A couple of observations on ISO levels and printing.

[I @ M]

I have witnessed the extraordinary advances in technology over the years that camera makers have managed to squeeze out of their products when it comes to iso levels.
Many times I have been somewhat underwhelmed by images posted on the 'net that have been taken at high iso levels and even more disappointed by the "rescue" attempts with noise reduction software.
The balance between non smeared detail and noise seems to be a fine line when viewed on high contrast monitors and even more so when viewed at pixel peeping levels.
Going back in history I took a photo at iso 400 with a Nikon D200, that model was never renowned for having good high iso ability but after processing the image with very moderate noise reduction and deciding it should be printed I was quite pleased that the resultant 16x24 inch monochrome print showed hardly any noise.

Fast forward a few years and whilst I try to stay at the camera's base iso of 100 for the majority of my images I found myself wanting to produce an image under the lights of our kitchen which consist of a 13 watt LED light in a shade which gives mostly downward light, a couple of 50 watt halogen spots above the sink and a very dim rangehood light.
To get an acceptable shutter speed for both hand holding, allowing 1/2 stop overexposure and to keep subjects from displaying any slight movement I needed to have 5000 iso at the aperture I wanted.
Processing the image and after experimentation with varying levels of noise reduction resulted in noise that was, to my eyes at least, somewhat ugly in the darker areas of the image. That is when viewed on a monitor at up to 100% magnification.
I went ahead and had the image printed as a 16x24 glossy and to say I was surprised at the finished image would be a huge understatement.

Looking closely at a print that is meant to be looked at from a reasonable viewing distance shows a little noise where I KNOW it is but in keeping with the slightly oddball subject of the photo it is barely noticeable to anyone else.

Moral of the story is ----- don't fear high iso levels, stop pixel peeping at pictures on monitors and head forth and print ALL your images.

The current cameras here are a couple of generations old now so the newer models with even more stratospheric iso levels must produce better again.

Aint technology marvelous.