What Is ISO?

What Is ISO?

ISO is a measure of how sensitive the camera sensor is to light. In the film days, ISO was a physical property of the film you loaded into the camera. You bought ISO 100 film for bright sunny days and ISO 800 film for indoor or low-light situations. In digital cameras, ISO is an electronic amplification of the signal coming from the sensor. You can change ISO from one shot to the next, which gives you enormous flexibility that film never had.

At its base ISO, usually 100 or 200 on most cameras, the sensor produces its cleanest, most detailed images with the least amount of noise. As you increase ISO, the camera amplifies the signal from the sensor. This makes the image brighter, allowing you to use faster shutter speeds or narrower apertures in low light. But amplification also amplifies the random electronic noise inherent in all sensors, which appears as grain or speckles in the image.

The amount of noise at a given ISO varies greatly between cameras. A full frame camera at ISO 6400 might look cleaner than an entry-level smartphone at ISO 800. This is because larger sensors have bigger photosites that capture more light and produce a stronger signal with less amplification needed. Modern cameras have become remarkably good at high ISO performance, and many can shoot at ISO 12800 or higher and still produce usable images.

Noise is not always bad. Some photographers actually like the look of film grain, and digital noise can add a gritty, textured quality to an image. Black and white photos tend to handle noise better than color images because color noise looks more distracting. The key is knowing when noise becomes objectionable for your particular use case. A noisy image shared on social media looks fine. A noisy image printed poster-size does not.

The practical approach to ISO is simple. Always use the lowest ISO you can get away with given your shutter speed and aperture requirements. If you have enough light, keep ISO at 100 or 200. If the light drops and you need a faster shutter speed to avoid blur, raise ISO without hesitation. A sharp image with noise is infinitely better than a blurry image with no noise. Modern noise reduction software like Lightroom and Topaz can clean up moderate noise very effectively.