Updated: Dec 30, 2019
If you're into landscape photography then you've probably heard of (or use) gradient neutral density filters. They're commonly used by landscape photographers to help balance out an exposure when dealing with very bright skies and darker foregrounds. The filter (glass panel mounted to the front of the lens) has a darker tint at the top and progressively fades to clear glass at the bottom. The idea is that it acts a lot like a pair of sunglasses for your lens by shading the top half of your exposure while allowing full light to the bottom of your frame.
Cameras are not like our eyes; they can't handle a strong variation of brights and darks. The camera will often result to exposing either the highlights or the shadows correctly, but not both - allowing the opposite to either fall into dark shadows or overexposing bright areas within the frame. This creates a significant challenge for landscape photographers who want to be able to show the full scene accurately. So, landscape photographers often resort to using a gradient neutral density filter to help the camera balance the scene and render a more evenly exposed image.
Inherent Problem with Using Neutral Density Filters:
Although the filter does enable a more balanced exposure, it can often come at a cost to the photographer - depending on the scene they're trying to capture. They typically work great when working with a flat horizon as the gradient transition (hard or soft) is linear. Meaning that if the horizon has angles (mountains or other elements protruding into the sky) then the gradient (shading) in the filter also darkens those elements in the exposure. This presents a challenge when brightening those elements in post processing (editing) without causing haloing or noticeable editing strokes in the final photo, depending on the editing tool used.
Enter the Range Mask Tool in Lightroom:
To avoid the issues presented by the gradient filter, a tool has been introduced into Lightroom that will help solve the issue of landscape photos with elements pushing beyond the horizon line. Now, landscape photographers can take a single RAW exposure out in the field without using any physical (glass) gradient ND filters, and make relatively similar exposure adjustments in Lightroom after the fact without compromising the elements above the horizon line when using a glass filter over the lens.
Here's how the new workflow might work should you want to try it:
Take a RAW single exposure of the desired scene. Do not use any gradient neutral density filters and simply use the histogram on your back screen to ensure you're not clipping either the highlights (far right) or shadows (far left). I typically expose slightly to the left, ensuring I'm not clipping any highlights while rendering my shadows a little darker then normal. My camera, along with most mirrorless/DSLRs these days, tend to be able to recover detail in the shadows better then the details in overexposed highlights. So I simply expose "to the left" to be sure I don't have any highlights that are too bright. In short, I ensure I have a decent exposure of the brighter sky and allow the foreground to be in deeper shadows when taking my shots.
Once I return back from the field, I open the photo in Lightroom and select the Gradient Tool in the Lightroom Basic Panel that can be found in the Develop Module right bellow the Histogram display at the top-right side of the screen.
I then pull down a gradient over the brighter area of my photo, usually overlapping all of the horizon to ensure I avoid any haloing effects once done editing. This is essentially replicating the darkening effect that a gradient neutral density filter would do when used out in the field by darkening the sky (highlights) to render a more evenly exposed image.
The next step is to toggle the Range Mask option to "Luminosity" in the drop down, as seen here:
Once the "Luminance" option is selected, a new set of sub sliders appear, as seen here:
Now, with the gradient in place over the desired area of the image and exposure, clarity, dehaze or any other adjustment made within the gradient, one can drag the "Range" left and right sliders inwards to make adjustments.
By dragging the left side Range slider in (towards the center) you'll notice a reduction of the adjustments made within the gradient you applied. The reductions are more intense for darker areas (areas where objects such as mountains, trees, etc.) overlap into the brighter background (the sky). In short, the left Range slider reduces the applied adjustments on elements interplaying with the horizon line, thus reducing their effect on those specific elements. So, in essence, the Range slider is reducing the effects of any adjustments (especially the reduction of exposure) to any elements interacting with the sky. Now, when using a glass filter over the lens, the darkening effect influences any elements the darkened glass interacts with. In the case of the applied gradient in post, the Range slider allows the user to dictate what parts of the image the reduction in exposure (and other adjustments applied within the gradient tool) are applied to. This is a huge benefit in that the photographer now has greater control over adjustments and what elements are influenced by those adjustments. With a glass filter, you lose this ability and have to work to de-influence elements within the photo in post - which can prove more difficult to do well (so the viewer can't see the edits).
By dragging the right side Range slider in towards the center you create the opposite effect. Meaning that the gradient applied will now increase the adjustments to the darker areas of the image. This can be useful when taking the reverse approach to balancing out your exposure in that you can apply a gradient to the lower half of the image and bring up shadows without touching the brighter areas within the photo. This would be best used if you tend to "expose to the right" on your histogram - meaning you exposed more to the shadows than the highlights.
Okay, so I know I might be getting a little technical at this point, but the general idea is that by using the Luminance Range Mask tool in Lightroom a photographer can maintain more control over highlight reductions and/or shadow recovery without having to rely on physical glass filters. Again, the glass neutral density filters apply their exposure reduction indiscriminately across the top of the frame, regardless of what elements are behind the shading effect. The Range Mask in Lightroom simply provides an alternative that gives the photographer more control over what's influenced in the photograph while also allowing for additional adjustments within the gradient applied.
With that said, a glass gradient neutral density filters can still be useful. There are times when we as landscape photographers get up early enough or stay out late enough to capture incredible sunrise and sunset skies. As we all know, sometimes the best colors appear either well before sunrise or well after sunset. This means that our foregrounds are often very dark, if not virtually black. This is when a glass filter can save the photo by helping reduce the drastic differences between the shadows and highlights at the time of taking the exposure. If we didn't use a glass filter in these scenarios, we might not come away with a salvageable image at all. But in all other cases where the differences between shadows and highlights isn't quite as extreme, it may be time to lose the physical glass filters and use Lightroom's Masking Tool to maintain greater control of your photos.
I hope this read proved useful to you. Feel free to shoot me any questions you might have and I'll do my best to answer them.