The important difference between the highlights slider and whites slider is in the tonal ranges of an image each slider affects.
The highlights slider is designed to bring back detail [moving slider to the left] in the brightest areas of an image or to brighten [moving slider to the right] highlights while protecting against clipping. The whites slider sets the ‘white point’ (brightness) or extreme tonal range of an image, by either lowering [slider to left] or raising [slider to the right] this ’white point’ value. In other words, the whites help you to define the true white in an image and the hightlights help to recover lost detail in the highlights of an image.
Understanding these differences is key to knowing when to use each slider effectively. Fortunately, Lightroom (as should every ‘image tool’ worth its salt) provides you with a histogram to help you with these decisions.
If you are unfamiliar with histograms, this explains it very well. You’ll find histogram types will vary; Lightroom uses 5 categories (Black, Shadows, Exposure, Highlights & Whites) to define the areas of a histogram, however, throughout, the concept remains the same.
We can use Lightrooms histogram directly to provide a visual representation of what is happening to the pixels of an image as you adjust the highlights and whites.
By manipulating the sliders in this way, you’ll see exactly what part of the tonal range the hightlights and whites affects. In both cases, moving the slider to the right causes an expansion of the images tonal range. Moving each slider to the left causes a compression of the images tonal range. For this reason it’s often useful to watch these changes on the histogram alongside the image itself to make sure your image doesn't lose detail through clipping.
Both sliders are part of the ‘Basic’ panel in the Develop module of Lightroom and are considered global adjustments. These sliders have come under a number of changes since being introduced. Prior to Lightroom version 4 the highlights and whites sliders were called recovery and brightness respectively. This in a way helps to give us some indication from their wording, to what each slider’s originally intended purpose was for.
Not all pixels are (necessarily) created equal.
In earlier versions of Lightroom you had to be very conservative with your adjustments as they were rather cavalier, affecting a much large range of pixels in an image somewhat equally. With each subsequent update of Lightroom, the effectiveness/usefulness of these ‘basic develop tools’ have been vastly improved. The changes they adopt are now much more subtle and provide much better protection for the pixels outside their respective tonal range ‘zones'. Under the hood, the algorithms used to determine what pixels are affected have become more and more complex and increasingly more accurate. The benefit is that these sliders now work hard to maintain the integrity of your image while allowing for some extreme adjustments.
The highlights as with the whites only affect the luminosity of the pixels of an image. As this is the case, it’s often easier to see the subtle differences between the two when ignoring colour altogether.
Now let's look at that same grayscale 16-step wedge, but this time changing the highlights slider in Lightroom to +50…
Now let's look at the original grayscale wedge, but this time with the whites set to +50…
In this example, I've chosen to use adjustments of +50/-50 as they are considered normal adjustments. Beyond this, Lightroom uses a different set of filters/algorithms for more extreme adjustments. As you can see there are differences, albeit subtle ones. The whites have a greater overall effect on the expansion or compression of the tonal range of the image. You’re more likely to overdo it when applying extreme adjustments with the whites than the highlights as a result. In both cases they do a great job of protecting pixels from the other tonal ranges.
When to apply ‘highlights’ and when to apply ‘whites’
It’s all very well showing us neat grayscale images but how does that help me knowing when to apply each slider to my actual images?
Highlights – every cloud has a silver lining.
Recovering detail from a blown out cloudy sky is a typical example where the highlights slider comes into its own. By applying a -100 highlight adjustment to the above image the detail in the clouds/sky was recovered very successfully without visible image degradation. Recovering blown out detail in water reflections is another great useage of the hightlights slider. If your image has on the whole been exposed correctly but the highlights are subdued, then increasing the highlights slider can recover this detail without changing the overall brightness.
Whites – When white isn’t white.
Ansel Adams believed that every image should contain a full range of tonality from something true black to something white to truly stand out. This concept remains as true today in the digital world as it did in the film era.
The below image was exposed to the left and as a result had very little highlights and no whites to speak of. It’s a rather flat poorly exposed image.
Extending the whites (tonal range) of this image by moving the whites slider to the right will go a long way to improving it. By how much can be a bit hit and miss at times with the naked eye.
Fortunately, Lightroom provides a handy trick to make things clearer. Holding the ALT/Option key whilst moving the whites slider provides a threshold screen that displays the exact level of whites in an image at any given adjustment.
It’s a good example of what can be done to rescue an image that was destined for the recycle bin.
In reality, these adjustments are rarely carried out solely on their own but usually in combination with the other basic panel sliders found in the Develop module. E.g. it’s common to bring up the whites after reducing the highlights for better contrast. However, the amounts to adjust will all depend on your style & vision and the individual image in question. Lightroom being non-destructive gives you the opportunity to experiment to your heart’s content at no cost (apart from time well spent!).