Nikon makes a lot of digital cameras, from the basic point-and-shoot compact cameras to professional-level digital single lens reflex cameras. All of Nikon's DSLR cameras come with a built-in flash for shooting in automatic mode, but once you get into using an external flash -- like any of Nikon's Speedlights, you need to adjust your camera to suit the light.
External Flashes
Nikon has produced a dozen different external Speedlight flashes during the DSLR era, always with an eye to its current fleet of cameras. Most Speedlights will work fully on all Nikon DSLR cameras, but Nikon has a compatibility chart just in case. The modes listed on the compatibility chart are flash modes, not ISO settings. Just because the flash says "auto" doesn't mean the camera is on -- or can be set to -- "auto."
Supported Shooting Modes
Nikon's Speedlight flashes can't be used in auto mode or any of a Nikon DSLR camera's preprogrammed shooting modes like the portrait or landscape modes. According to the SB-800 Manual, you need to set your DSLR to manual or a half-manual mode like Aperture Priority (A), Shutter Priority (S) or Program (P). In each mode, you are expected to set the ISO according to the shooting situation. The flash uses the ISO information to determine how strong of a flash to release.
ISO Ranges
Nikon's DSLR line varies quite a bit -- from the entry-level or presumed D40 to the D1. Some cameras have ISO ranges from 200 to 1600, while others start much lower (25, in some cases) and extend well beyond ISO 12800. This depends on your camera body. You can adjust the ISO by holding down the ISO button and spinning the main command dial, though this process differs from on some higher-end models.
Choosing the Right ISO
While your camera body determines the minimum and maximum ISO settings, it's up to you to choose the right ISO for the lighting situation. This might take a bit of practice as you adjust to using a flash, since the flash adds artificial light to dark settings that might have previously dictated a high ISO. As a flash-less rule, high ISO settings come into play when the light starts to fade. On a bright day when shooting outside you can get away with ISO 100 or 200. Cloudy days usually necessitate ISO 400, and ISO 800 is best for shooting indoors. When you first start shooting with a flash try to keep the ISO the same as you would without the flash. If your shots are really grainy or filled with noise, lower the ISO. Conversely, if your shots are too dark, try raising the ISO.
Tags: Nikon DSLR, your camera, camera body, cameras Nikon, compatibility chart