Today’s Question: I am curious about the option on my Nikon D7500 to use an ISO setting BELOW the last numbered ISO of 100 (Lo 0.3 to Lo 1.0, equivalent to ISO 80-50). I am wondering what if any effects on recorded image quality this extra-low ISO option might have?
Tim’s Quick Answer: In general, I would opt not to use special ISO settings that go below the base level for a camera, as it will generally result in reduced dynamic range.
More Detail: Various camera models from several manufacturers include special ISO settings that are below the base level. When an extended ISO setting that is below the base ISO for the camera is used, the image is over-exposed and then adjusted in-camera to darken the image. This results in slightly reduced dynamic range and a risk of clipping in bright highlights. Noise levels are not impacted by using these lower ISO settings.
While in general these extended low ISO settings produce good results, I personally prefer to avoid them in the interest of maintaining dynamic range. The main reason to use one of these extended low ISO settings would be to achieve a longer exposure duration. However, with most cameras the most you can go below the base ISO by up to one stop, which I don’t find to be compelling considering the minor risks to image quality.
Instead of using the ISO settings below the base setting, I recommend using a solid neutral density filter when you need a longer exposure. Otherwise, I would stick with the base ISO setting whenever possible to minimize noise, and a higher ISO setting as needed for a faster shutter speed.
