Why does my noisy high-ISO photo show banding when converted to black and white?
Asked 3/30/2015
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I have an older Nikon D70 image shot at ISO 1600. In color it looks noisy but acceptable, yet when I simply desaturate it in Photoshop, obvious banding/posterization appears in the guitarist’s fingers and some highlights. Why does this happen, and what’s the best way to make a better black-and-white conversion?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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The image has some highlights where there is some blue mixed in, where there is otherwise only information in the red and green channels. It's those highlights that doesn't balance well when you desaturate the image.
You can use the Channel Mixer in Monochrome mode to convert the image, that allows you to balance the channels to avoid the posterising.
Example:
Originally by user149. Source · Licensed CC BY-SA 4.0
user149
11y ago
0
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This happens because a simple desaturation can expose uneven tonal information between the color channels. In your image, some highlight areas appear to contain a bit of blue mixed into regions that otherwise rely mostly on the red and green channels. When Photoshop removes color without carefully rebalancing those channels, those tonal differences can turn into visible banding/posterization.
A better approach is to convert using the Channel Mixer in Monochrome mode (or an equivalent black-and-white conversion tool that lets you control channel contribution). By adjusting the red, green, and blue channel mix, you can smooth those highlight transitions and reduce the posterized look.
In short:
- simple desaturation = poor channel handling
- channel-based B&W conversion = better tonal control
- rebalance the channels, especially in the affected highlights
That should give you a cleaner black-and-white result from a noisy older high-ISO file.
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AI11y ago
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