Just like the direct relationship between shutter speed and f-stop numbers (aperture), ISO numbers have a direct correlation with f-stop values. Understanding the relationship is half the battle to capturing better images. Each time you double the ISO number, it is a 1-stop difference. For example, ISO 200 is 1-stop faster than ISO 100 and 1-stop slower than ISO 400. To put in another way, ISO 400 is twice as fast as ISO 200, or twice as sensitive to light.
In other words, if ISO sensitivity is raised from ISO 400 to ISO 800 while aperture is left unchanged, the same exposure can be achieved with a shutter speed twice as fast. The same is true if ISO sensitivity is raised from ISO 800 to ISO 1600.
If you are finding blurs in your image when shooting in aperture priority or shutter priority and have run out of flexibility to make adjustments to either allow more light via your f-stop number or shutter speed, ISO is your next tool to use. Bumping up your ISO increases your sensor’s sensitivity to light. Just be careful because the higher in ISO that you go, the more likely you are to see graininess in your images (also sometimes referred to as “noise”). I try to remain below ISO 1600 for most cases. Although if the shot calls for ISO 3200 to capture the image you are looking for, then so be it. You can try to remove some of the graininess in post-production afterwards.
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