DEVELOP - Adobe Photoshop Lightroom 3: Streamlining Your Digital Photography Process - page 237

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Figure 4-126
h is choice is important. Remember that Develop settings are absolute. h us,
a preset that contains a value for a given adjustment will override the current
setting when the preset is applied. Any adjustment that isn't included in the
preset will not be changed when the preset is applied.
For example, if you have a photo with a Contrast setting of +25 and you apply
a preset that contains a Contrast setting of +35, the resulting value will be +35
(not +60). But if you apply a preset that doesn't contain a setting for Contrast, it
will remain +25, regardless of the other adjustments that are changed.
Since each adjustment checked in the dialog box will be saved into the preset
using the current value, you should carefully consider which settings to include,
and in most cases, it's best to include as few adjustments as possible within each
individual preset so you can apply them progressively.
Eventually, you'll likely make some presets with lots of settings included, and
others that include only a single adjustment. It all depends on the purpose of
the preset.