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What’s an ISO? How does ISO work? And does having a camera with high ISO number matter?

ISO is one of the most widely debated merits of one camera over another when it comes to internet debates. For someone new to serious photography they may be confused as to what ISO even does besides make images brighter and that’s what I hope to remedy today.

What exactly is ISO besides a setting?

In simple functionality, the higher the ISO the brighter the image, but what is increasing the ISO actually doing? ISO gets it’s name from the International Organization of Standards. In the old days there were standards for “film-speed” from which the film would be called and labeled things like ISO – 100 or ISO – 400 for instance. Specialty films could go as low as ISO 32 and some went as high as 3200. In the case of film the ISO was defined by the size of the little grains of photosensitive material. A few photons were all that were needed to activate a grain and make it change color. Finer grain meant a smaller target for a beam of light to hit the film grain and therefore decreased the sensitivity of the film. If you look at the work of masters such as Ansel Adams and wonder how he could create such silky smooth photos, it was partly because he used film of very low ISO rating and shot very long exposures to make up for that. You see, the shortcoming of film is that the higher the ISO the more grain is visible.

In digital ISO refers to the same thing that film did, the sensitivity of the film, or in this case the sensor. ISO settings for a digital camera today is actually pretty close to that of film. Today we can easily push way beyond an ISO to much higher numbers such as ISO 3200 without much problem. A good digital sensor can go even higher thanks to noise eliminating algorithms. With digital we don’t get more film grain at high ISOs like film, instead we get something very similar called “noise.” As the digital sensor gets its sensitivity turned up its detected signal to noise ratio changes for the worse. Given that at high ISOs the subject is usually very dark, the number of photons hitting the sensor compared to the amount of noise generated from various sources such as heat makes for the camera to have a very hard job producing a crisp image. Algorithms can guess and approximate only so far before your image looks like snow on an old TV.

The exposure triangle

Now that we know what ISO is and what it does, we need to ask when we should change it in the camera settings. As part of the exposure triangle ISO doesn’t usually play antagonistically against aperture and shutter speed, we generally want a lower ISO because we want the sharpest possible picture. Instead ISO supports the flexibility of changing shutter and aperture settings by taking a photo that might otherwise be too dark and adding more light gathering ability to the sensor to allow for shorter exposures with higher aperture settings. Both of these things let less light in and the sensor can actually pick up the slack. Consider an example of an indoor sporting event being photographed. The players are moving quickly and blurry players isn’t an option so the exposure may need to be extremely short to catch the players sharply. In a dark environment like an arena or night game the available light may be very low. We can help let more light in by increasing the aperture but that may not be enough. The only setting we have left to adjust is our ISO, it’s gotta go up to allow for the correct exposure, we have no other choice left. Cranking up the ISO should always be the last choice in getting the correct exposure because it adds more noise, but these days doing so matters a lot less than people would think.

The primary difference between a photo taken at ISO 100 and ISO 1600 in a modern camera is barely visible. But the difference in brightness sure is. For most professionals on modern DSLR cameras, an ISO of 3200 may be just fine for a professional photo. The noise will be visible, but it’s usually acceptable and can be decreased at the expense of sharpness. The other thing to be aware of is that when you get to higher ISO settings the shadow detail (details in the shadows) will become washed out and muddy.

Below is a comparison of different ISO settings on the same subject. ISO was increased in each image but the shutter speed had to be increased as well to make a correct exposure.

Should you go for one camera over another based on how high its’ ISO can go?

As with all things the answer is, “it depends.” But generally no, you don’t need to make ISO performance a buying point. Any modern camera with adjustable ISO from any company will not limit you from taking great photos. Since high ISO introduces noise which creates a “grain” as the ISO increases, you will simply have less flexibility to use higher ISO in some situations. This is where the one exception is. If you are shooting in dark locations where you can’t use a tripod and longer exposures, you might need to choose one camera over another in this area. But this is a niche area for most people. For the vast majority photos you will never need to go above ISO 1600 and pretty much every good camera on the market these days can do great photos at that ISO.

Here's a comparison of my highest and lowest ISO settings on my Canon T6S.

One last thing to mention. High noise from high ISOs is usually not a problem when viewing an image on a screen or even matte photo papers. Where the grainyness becomes a problem is on large photo prints. The noise gives the photo a “grungy” feel. In cases of street photography or otherwise grungy settings this can be a great thing to add, however it can make a photo ugly when printed on glossy photo paper.

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