Not quite sure what the Fill Light slider is doing? Well, think of it this way – when you adjust your Exposure setting, Lightroom will tend to make the entire photo brighter. This may be fine most of the time but it may eventually push some of your highlight areas to the point where they’re clipped and become all white. If you reach that point and you’d still like to make the photo brighter, you can resort to the Fill Light slider. This will adjust the midtone areas of the photo and will, for the most part, leave your extreme highlights alone. It’s a good way to brighten the overall photo without blowing out your highlight areas.