Reply #2

It was partly cloudy with the sun peeking out and then hiding behind a cloud, and a nearby forest fire filled the air with haze.

Yes, a closeup target and constant lighting would be better. As for using the entire camera frame, that's a HUGE picture, 4912 x 3264 on my camera. It's hard to look at a picture that big on the monitor screen. And that doesn't duplicate normal viewing conditions. A picture is normally either cropped, or reduced, or more commonly, both. Reduced to 50% size (2456 x 1632) and given a noise reduction pass in Paintshop Pro there's really no difference that I can see between an ISO 100 and an ISO 800 picture.

Reduced to 10% of out-of-camera size (491 x 326) there's hardly any noticeable difference between the 100 ISO and the 25600 ISO! So for small web pictures high ISO is perfectly OK. The noise will disappear when the picture is scaled down.

What I need to do next is test different size prints and find out what ISO settings are suitable for what print sizes. I know ISO 100 printed at 16 x 20 shows absolutely no sign of noise even under a strong magnifying glass.