SUMMARY
Base ISO represents the lowest native gain setting on a camera sensor, typically ISO 100 or 200, where it performs optimally. At this setting, the signal from the sensor is amplified the least, preserving the widest dynamic range, color information, and editing latitude. While base ISO provides the cleanest files in good lighting, it's not always the best choice; raising ISO is necessary in low light to achieve proper exposure and avoid blur, even if it introduces some noise. Extended ISO settings below base are achieved through post-processing and typically reduce dynamic range.
TAKEAWAYS
Base ISO is the sensor's native setting with minimal signal amplification for the cleanest image.
It offers the widest dynamic range and best editing latitude when properly exposed.
Higher ISO is necessary in low light to achieve usable shutter speeds and apertures.
Extended ISO settings below base ISO are not native and can reduce dynamic range.