How can I mitigate edge halo when darkening skies in post-processing?

Question

I process all my photos to black and white. Although I use a polariser where possible, I often find myself wanting to drop the luminance of the sky further which I do using a Black and White adjustment layer (in Photoshop). This usually results in a lighter halo following the line of the horizon, where the blues of the sky meet whatever the colour of the land is.

My current solution to this problem is to run a brush along this margin with a colour set to the same as the sky and the blend-mode set to Darken. This effectively paints in the lighter band with the sky colour while not laying down the colour on top of the darker horizon. Whilst this is effective, it is very time consuming.

Is there a better approach to dealing with these halos?

Here is a video that illustrated the Darken solution that I mention: https://www.youtube.com/watch?v=wcOZcwHdqz8

Answer 1

The easiest way to do this is through single-image exposure blending. It works well for any image where the sky is clearly separated from the other elements of the picture.

You can do it with one original (colour!) photo, but you need to develop two more separate images from it. Image 1 is the unedited original in colour. Image 2 is edited to have a correctly-exposed foreground/rest of the image, Image 3 is edited for the sky only. Note that any canges to exposure must be applied to the whole image. If you want to darken your skies, you must reduce the overall exposure of the image; do not just reduce the luminance of the blue tones in the image, as this is what creates the halos.

Now that you have three versions of the same image, stack them like this (top-to-bottom):

• Layer 3: Black-and-white image exposed for the foreground
• Layer 2: Black-and-white image exposed for the sky
• Layer 1: Original image (colour)

Apply a white layer mask to Layer 3. Go to Layer 1 and select the whole sky, using the colour selection tool. Now increase your selection by 1 to 2 pixels and feather it. Go back to the layer mask you have applied to Layer 3 and fill the entire selection with black colour.

Admittedly this still needs a bit of work to create the two versions of the image, but if done correctly, it will still save you a lot of manual healing brush work.

For more on exposure blending, check out this website.

Answer 2

Traditional Technique

An in the field technique is to take the photograph through a graduated neutral density filter (or several stacked filters). A similar effect can be achieved in post processing using the graduated filter tool in Photoshop, Lightroom or similar software.

Digital technique

The advantage of the graduated filter tool is that it is designed to more or less replicate photographic practice and tends to make it relatively easy to achieve a reasonable effect.

Example

This is an image I shot yesterday:

This has the graduated density tool applied:

Notes:

1. The RAW image was processed in Darktable, not Photoshop
2. The gradient filter is rotated to roughly follow the tree line.
3. Applying and tweaking the graduated density filter was less about five minutes work...and it probably shows.
4. I don't usually shoot this kind of scene. I like the image with the filter more, your mileage may vary.

Answer 3

For monochrome images, often applying a color filter to a raw file will do this for you without having to use a mask at all. Applying an orange filter, for instance, will darken a blue sky relative to a non-blue landscape. For a predominately yellow/orange/red sky, use a green filter.