ISO refers to the level of sensor gain, taking the name from “film speeds” back when we photographed on analogue film. Higher ISOs make the camera sensor more “sensitive”, allowing for photography in lower light levels. However, increasing the ISO beyond the camera’s “native” ISO (this varies between model, but is often around 200-400) will increase the level of noise, which in turn reduces the dynamic range of the images. ISO should therefore ideally be kept constant throughout your entire study. Different models of camera vary substantially in the quality of noise produced at different ISO levels, so comparisons between cameras should be treated with caution (e.g. the bandpass pattern analysis will be very sensitive to noise levels in images at the very high spatial frequencies).