Some Pentax DSLRs have a nifty exposure mode called Shutter and Aperture Priority (TAv), which allows you to dial in the aperture you want, the shutter speed you want, then let the camera pick the ISO automatically. This is useful if for example you have a specific aperture and shutter speed combination that you want.
At least some Nikon cameras can emulate that TAv function to some extent. Here's how:
1. Switch to manual exposure mode.
2. Turn on Auto ISO.
3. Dial in the aperture and shutter speed that you want.
You'll notice that Auto ISO will adjust your exposure upward or downward, up to as low as the base ISO to as high as the ISO limit that you set in the menus. You'll also notice that the light meter won't move as it usually does, instead showing 0 until you go beyond the minimum or maximum ISO. If you do want to adjust exposure to match your intent, you can use exposure compensation.
Here are test shots. The baseline shot is at ISO 800, f/4, 1/25, taken on manual exposure mode, matrix metering, with the light meter at 0. Let's assume that this is the correct exposure.
In the next shot, I activated Auto ISO, then changed the aperture and shutter to f/5.6, 1/50, with ISO initially at 800. ISO automatically adjusted to 2800, showing that Auto ISO will adjust upward as needed.
In the following shot, I changed the aperture and shutter to f/2.8, 1/13 with ISO initially at 800. ISO automatically adjusted to 220, showing that Auto ISO will also adjust downward as needed.
In this shot, I changed the aperture and shutter to f/2.8, 1/6. ISO could not adjust below the base ISO (ISO 200 for the D300), resulting in overexposure (you do get a warning of sorts because the light meter will show an overexposure).