Just curious, what is the limitation of auto-ISO that explains this frequent need to change the ISO manually on the fly?
The only time when I change ISO manually is prior to shooting panos (I switch off auto ISO and set it manually to the value I need ti get correct exposure with the desired speed/aperture), otherwise I have been 100% auto-ISO for almost 10 years (since the D2x in fact).
Cheers,
Bernard
Auto-ISO almost always selects a value that leads to over-brightness.
For example, a butterfly may have white spots on a darker background, and the Auto-ISO will expose the image for the "general value" of the entire photo, which (if dark background) will lighten the general darkness to the point of blowing the highlights of the very light areas.
For this reason, it is preferable to be able to manually-set the ISO and create my own (slightly-dark) exposure, because I can keep the details in the light areas, and move the shadow-slider in Lightroom to balance the image out later.
Same thing with taking photos of very light flowers against a darker background: Auto-ISO simply doesn't do what I, manually, would have chosen. Almost invariably, Auto-ISO will jump to a higher value than where it really needed to go.
In even light, Auto-ISO works fine. But if your subject is very light, and the background is very dark, it doesn't work quite so well.
Jack