth of field.
Rajib Mukherjee
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