The ProsFirst, Develop defaults are user- and machine-specific, which means they will apply to any Lightroom catalog that's opened, or any RAW photo that's opened using Camera Raw, under one user account, on any given computer. This can be useful for any facility where several photographers share the same computer, such as at a newspaper or in a school computer lab. In these cases, there are almost always photographers with more than one of each of the popular camera models. By turning on the preference to make defaults specific to the camera serial number, each photographer with a Nikon D3, for instance, will have his or her own Develop defaults (Fig. 5).
Second, the option to make defaults specific to the camera ISO setting is particularly useful for photographers who do a lot of shooting at high ISO settings. By default, Lightroom applies the same amount of sharpening and noise reduction to every photo imported. Many photographers may want to customize that default noise reduction for one or more ISOs, and this preference gives you that capability.
When you turn on these two preferences and start with a RAW file, the Set Default Develop Settings dialog adds that very specific information, telling you precisely what default you're about to create.
Figs. 6a & 6b: Here, Lightroom is about to create a default for RAW files coming from just one Canon EOS 5D Mark III serial number and only at ISO 1600. Photos from any other 5D Mark III or shot at any other ISO won't be affected. (Note: JPEGs get their own default.)