Camera manufacturers increasingly like to crow about ISO ranges when launching a new camera, with high ISO numbers now reaching into the hundreds of thousands. But what do these numbers really mean? Here we look at what ISO is, and how you can use different ISO settings to not only to shoot in varying lighting conditions, but also take more control of your photographs as you move out of auto mode.
What is ISO?
ISO refers to the sensitivity setting of your digital camera's image sensor. The measure gets its name from the International Organization for Standardization, which took over from the American Standards Association, which is why some old-school photographers still refer to ASA.
It's this sensitivity to light which decides what image is captured by your camera, as has been the case since the early days of photography. If you're shooting in an environment with plenty of light, you don't need your camera to be as sensitive as when shooting in a poorly lit situation.
For those of us who are old enough to remember learning to shoot with film, you selected your ISO when buying your film, and popped it in your camera. The beauty of ISO settings on digital cameras where weak electronic signals can be amplified, is that you can change the ISO level in-between shots.
What do different ISO numbers mean?
Unlike some of the numbers you encounter in photography, ISO is relatively simple to understand. The bigger the number, the more sensitive your camera will be to light. For example, if you set your camera to ISO 200, it will be twice as sensitive as at ISO 100, as there is a one stop change. As you scroll your ISO dial or button, you are likely to see a progression in 1/3 stops, such as 100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 1000, 1250, 1600, 2000, 2500, 3200, 4000, 5000, 6400 (and onwards).
If you were to only change your ISO setting, the brightness of your image would change, as shown above. But as we already know, photography is a balancing act of different settings, so it's important to think about ISO in terms of what other settings it will let you adjust, such as your shutter speed and aperture.
For example, a higher ISO means less light is needed for a photo, this could allow you to select a faster shutter speed (to freeze quick action) or adjust aperture and give a wider depth of field in your photo. High ISO settings are frequently used as a way of shooting in lower light situations.
While we've previously touched on shutter speed and aperture in our guide to the different modes on your camera, we will look at these settings in more depth in upcoming articles.
ISO and image quality
Unfortunately, ISO is something of a setting of compromise. Yes, using a higher ISO number will make your camera more sensitive to light, but it also reduces overall image quality. Higher ISO settings result in more image noise and less color depth. Film shooters often like the organic-looking grain of high ISO films, but very few would argue digital noise is ever a good thing.
As you can see from the cropped sections on the image above, once you start moving up into the higher ISO settings, colors don't look right, color gradients become blocky and there is visible noise present.
This means that for the vast majority of shooting, a low ISO setting will be preferable. However, if bumping the ISO is the only way to get the shot you want in terms of shutter speed and aperture, then it's a sacrifice worth making. Again, we'll look at balancing shutter speed, aperture and ISO in a future article.
High ISO settings on modern digital cameras
It's also worth noting that high ISO settings on modern cameras are now a lot better and more forgiving than they were just a few years ago. This means better low light performance and the ability to shoot images that quite simply wouldn't have been possible in the past. Not long ago, shooting at ISO 800 on a DSLR would result in levels of noise and color loss that you nowadays don't suffer until getting to ISO 6400 or above.
As such, you can now get perfectly usable images at ISO 12,800 on a number of cameras, though mega-high ISO settings (which can reach into the millions on cameras like the Nikon D5) are still best reserved for the times you absolutely must, as they are there for a combination of specialist uses, and specification sheet grandstanding rather than everyday photography.
As a general rule, you'll get better high ISO performance from cameras with larger sensors (with the almost given proviso of assuming all other things are equal). However, new cameras can outperform older shooters, even if they have smaller sensors. For example, the APS-C Sony A6300 could do a much better job in low light than a full frame DSLR from a few years ago. Equally current Micro Four Third cameras likes the Olympus PEN F can trounce the high ISO performance of older DSLRs with bigger sensors, like the Nikon D200.
Image processing has also improved greatly. In-camera processing of JPEG images can apply noise-reduction which makes high ISO images look better. For RAW-shooters, who want the best quality images, post processing software like Lightroom can offer even more sophisticated noise-reduction.
When to use low ISO
If you are photographing in a situation with plenty of light, you probably don't need to bump the ISO too much to use the shutter speed and aperture needed to give you the desired shot of your subject. For example, we were able to have a wide depth of field for this cityscape while still shooting at ISO 100.
As we've said, for the majority of photographers, for the majority of the time, you'll want to shoot at low ISO settings (typically ISO 100 or 400) to give you the best image quality.
Other situations where using low ISO settings will give you the best results include if you're shooting a static subject on a tripod, where you can use a slower shutter speed to get the right amount of light into the camera without having to worry about unwanted motion blur. Equally, in the studio where you are again in control of the light, you should almost always be able to keep the ISO low.
When to use high ISO
Many photographers will say that the only time to use high ISO settings are when you have to. That's because doing so comes with the loss of overall image quality. That said, once you begin to lose light, or move indoors, you might need to start dialing up the ISO to keep other camera settings as you want them.
The cycling image above was taken at an evening race when the light was beginning to fade faster than we'd have liked. To freeze the face-paced action I needed a shutter speed of 1/400th of a second, and even with the lens at its F2.8 maximum aperture I still needed to increase the ISO to 3200 to get the shot.
Equally, you might need to bump the ISO to settings like 1600 to get an Instagram-able shot of your meal in a restaurant with less than ideal lighting, or take it up to ISO 6400 to shoot a quick night-time or lower-light portrait with a quick enough shutter speed to keep the important things sharp.
As we have seen, you are probably going to want to keep your ISO setting as low as possible when shooting. Shooting at low or base ISO settings will deliver the clearest images, with the best colors and dynamic range.
However, dialing up the ISO isn't as big of an issue as it once was. You can now get great images shooting at settings which would have been unthinkable just a couple of years ago, and with technological advancements it's just going to get better.