What is ISO invariance?
The term ISO invariance is increasingly used in articles about image noise, dynamic range and ISO settings.
With a largely ISO-invariant camera, an underexposed RAW image taken at a low ISO setting can be brightened considerably in post-processing without showing much more noise than an image taken with the same aperture and shutter speed at a much higher ISO setting.
Suppose you take two photographs of the same subject in low light, using the same aperture and shutter speed. The first photograph is taken at ISO 100 and therefore appears dark. In post-processing, you digitally increase its exposure until its brightness matches the second photograph.
The second photograph is taken at ISO 3200. With a strongly ISO-invariant camera, the visible noise in both images can be surprisingly similar after the ISO 100 image has been brightened. This behaviour is known as ISO invariance and is also commonly described as ISO-less.
What exactly is image noise?
Photographers often associate higher ISO settings with more image noise. It is important, however, to understand that increasing ISO does not physically make the sensor more sensitive to light. The amount of light captured by the sensor is determined by the aperture, shutter speed and the light in the scene.
The ISO setting mainly affects how the electrical signal from the sensor is amplified and processed. Noise has several sources, including the light itself (photon noise) and the camera's electronics. For more information, see the page about ISO settings.
What are all those ISO settings for?
Many cameras distinguish between a normal ISO range and an extended ISO range. A camera might, for example, have a normal range from ISO 100 to ISO 6400, while ISO 12,800, 25,600 or higher settings are offered as extended values.
Depending on the camera, some extended ISO settings are partly produced through additional digital processing. This does not mean that the sensor itself becomes physically more sensitive to light. Exactly how amplification is performed depends on the camera and sensor design.
Is that clear?
ISO invariance does not mean that ISO has become irrelevant. Instead, it means that with certain sensors, shooting at a low ISO and brightening the RAW file later can produce a noise level similar to shooting at a higher ISO in the camera.
This can be useful when you want to protect highlights by deliberately keeping the exposure darker. Aperture and shutter speed remain crucial because they determine how much light actually reaches the sensor.
Is every camera ISO invariant?
No. Not every camera is ISO invariant to the same degree. ISO invariance is also not simply an on-or-off characteristic. A camera may behave almost ISO invariant over one range of ISO settings while behaving differently at other settings.
ISO invariance depends on factors such as sensor design, analogue amplification and readout electronics. It is therefore useful to test your own camera or consult reliable measurements for your specific camera model.
Mainly relevant when shooting RAW
ISO invariance as a shooting technique is mainly useful when photographing in RAW. A RAW file retains considerably more unprocessed image information and therefore provides more latitude for brightening dark areas during post-processing.
JPEG files have already been processed and compressed by the camera, so they offer much less flexibility for substantial exposure corrections.
Post-processing is necessary
If you do not want to post-process your photographs, experimenting with ISO invariance offers little advantage. A deliberately dark RAW image taken at a low ISO setting needs to be brightened later in a RAW converter.
ISO invariance is therefore particularly useful for photographers who deliberately control their exposure and process their RAW files afterwards.