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What is ISO?

Updated: Jan 4

ISO, is the most underrated and confusing part of the camera exposure or exposure triangle for most photographers. Understanding Aperture and shutter speed is easier than ISO, even for experienced photographers. Photographers have often asked each other “Hey! Tell me what ISO are you using”.

Often photographers consider ISO as the worst enemy for their photography and try to keep ISO as low as possible or even mostly at the BASE ISO value of their camera.

Read my book "ISO: A true friend of the photographer"

After reading this eBook, you will start considering ISO as your true friend; believe me, this will always help you in critical lighting conditions.

In simple terms, ISO is a camera setting out of three settings (Shutter speed, aperture, and ISO) that affect the brightness of the final image. As the ISO values go higher, the final images become brighter. With the help of ISO, the camera can capture images even in darker environments.

ISO in Film Photography

In film photography, ISO or Film speed is a value or measurement of sensitivity to light. In film photography, ISO is responsible for the amount of light captured on film and the aperture and shutter speed. A high-speed film or larger ISO on film will capture more light than a slow-speed or low ISO number film.

Normally films are coated with silver nitrate, and the ISO number/ speed represents the size of the silver nitrate molecule or particle. The larger the number bigger the size of the particle. The large particle will capture more light than a small particle, thus you will get a brighter image on a fast film compared to a low-speed film.

Now with this high speed or fast speed or high ISO due to the large size of particles, noise also comes as unwanted trouble. You will get more noise on an ISO 3200 compared to ISO 100 film.

But in digital photography the story is different, in most consumer cameras, ISO has nothing to do with the sensor, or I can say with the exposure triangle, in modern digital cameras only lens aperture and shutter speed are responsible for controlling the volume of light that hits the sensor of the camera.

Some high-end cameras (normally video cameras) can control the camera sensor’s sensitivity with the help of ISO (or Gain they call it). In these most expensive cameras, the sensitivity can be increased by increasing the sensor's input voltage and getting a much cleaner image to any consumer / professional DSLR/mirrorless camera.

ISO in Digital Photography

In a Digital camera, lens aperture, and shutter speed control the volume of light, and the camera sensor captures available light in the format of an analogue signal, (a microvolt signal) after that this analogue signal travels to a controller/chip called Analog to Digital Converter (ADC), this chip converts analogue signals to digital signals and during this process, the ADC increases (or even decreases) brightness level of captured light by multiplication of ISO value.

In some modern cameras, the ADC unit is built into the sensor chip, which helps to reduce transmission noise.

Even some top model cameras are equipped with dual gain (they call it), these cameras used to have two base ISOs. Cameras like Nikon D850, Nikon Z6II/Z7II/Z9, Sony A7M3, and Canon R5 are examples of dual gain or dual base ISO.

How is it measured?

ISO is measured in numbers, usually multiples of 100. While manufacturers used to stick to ISO 100, 200, 400, 800, 1600, and so on (doubling in value), things have changed with more recent cameras. Smaller increments have been introduced for better refinement, and now it’s common to see ISO measurements like 125, 160, and more. The concept remains the same, though. ISO 100 is half as bright as 200, which is also half as bright as 400.

Why on earth do we need ISO in the camera?

From what you have read till now, you may think that if this is only about the brightness then why do we need ISO in the camera we can increase the brightness in the post process or while editing.

Yes, certainly you can do that. But this approach has some practical issues and cannot be useful every time.

The Benefit of High ISO

Well, the First benefit of using high ISO in photography is you can capture photos even in darker lighting conditions without using flash or any other light source.

High ISO gives you the freedom to use a higher shutter speed or a small aperture to get the desired results.

For sports, for example, you need a higher speed to freeze the movement of the player or in wildlife photography, you need a higher shutter speed to freeze the movement or maybe you want to a use smaller lens aperture to increase DOF. High ISO simply allows you to configure your camera to capture properly exposed and composed.

Analogue Noise: An analogue signal is subject to electronic noise and distortion introduced by communication channels, recording and signal processing operations, which can progressively degrade the signal-to-noise ratio (SNR). As the signal is transmitted, copied, or processed, the unavoidable noise introduced in the signal path will accumulate as a generation loss, progressively and irreversibly degrading the SNR, until in extreme cases, the signal can be overwhelmed. Noise can show up as hiss and intermodulation distortion in audio signals, or snow in video signals. Generation loss is irreversible as there is no reliable method to distinguish the noise from the signal. Source:

We get a real-time preview of the results based on our camera setting.

Camera company understands its products better than any other software company and it can manage all this better than Adobe or any other third-party software company.

Hence, we will be getting better results / higher quality images than the post process.

Having the correct exposure information, you can fine-tune other settings like aperture and shutter speed.


When low ISO is used to capture a photo such as 400 or lower, usually the results are of higher quality. Low ISO can be used in good lighting conditions like a bright sunny day or with flash. Low ISO, if used properly with the right techniques will give a cleaner-grained image, every time the ISO is increased it will decrease the image quality and add grain or noise to the final image.

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