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Understanding ISO: How it Works, What it Does, and How to Make the Most of It

Understanding ISO is the third post in my Understanding Photography series. The first two include Understanding Aperture and Understanding Shutter Speed. Upcoming will be Understanding the Exposure Triangle, tying the three elements together. Mastering these elements will make learning photography a much easier process!

understanding ISO in photography
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Intro to Understanding ISO

Of the three exposure triangle elements, ISO is one of the simplest. Particularly in travel photography, ISO is often adjusted more often than in many other types of photography. Travel photography encompasses multiple genres and widely varied scenarios.

There is no single “best” ISO setting for travel photography.

However, there is a best (or ideal) ISO setting for individual photographic situations. Understanding ISO thoroughly is an essential part of developing the skill and confidence to make an ISO choice.

So WHAT is ISO in photography (or ISO in digital photography) exactly? ISO refers to the sensitivity of the film or digital camera sensor. This is also called your ISO speed. From the tiniest Smartphone sensors to massive sensors found in high end digital medium format cameras, ISO is used in all of them. 

History of ISO in Photography

ISO didn’t come about with digital cameras and sensors. ISO stands for International Standards Organization – which creates global standards for a number of industries. ASA stands for American Standards Organization and is the same thing. ISO is the more commonly used term between the two. ISO was introduced in the 1940s to standardize the sensitivity rating of photographic film.

Prior to this, each film manufacturer used its own system for rating film speed. This made it difficult to compare different films and create correct exposures consistently. Establishing the ISO rating meant photographers could choose film knowing that a film’s stated speed would be consistent across brands.

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Available ISO Ratings (and what they mean)

ISO is typically referred to as fast or slow, referring to how sensitive it is. Film is available in a range of sensitivities, from as low as 25 (barely sensitive to light) to 3200 (extremely sensitive). The most common ISO ratings (both in film and digital) range from about 100 to 6400.

Fast film (such as ISO 800 or higher) is more sensitive to light, meaning you can use it in low-light situations and still get a properly exposed image without resorting to extreme wide aperture or slow shutter speed. 

Slow film (such as ISO 200 or lower) is less sensitive to light and produces finer grain, making it ideal for situations with plenty of light and/or when maximum sharpness, detail, or dynamic range are critical.

So why not just use the highest ISO possible every photo? Fast ISO has the tradeoff of more significant grain or “noise,” which can make the image look less sharp, impact dynamic range, and muddy fine detail. This is true whether using an analog film camera or digital camera.

effects of different ISO settings on a photo

Film is an entirely chemical process: the only way to control the sensitivity of the film is to change the chemical makeup of the film. Very high sensitivity film such as 1600 ISO or even 3200 ISO is very rare and hard to find these days. It’s not easy to make those films because the chemicals in them are highly reactive.

Most modern films are designed for the 100-800 ISO range of sensitivities. Film photographers have to consider their selected ISO more carefully, committing to it for 24 or 36 frames. Back in my own film days, I’d usually use ISO 400 or ISO 800 as my standard film speed.

Understanding ISO in Digital Camera Sensors

In digital photography, the ISO of a digital sensor can be changed as much as you like with the flick of a dial or button. Just like with film, a higher ISO setting on a digital camera makes the sensor more sensitive to light, while a lower ISO setting makes it less sensitive. 

The most common digital ISO range for the majority of users (and camera types) is 200 to 1600. More advanced digital cameras boast ISO settings ranging from as low as ISO 50 to as high as ISO 12,800 or even beyond! However, there are definite image quality tradeoffs for ultra-high ISO, and in most situations it’s best to keep it at or below 1600.

When it comes to pros and cons of using high ISO settings on a digital camera, there are a few things to consider. On the one hand, a higher ISO setting allows you to take photos in low-light situations without needing a flash, wide open aperture, or long exposure time. However, using higher ISO setting starts to introduce noise or grain into the image. This isn’t inherently bad, but does need to be considered.

In general, it’s best to use the lowest ISO setting possible for the given situation to minimize noise and get the best image quality. But in some cases, such as when shooting in low light or when in need of a fast shutter speed, increasing the ISO becomes necessary to create a properly exposed image.

Hover over or click on the images below to see the difference between RAW noise and after it’s been reduced in the editing process.

Unlike film, a digital camera sensor doesn’t have a predetermined ISO; it’s adjustable. Think of it as gain in audio: amplifying the signal and increasing volume. Increasing the ISO setting increases the amplification of the signal from the sensor.

What’s actually happening as ISO is set higher on the camera, is the signal is amplified by varying amounts. Much like taking a low volume recording and playing it back with the volume cranked up – you’ll get distortion, crackling, or hissing. Except in photography, it becomes visual noise, loss of fine detail, and reduced dynamic range.

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ISO and the Exposure Triangle

Along with aperture and shutter speed, ISO is one of the three factors that make up the exposure triangle in photography. The exposure triangle refers to the relationship between these three factors and how they work together to create a properly exposed image.

To get a properly exposed image, ISO, aperture, and shutter speed settings need to be balanced. This ensures that the right amount of light enters the camera and reaches the sensor (or film!) – resulting in a photo that is neither under nor over exposed.

Understanding ISO – as well as aperture and shutter speed – will lead to understanding the exposure triangle. Mastering the exposure triangle and its elements is a key factor in going from beginner to intermediate photographer.

Choosing the Best ISO Setting for a Scene

Below are common travel photography scenarios that typically have an ideal ISO setting or range to use and why. As you develop confidence with understanding ISO and how to leverage it, you’ll find your own “best” ISO settings for numerous situations.

What is the best ISO for daytime photography?

Most daytime situations can be shot at lower ISO settings. ISO 200 works well as a starting point. If it is more overcast, ISO 400 may be needed.

What is the best ISO for night photography?

If shooting handheld in a city with lots of lights, ISO 800 or 1600 are good choices. On a tripod you can go lower of course, and create light trails and motion blur. Night landscape photography will typically be shot on a tripod at ISO 100 – 400 (excluding star or night sky photography) for longer exposures.

What is the best ISO for travel photography?

For the most common travel photography scenarios, ISO 400 – ISO 800 is suitable. I often leave mine set to 400 unless photographing in very low light or a unique circumstance.

What is the best ISO for street photography?

Most street photography is shot at smaller apertures (like f/8 or f/11) and with faster shutter speeds (1/100th or so), a higher ISO is often needed – around ISO 800.

What is the best ISO for landscape photography?

In most cases, it is ideal to have as little noise (to get clean detail) & as much dynamic range as possible in landscape photos. So ISO 100 or ISO 200 is broadly considered ideal when taking landscape photos.

Is it bad to use high ISO for landscape photography?

While not inherently ideal, it isn’t automatically bad to use high ISO when photographing landscapes. If it’s the only way to get the shot, then use the ISO necessary. The goal (in all scenarios) is to use the lowest ISO possible –

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