It’s no secret that all lenses have a “sweet spot” aperture that produces maximum image quality. I like Sean McHugh’s take on this: A lens’ “sweet spot” increases for successively narrower apertures, reaching maximum for intermediate apertures, and declines for very narrow apertures. Depending on the lens, this is typically between f/8 and f/16 on a full frame or APS-C sized sensor but is independent of the number of megapixels.
There are other kinds of sweet spots, such as ISO. When testing Rotolight’s Anova LED lighting system for Shutterbug I wanted to determine a sweet spot of the camera simply because most LEDs don’t provide the same mount of punch as electronic flash and while the latest Anova may be an exception to that rule, I still knew that I’d be shooting at higher than normal ISO settings. (I prefer to work at ISO 100-200, depending on camera, in the studio.)
When testing Anova I used a Nikon D600 and shot an ISO bracket to see what ISO settings would work best in the studio, Using the traditional concept of a test strip, ultimately shooting exposures from ISO 400 through 500, 640, 800, 1000, 1250, and 1600. With the D600, the differences between 400 and 1600 is so small that I could only see it at large magnifications, so I shot from 640 to 1600 depending on the specific application.
For the test strip that you see at right, I did my best to align the section allowing for subject and camera movement between shots. Your camera will be certainly be different, so I suggest that you shoot a similar bracket of ISO settings, using a real portrait subject, and evaluate them on a monitor and make your decisions based on real data.