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

Hello! You're not the only one with that dilemma. Unfortunately, you're running into the limitations of manipulating the image data. You can only lower the luminance value of one color so far before those bands start to show at areas were that color ends or other colors are also present. That's just how digital images work.

Here's a couple things to try, none are quick fixes, but might help you!

First, make sure you editing RAW images, not JPEG. JPEG is an already compressed format that lowers image quality and those bands will show much MUCH sooner than if you were editing a RAW images.

Second, if you're ONLY shooting b/w, consider using a red filter or buying a camera dedicated to greyscale imaging—Leica makes one, as do a few others. They aren't cheap, but the image quality is second to none. Seriously, they are insanely sharp and smooth in their tones. Using a red filter on your current camera will tone everything red and you will loose a few stops (2 ish) of exposure, but the red will filter out blue light, i.e. the sky. So once you convert to b/w the sky will be darker to begin with. This is't always the best solution on a color sensor though. Try it out regardless, red filters are cheap.

Finally, try a gradient ND filter if your horizon line in mostly clean (i.e. no messy, prominent object sticking up into the sky). You can also stack this with a polarizer.

Hopefully one of these helps! Good luck.