Which ISO to use

[Major Nikon]

4. Pixel peeping doesn't always provide all the information you need

As you noticed, differences that are readily apparent while viewing at 100% resolution don't necessarily translate to the finished product. So a lot depends on how you're actually going to use the images when it comes to what is acceptable vs what is not.

Generally speaking with a relatively constant light source there are only 3 things which determine exposure which are shutter speed, aperture, and ISO setting. Typically I have a pretty good idea of the range of aperture and shutter speed settings I want to use. From there I select the ISO setting that puts me in that range. If I'm using artificial light, I may also up the ISO if my strobe(s) are putting out full whack. I use an app called Light Meter for my iPhone which gets me in the ballpark pretty quickly.

If I'm in a situation with rapidly changing light conditions, I may be tempted to use auto-ISO, but this is generally a last resort for me because I want to know when ISO levels are ranging into unacceptable territory. My D7000 can change ISO pretty quickly manually, so this is almost always my preferred method.