How does ISO affect Image Sharpness?

How does ISO affect Image Sharpness?

As photography enthusiasts, we all strive for the perfect image. Whether it’s a stunning landscape, a beautiful portrait, or a captivating still life, we want to capture the moment and make it last forever. 

One crucial factor that can make or break the quality of an image is ISO.

As an award-winning photographer, I always strive to use the lowest ISO possible to capture the sharpest, and cleanest images possible. That way, I can edit the images as much as I need to in Lightroom or Photoshop, without worrying about reducing the image quality too much. 

In this article, we’ll explore the impact of ISO on image sharpness, how to use it to your advantage, and how it can affect the overall quality of your photos. 

The Angel Oak Tree in Charleston South Carolina, submitted to the MPI competition in 2023.
ISO is the camera sensor’s sensitivity to light. Low ISO settings create cleaner, sharper images, but require longer shutter speeds.

What is ISO?

ISO is an acronym for International Organization for Standardization, which is a governing body that sets standards for a variety of industries, including photography. In photography, ISO refers to the sensitivity of your camera’s sensor to light. 

The higher the ISO number, the more sensitive your camera sensor is to light, at a cost of more noise in the resulting image. A high ISO setting can allow you to use a faster shutter speed in low-light situations, but the extra noise can reduce the overall image sharpness.

If you’re a wedding or event photographer, the extra noise will be worth it if it means you can use a fast enough shutter speed to freeze action. Photographers who need fast shutter speeds in low light prefer using full-frame cameras and lenses with large aperture values like f/2.8 or f/1.4 that capture more light. This allows them to capture candid smiles and laughter in the audience without needing to use a high ISO value. 

Cuiping Hill in China, submitted to the MPI 2023 landscape photography competition
Enchanted Hill by Tim Shields, taken at Cuiping hill in China. Most landscape photographers use a tripod to keep their ISO settings as low as possible.

Understanding ISO and Image Sharpness

When you increase your camera’s ISO setting, you’re essentially telling it to amplify the signal from the sensor, which makes it more sensitive to light. However, this amplification can also amplify the noise that’s present in the image, which can reduce sharpness. 

Noise can appear as a grainy texture or colored speckles that are scattered throughout the image. The higher the ISO setting, the more noise you’ll likely see in your images.

However, every camera will be a little bit different. Professional cameras tend to capture less noise at higher ISO settings than entry-level cameras. For example, ISO 1600 on a Canon Rebel with a small sensor might introduce so much noise that the images completely lack discernible sharpness, while a pro camera like a Nikon D850 with a larger sensor can shoot up to ISO 6400 before the noise starts reducing sharpness. 

A photo of an Osprey building a nest with a fledgling
Using a high ISO will allow you to use a faster shutter speed for capturing action like this

Using ISO to Your Advantage

Despite the potential drawbacks, ISO can be a powerful tool in photography. In low-light situations, increasing your ISO can help you capture images using shutter speeds that are fast enough to freeze action.

But it’s important to remember that there’s a trade-off between ISO and image quality, so you should use the lowest ISO setting possible in any given situation to maximize sharpness. 

If you’re photographing people, it will be more important to use an ISO value high enough to use a shutter speed fast enough to capture smiles and laughter. But if you’re outside taking landscape photos in low light, you’re better off using a slower shutter speed and a low ISO to capture the highest-quality image possible. 

Using a low ISO makes it possible to take long exposures that can blur the water like this.

The Relationship Between ISO and Other Settings

It’s important to note that ISO doesn’t operate in a vacuum. It’s just one of several settings that affect the exposure of your image. Shutter speed, aperture, and ISO all work together to determine the amount of light that reaches your camera’s sensor. 

When you adjust one setting, you will need to balance the exposure using one or both of the other settings in the exposure triangle. For example, if you increase your ISO setting, you may need to use a faster shutter speed or a smaller aperture to compensate for the increased sensitivity to light.

Camera settings work in 1/3rd of a stop. That means if you make the shutter speed 1 click slower, you will need to decrease the ISO by one click, or increase the aperture by one click. This is called reciprocity, where changing one setting means changing another in a reciprocal fashion to balance the exposure.

The lower your ISO, the sharper your images will be. That may mean you will need to use a tripod to make the image as sharp as possible. Lavender fields in Valensole, France by Tim Shields.

How to Achieve Maximum Image Sharpness

To achieve maximum image sharpness, you’ll need to use the lowest ISO setting possible. This will not only give you sharper images, but it will also increase the dynamic range of the images, allowing you to brighten the shadows or darken the highlights without decreasing the image quality. 

This is especially important for landscape photography, where you’re often photographing at sunrise and sunset. At these times, the low angle of the sun casts long, harsh shadows over the landscape, which helps to bring out textures through contrast, but it also means that the shadows will be very dark. 

By using a tripod to keep your ISO low, you’ll be able to lighten the harsh shadows without adding noise much easier in post-production. Your camera will also capture more information in the darker areas that can be more easily brought out. 

Using a low-ISO like this was especially important when I was taking photos at the grand canyon. The canyon walls are very dark, while the sky is bright. Without using a low ISO, it’s impossible to capture a single image that has both the sky and the canyon walls perfectly exposed. Using a low-ISO meant even if the walls inside the canyon were underexposed, I would still be able to recover them in Lightroom after the fact. 


Tim Shields

Tim Shields is the founder of Photography Academy, the author of The Photo Cookbook, and the creator of the Photography Transformation 4-Step System. He holds the designation of Master Photographer in Fine Art from Master Photographers International.