Ever wondered what camera ISO is? It is a measure of how sensitive the camera sensor is to light. How that sensitivity affects the image you are taking and how that impacts the exposure triangle is something that you will need to know if you are interested in photography. Read on to learn more!
What is an image sensor?
An image sensor is a fairly complicated device with various implementations and methods for converting visible light into electrical impulses, each with its own set of pros and cons. We will not be discussing the various implementations of image sensors, rather we will discuss image sensors generally and in the context of photographic cameras.
An image sensor is the part of the camera that actually “sees” light and converts that light into electrical impulses that can then be stored as information. The sensor is divided into numerous separate photosites that we refer to as pixels. Each pixel on a sensor is the same size as the other pixels on the sensor. The number of pixels on the camera sensor is where the term megapixels comes from and is a reference to the number of million pixels on the image sensor.
Sensors themselves come in different sizes as well. Obviously, sensors used in cellular devices are smaller than sensors used in professional photography equipment due to size restrictions. Given typical image sensor sizes at the time of this writing, a sensor in a professional mirrorless camera might be twenty-five times the size of the image sensor in a cellular phone camera.
What is ISO?
The term ISO comes from the International Organization for Standardization. The organization chose ISO for their organization based on the Greek term “isos”, which means equal. The organization sets and maintains numerous industrial standards around the world, including photography standards. The previous camera film standard, ASA, was used in creating the ISO rating for camera film back in 1974 and ISO has been in use for determining the light sensitivity of film since then. Since the light sensitivity of film (or a sensor) is a pillar of photography, the ISO convention carried over into the digital world.
Unfortunately, there is no perfect ISO setting to use. If there was, I’d have put it up at the beginning of this article and life would be much simpler. Instead, you will find that ISO is very much situational depending on the type of photograph you are trying to take as well as the lighting conditions of the scene.
Originally, ISO referred to the sensitivity of camera film to light. A film with a low ISO, like ISO 100, was less sensitive to light than say ISO 1600 film, which was more sensitive to light by a factor of five times more sensitivity. The ISO 1600 film was considered a “fast” film because it was more sensitive to light and needed a “faster” shutter speed to properly expose the image.
In the film days, the speed of the film was controlled by the size of the “grain” in the film. A slower film (ISO 100) would have smaller “grains” than a faster film (ISO 1600). Using an excessively fast film would lead to having more grain in the image than necessary. Using too slow of a film, which is less grainy overall, might lead to other issues such as camera shake introduced by lower shutter speeds.
In modern cameras, of course, we no longer use film. We use digital image sensors in place of film and in many ways these pixels behave much like the grain structure in film. When the ISO sensitivity of the image sensor is increased, each pixel becomes more sensitive to light. The mechanism of how this happens isn’t as important as knowing that, just like with film, pushing an image sensor ISO higher will introduce unwanted noise into the photograph.
As ever, it is always advisable to shoot at the lowest ISO possible for the conditions of your scene.
What do ISO numbers mean?
You will find that the ISO of film or the ISO sensitivity of an image sensor is listed as a number, but what does the number mean? Let us first talk about the base ISO, which we will call 100. ISO 100 is the standard ISO that film cameras used and it is mostly true of image sensors in modern cameras as well. ISO 100 will therefore be the starting point for this brief discussion.
As you can see in the image above, the ISO numbers are chosen in such a way that they double or halve as you move from on end of the chart to another. For example, after ISO 100 comes ISO 200. After ISO 200 comes ISO 400. The meaning of this is simple: the sensitivity of the image sensor (or film) doubles or halves as the ISO is increased or decreased, respectively.
So, an ISO 200 setting is twice as sensitive as an ISO 100 setting. An ISO 400 setting is twice as sensitive as ISO 200. Conversely, ISO 400 is half as sensitive to light as ISO 800, and so on. What an ISO does and how it affects sensitivity to light is one of the most important concepts to the beginning photographer.
You can also get an idea from the above image for the relative effect of increasing ISO on the quality of the resulting image. There are always tradeoffs, and the tradeoff for increasing ISO too much is reduced image quality. The rule is to keep ISO as low as practical to achieve the image you seek.
As a side note, you will notice in your camera journey that settings on a camera often halve or double light (or sensitivity to light).
What does having a low ISO do for my photograph?
Shooting at a lower ISO does some desirable things for your photograph:
- Tones and colors will be of the highest quality
- Minimal noise
The colors in your photo will simply look better at lower ISO ranges and noise in the image will be minimal. Low ISO settings (100-200) are recommended for well-lit outdoor scenes during the day where light is abundant and the light-gathering ability of your camera’s image sensor is not a factor, such as a ball game during a cloudless or partly cloudy day.
There are two main disadvantages to shooting images with a low ISO:
- Camera shake when hand-holding causing blurriness
- Slow shutter speeds cause motion blur of moving subjects
In very well-lit scenes these will mostly be non-issues. Photographing a baseball game outside on a sunny day might present some challenges, but using an ISO of 100 will not be one of them. Using an ISO of 100 to shoot that same game at sunset or under artificial light could be a problem. As the light at the field begins to fade, the shutter speed will necessarily get slower to the point where too much motion blur is introduced into photographs and they become undesirable. One solution to this problem would be to increase ISO.
What does having a high ISO do for my photograph?
There are some advantages to a higher ISO as well:
- Less camera shake when hand-holding for sharper images in low-light
- Faster shutter speeds to freeze moving subjects
These advantages allow you to get better photos in less than ideal light. Instead of a photo that is blurry, the image can still be captured.
Of course, a higher ISO has some disadvantages as well:
- Less realistic colors
- A decrease in image sharpness and quality
These are the trade-offs that must be weighed when selecting an ISO on the higher end of the spectrum. The higher the ISO goes, the more pronounced the disadvantages become.
How does all this impact the exposure triangle?
ISO is one of the three sides of the exposure triangle, the other two being shutter speed and aperture size. In my opinion, you should make changes to the ISO last out of the three sides of the exposure triangle. In other words, keep the ISO as low as possible while still being able to capture the image the way you imagine.
This is easy on landscape or architectural photography, the subjects are static and unmoving so ISO isn’t much of an issue. Place the camera on a tripod and use as long of a shutter speed as necessary to capture that image. ISO could be kept very low in those instances where the subject doesn’t move and the scene doesn’t change.
Where ISO becomes more of a consideration is with moving subjects or scenes with moving parts, such as tree leaves or cars in the background. In these cases, simply putting the camera on a tripod would only result in a blurry subject or background.
The ISO setting on your camera is one of the three ways to compensate for less than ideal scene lighting. In well-lit scenes, ISO isn’t much of a concern since light is so abundant. In dimly-lit scenes, ISO becomes a concern to the photographer but it is dependant on the type of scene that is being photographed.
As a general rule, ISO should be the last setting used to compensate for a dimly-lit scene.
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