What is ISO in photography? A complete guide to understanding ISO

What exactly is ISO in photography? You’ve probably heard of it when someone shouts random numbers at you at a camera club. It seems important. Maybe it influences exposure? Can it make your images brighter or more grainy or less sharp, or all of those?

It might seem a bit mysterious at first, but ISO is pretty straightforward, and is the final key to understanding the exposure triangle. Here we will chat about all things ISO related and demystify this final clue to the exposure puzzle.

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What is ISO?

In its most basic sense, ISO is a camera setting that affects the image’s light or dark. It directly affects exposure and is one of the three corners of the exposure triangle, along with aperture and shutter speed. By altering one of the three of these settings, you will change the exposure of the image. However, increasing the ISO to extremes can have a detrimental effect on the quality of your images, as we will see.

However, ISO does get a little more complex, and to really understand it, it helps to know a little more about how your camera works. The interesting thing about photography is that it’s as much a science as it is an art. Understanding the nuts and bolts behind the lens can actually help you to take better photos in the long run.

What does ISO mean?

ISO is an acronym for International Organization for Standardization. If you’re still confused, don’t worry, this fact isn’t too important. It merely refers to an independent international organization that sets standards for different things.

It first started to be used in photography terms in 1974 and referred to standardized film speeds, replacing the older words of ASA and DIN. It later came to be adopted almost entirely by manufacturers of digital cameras. ISO 200 on one brand of camera or film, for example, will be identical to ISO 200 on another brand. That is the point of standardization.

What is the difference between ISO and ASA?

If we are talking about film, then there is no difference at all between ASA and ISO. They both mean the exact same thing. However, most of the time, ISO refers to digital cameras and photography settings. It essentially means something similar, but it does work slightly differently.

ASA and ISO in film refer to the sensitivity level of a particular film. For example, ASA 100 film is less sensitive than ASA 400 film. That means that you can shoot in lower light conditions with the ASA 400 film while keeping the same shutter speed and aperture settings.

Is ISO sensor sensitivity?

One myth around ISO with digital cameras is that it changes the sensor sensitivity, just like it does with film. This is actually not the case. A digital camera sensor will always have the same sensitivity level that it was manufactured with.

That being said, the ISO does affect the level of amplification of the sensor’s sensitivity. It works in a similar way that turning up the volume on TV amplifies the sound. ISO amplifies the signal, or the light information, that reaches the sensor. This does mean it’s simpler to think of higher ISOs as being more sensitive in a similar way to film photography, although now you know that it’s slightly more complicated than that!

How does ISO affect exposure?

As mentioned above, ISO is the third point on the exposure triangle, so it directly affects the overall exposure of an image. However, it doesn’t affect the amount of light that comes into the camera and hits the film or sensor in the way that aperture and shutter speed do, so it’s a little bit different. Because of this, ISO affects how dark or light an exposure appears on the film or sensor.

How does ISO affect a photo?

A higher ISO increases the camera’s ability to take an image in low light. You can dramatically change the overall lightness of an image just by altering the ISO. You’ll see in the example below how this looks:

Generally, the higher the ISO value, the more risk there is of introducing digital noise into the image. Noise is similar to film grain, however is most notable in the shadow areas of an image and can present as ugly red or blue-colored grain.

What are standard ISO settings?

Most digital cameras have a range of ISO to choose from as low as 50 all the way up to 3.28 million! The table below shows the most usual ones, however. Just like aperture and shutter speed, the numbers go up in stops of light. You will often find that base ISO is around 100.

How do I choose the right ISO?

There’s no right or wrong answer to selecting your sensitivity. The answer is always dependent on the lighting conditions. One thing to take into account when choosing ISO is that the higher the ISO the greater the impact on image quality. Higher ISO means a higher risk of having visible digital noise in the image. Because of this, many people prefer to select the lowest possible ISO number for any exposure. However, these days it’s not such a concern because the most modern mirrorless digital cameras have very little digital noise, even at very high ISOs.

For example, anything up to around 3600 on my Canon EOS R is relatively noise-free. In the past, this was much more of an issue, and of course, the more budget-level DSLRs will have noticeably more noise at lower sensitivities. My first Canon Digital Rebel had very noticeable noise issues for anything above ISO 400.

What is digital noise?

In digital photography, the term ‘digital noise’ refers to a type of visual distortion. It can be visible in photographs when using high ISOs or shooting in low light. It usually presents similarly to film grain. However, in color images, it can produce unsightly colored speckles, which can detract from the quality of the image.

To a degree, noise can be reduced in post-processing. However, only to a certain extent. Generally, the smaller the sensor and the lower the pixel count, the more noise you will have. Very long exposures can also produce significant amounts of noise.

What is the best ISO to use?

The answer to this, obviously, is “it depends”. Like all areas of photography, it will depend on what conditions you’re shooting under, what you’re photographing, and what outcome you are trying to achieve. Generally, though, the answer should be “the lowest ISO you can get away with for the correct exposure”. But don’t be afraid to boost your sensitivity. A higher ISO and slightly noisier image will always be better than an underexposed or shaky image. That being said, there are a few guidelines.

Indoors versus outside

Sunny day

Generally, on a bright sunny day, you can be safe at shooting at ISO 100. Use the Sunny 16 rule to determine shutter speed and aperture if in doubt.

Indoors

For indoor portraits using natural window light, I find that I can shoot happily at ISO 640 or 800 without too much noise causing problems. Of course, I would be using a fast lens and wide aperture of around f/2.8 with a shutter speed of 1/100.

For photographing concerts or indoor events, be prepared to push your ISO to 25,000 even. Once again, you don’t want to miss the decisive moment, so choose a high sensitivity if you have to.

With flash

If shooting with flash, you can generally choose the base level ISO for your camera since the amount of light isn’t a problem. Be aware that, just like with the aperture, changing your ISO will have an effect on the brightness of the appearance of the flash in your images.

Night time

For night sky photography, a general rule is to select an ISO of around 3200. Of course, you can go higher or lower if you wish. However, this seems to be a great starting point. It’s the sweet spot between being high enough to shoot an exposure length of 15 to 20 seconds (and thus avoiding star trails) but not introducing too much digital noise, which could interfere with the star patterns in the sky.

Does ISO affect sharpness?

If you shoot at very high ISOs, you will notice an increase in digital noise. This doesn’t in itself result in a less sharp image per see. However, it does create the effect of the image not being in pin-sharp focus. To take advantage of your high-quality lenses, always try to shoot at the lowest ISO setting you can.

Remember, if you are shooting in low-light situations and want to avoid motion blur, then a higher ISO will enable a relatively fast shutter speed.

Using a tripod will help to avoid camera shake for a slower shutter speed, which means that you can dial in a lower ISO value. This is helpful in lots of situations, but especially with landscapes.

How to change ISO?

You can choose to either select the ISO manually or you can set your camera to Auto ISO. This is where the camera will select what it thinks is the best ISO for each exposure. If you’re unsure, then it can be a good option, and it’s very easy to override if the camera doesn’t quite get it right.

The actual control for ISO varies for each brand of camera, so look in your manual if you’re unsure. With most DSLRs and mirrorless cameras, its one of the easily accessible pad on the back. With more basic cameras, it can be a dial around the lens or via the LCD touchscreen.

If you are shooting in aperture priority or shutter priority modes, you will still be able to set different ISO values while the camera chooses the shutter speed or aperture respectively.

As a rule of thumb, it can be good to get into the habit of setting your aperture and shutter speed first, and then adjusting the ISO as necessary. Make sure you always select the lowest ISO that you can whilst still getting an appropriate exposure. This way, you will ensure that you have the highest quality image with the least amount of noise.

Now you know all about ISO, what it means, what it is (and isn’t).

Conclusion

ISO is the final point of the Exposure Triangle, which helps to balance the overall exposure of your image, alongside shutter speed and aperture. It does work in a slightly different way, although the eventual result is similar. It has little influence on creative effects in your photography. However, it does help control how dark or light your images appear. Therefore, it’s very useful to understand how it works when shooting in lower light conditions.

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