ISO: It’s Not What You Think

Overview

ISO is the camera setting that seems to cause the most confusion among both new and experienced photographers. The confusion is due, in part, to the plethora of misinformation on the internet about ISO and how it works.

In our two-part series on ISO, we hope to clear up the confusion and debunk the common myths surrounding ISO.

Here, we provide a general overview of what ISO is and what it is not, but without diving too deeply into technical details. The second article covers more details for those who want them.

We remain descriptive and non-technical throughout. These articles are for photographers who want to better understand the ISO setting and know more about how their digital cameras work.

The Basics

Before we get started, let’s dispel one (very) common misunderstanding right off the bat. When discussing solid-state detectors found in digital cameras, ISO settings other than base ISO do not change detector sensitivity. Yes, it’s true – let that sink in for a moment. You may have heard that ISO changes sensor sensitivity, but it does not.

The ISO setting changes the brightness of an image. The higher the ISO setting you use above your base setting, the brighter the image will be when you read it out from your camera.

That’s it. That’s all ISO is. Now that we have dispensed with that, we can get into the subject in more depth to understand how ISO works.

A familiar example from a similar system might help to get us started. You have probably tuned your radio to a station with a weak signal and found that you could barely hear the voices or music being broadcast. Instead, you heard a lot of static, with intermittent music coming through. The static is noise. It is what your radio’s output consists of in the absence of a signal.

Noise is always present, even when the signal is not. When the signal is weak, the noise tends to overpower it, making the music hard to hear, but when the signal is strong you might not be aware of any noise at all. Still, it’s there.

You can try to overcome the weakness of a signal by boosting the volume of the radio, or in other words, by increasing the output gain. But if you do that, you generally find that you cannot hear any better than before. This is because the static becomes louder right along with the signal you are trying to hear, drowning out any increased signal.

Raising the ISO setting on a camera has the same effect as increasing the volume on a radio. It makes the image “louder” if you like, but it does not boost the signal more than the noise. Sometimes, when the noise is not too bad, increasing the ISO (gain) can be worthwhile, just as increasing the volume of a weak radio signal can sometimes make it easier to hear.

However, if the signal level is too small in comparison to the noise level, no amount of amplification will help. The noise will be so strong in relation to the signal that you simply will not be able to compensate for it this way. If you try, you will see a bright but noisy (grainy) image, not necessarily a better image.

So if ISO really only increases the brightness of an image, not a camera’s sensitivity, why is there all this confusion about what ISO does? Well, a lot of it probably arises from the transition out of film and into digital photography. When this transition happened, camera makers and photographers brought over much of the terminology they had used for film and applied it to digital. That is sensible in some respects, but it also causes confusion. Digital sensors and film both allow us to record light, but they do it in very different ways.

We will first discuss how ISO is used for film, since that seems to be the root of current misunderstandings. After, we’ll discuss how ISO is used in digital cameras and see how the two can be compared. We assume that readers understand the basic ideas behind exposure and how shutter speed and aperture affect it.

Sensitivity of Film

There are still many types of film available, each used in different conditions. Some films are used in low-light conditions, some are used in full sunlight. Others are used in specific situations, like filming a movie. Film manufacturers do this because, depending upon the photographs being taken, it is not always possible to compensate for the available light by changing the aperture and/or shutter speed.

For example, when photographing a sporting event, a situation in which people and objects can be moving quickly, a fast shutter speed is often desirable; it can reduce motion blur or even stop the action entirely. But a fast shutter speed will decrease the amount of light entering the camera, and thus lower the exposure.

Opening the aperture is the usual way to compensate for decreased light, but it might not work in every case. In some situations, a photographer might want an image with a large depth of field. That demands a small aperture.

So constraints on aperture and shutter speed can prevent us from obtaining the necessary level of exposure given ambient light conditions and other aspects of the image being taken. To address these varying conditions, film manufacturers make films with different light sensitivities.

The film with the lowest sensitivity to light is given a low ISO number, say ISO 100 or even ISO 25. These are for relatively bright conditions. Films with higher light sensitivity are given higher ISO numbers, like ISO 400 or ISO 800. These high-ISO films are appropriate in conditions where exposure levels are low.

And who gives these films all these numbers? The International Organization for Standardization.

The International Organization for Standardization developed a standardized system of rating film sensitivities so that photographers around the world could compare films from different film manufacturers.

For example, a film with an ISO rating of 200 is twice as sensitive to light as one with ISO 100, which was twice as sensitive as a film with ISO 50, and so on. This organization rates lots of things, not just cameras and film. You can check their website to see all that they’re into. It’s actually pretty interesting.

So, you might ask, if taking photos in low exposure conditions can sometimes be a problem, why not just make all the film very light sensitive? Problem solved once and for all! Sadly, it’s not that easy. There are trade-offs between film sensitivity and image quality.

Image Quality and Film Sensitivity

To understand how these tradeoffs arise, we must know how film records light. A film is really a transparent sheet of plastic with a light sensitive, clear chemical emulsion on one side. The emulsion is spread into a thin film, hence the name. The details of how this emulsion reacts with light (the photo-chemistry) are not vital here. We just need to know that when light hits an area on the emulsion it induces a chemical reaction that causes that part of the emulsion to become dark.

In emulsions with high light-sensitivity, a small amount of light causes a large area to darken. That, in turn, allows a discernible image to be created. Because the high sensitivity emulsions create large areas of exposure that are easy to see individually, these images have a grainy, “noisy” look.

For film emulsions with lower light-sensitivity, a large amount of light causes only a small area to darken. So images made from these emulsions have finer grain, giving them a more detailed and smooth appearance. Over time, people came to appreciate the grain in its own right for the “artistic” aspects it gives to photographs in certain situations. So using low or high- sensitivity film is sometimes a matter of artistic choice, not just a practical one of exposure level.

ISO and Sensor Sensitivity

The aspect of digital cameras that most closely approximates ISO in film is the base ISO number for a camera. Like film, the base ISO determines the baseline light sensitivity of a camera’s sensor. The International Standards Organization tests each camera with a standardized procedure to determine the camera’s intrinsic light sensitivity, and then assigns a base ISO for every camera.

So you can think of film sensitivity and the base ISO of a digital camera as similar measurements of light sensitivity. For example, a camera with a base ISO of 100 should provide image tone similar to ISO 100 film at the same light exposure level. By “tone” we mean that the exposure should look right, not under or overexposed.

That explains the base ISO of a camera. What about all the other ISO settings? Here is where the confusion often arises.

What Do the Other ISO Settings Do?

You have probably noticed that you can obtain the “proper exposure” for an image by using the right combination of shutter speed, aperture, and ISO. Sometimes this trio is referred to as the “exposure triangle,” a problematic phraseology, for reasons that will become apparent.

The shutter speed determines how long the sensor is exposed to light. The aperture determines how large the opening is through the lens. Together they set the amount of light entering the camera and exposing the sensor.

Or, in other words, the shutter speed and aperture alone determine the light exposure level. They must be balanced to take into account the sensitivity of the sensor, whether film or digital. Changing the ISO setting on a camera away from the camera’s base ISO setting certainly does not change the amount of light the sensor is exposed to.

So what does the ISO do?

For all but the base ISO, the ISO setting just determines how bright your image will be for a given shutter speed and aperture combination: it does so by boosting the brightness artificially – not by changing the sensor sensitivity.

Sensor sensitivity is a fixed quantity given by the base ISO. Changing the ISO setting actually changes the output gain of the sensor upon readout. So changing the ISO setting from 200 to 400 makes the image twice as bright. This has the effect of adjusting the image such that the exposure looks more acceptable in terms of image tone. But just as with increased ISO in film (or the volume on a radio), it comes at a cost. Increasing the gain increases both the noise and the signal, resulting in an apparent graininess.

This is strikingly reminiscent of the increased grain in film at higher ISO values. However, the origin of the grain in digital images is different than it is in film. The two are related at the most basic level: you cannot get a high quality output signal if you don’t have a high quality input signal.

To understand more about how camera sensors work to amplify light signals and how digital noise is produced, please read the next article in this series.

Summary

Here are the main takeaways from this discussion on ISO.

1. In particular, and to emphasize greatly, changing the ISO setting does not change a camera’s or camera sensor’s sensitivity to light. The light sensitivity is a fixed property of the camera and sensor. And why would we ever operate a camera at a sensitivity lower than the maximum in the first place? What would be the advantage of that? If you cannot think of any, neither can I.
2. The base ISO of a camera gives a standardized indication of that camera’s intrinsic sensitivity to light. This intrinsic sensitivity is a fixed property of the camera. It cannot be changed by any of the camera settings. The base ISO allows us to compare different camera models in a useful way in terms of their light sensitivity, should we even want to do that.
3. The base ISO of digital cameras is a number that relates the light sensitivity of current digital cameras to the light sensitivity of the films that were used with the previous generations of cameras. The various ISO settings on digital cameras are created in a way that the resultant images are similar, in terms of tone, to the images that would be produced by film of the same ISO number.
4. In the case of film, the ISO number really does tell you the light sensitivity of the film in some standard way.
5. In the case of digital cameras the ISO tells you:
   • the intrinsic sensitivity of the camera (for base ISO) or
   • the output gain of the detector (for all other ISO settings).

Hopefully this article helped you understand how ISO works and why higher ISO settings tend to give grainier images. Clearly, understanding how ISO works will not be the difference between being a good photographer and not being a good photographer. Many, many great photographers are not experts at the technical details behind their cameras.

Nonetheless, having a deeper understanding of the technical aspects of cameras and how they work can improve how you think about making your photographs and give you more control over your results.

If you want to dive in a little bit deeper to learn about the sources of image noise, please enjoy our second article in this series on ISO.

If you have any questions about ISO, please leave them in the comments below!