Simon d'Entremont

Using high ISO values does not cause images to have high noise levels

Today's video can be viewed (here) and gives an excellent treatment of this subject.

It is a myth that using high ISO values in low-light conditions causes noisy images! Noisy images are caused by too little light reaching the camera sensor. If you shoot in those conditions with a high ISO value the camera will boost the sensor values, noise and all, leading to poor image quality. But it was shooting in low light, not turning up the ISO value to address that problem, that caused the extra noise.

If this seems mere pedantry it's not! Photographers have been known to avoid using a raised ISO value and so underexpose instead, because of this myth, and then planning to correct the exposure in post. This is likely to produce even more noise than getting the camera to boost the sensor signals by setting a high ISO.