Optimal ISO

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Today’s Question: Another photographer wrote that using 100 ISO, if possible, is better than 200 ISO as a standard choice. Do you agree?

Tim’s Quick Answer: In many (perhaps most) cases, it is better to use 100 ISO rather than 200 ISO. However, it is important to note that the lowest ISO setting is not always the best option for every camera, in part because the lowest ISO is not necessarily the same as the base ISO.

More Detail: The image sensor in a given camera has a native sensitivity to light. You can think of this in very general terms as the result you would achieve with no amplification applied to the signal recorded by the sensor. When you raise the ISO setting, you are under-exposing the image and then amplification is applied to effectively brighten the resulting image. The result of this amplification will be some degree of noise. A greater increase in ISO setting will generally yield a greater amount of noise.

It seems logical that the lowest ISO setting should, in theory, provide the least amount of noise, since the lowest ISO setting presumably involves less amplification. However, the lowest ISO setting is not necessarily the base ISO setting. Reducing below the base ISO setting can reduce dynamic range, which can also affect overall image quality.

For optimal image quality, it is generally best to use the base ISO for your camera. In many cases this will be 100 or 200 ISO, depending on the specific camera you’re using. So, if your camera’s native ISO is 100, then you can expect better overall image quality at 100 rather than 200 ISO. If the native ISO for your camera is 200, then you may see reduced noise at 100 ISO, but you could also expect reduced dynamic range.

So, as a general rule, using the native ISO will produce the least noise with the maximum dynamic range, all other things being equal. It is important to note, however, that it can often be preferred to have a little more noise caused by an increased ISO setting, rather than having motion blur in the image caused by a shutter speed that is too slow.