PSC 17: Sharpening Noisy Pictures

Sharpening your digital images is an essential step in the digital photography workflow, and I've been talking about that already back in Photoshop Corner #04. Before we dive into this specific episode, let me recap a few of the most important reasons for sharpening first.

All digital cameras capture the image on a sensor which has a very regular pattern of pixels on it. And in order to avoid so called moriée patterns, or stair steps in diagonal lines, most of the digital camera sensors are equipped with a filter, that slightly softens the image. In addition, image sharpness also suffers from resizing images, no matter if you upsize or downsize them.

But some images are easier to sharpen than others, and the reason sometimes is noise. If an image has a lot of noise, for example because you took it with a high ISO, then sharpening the the image the regular way can drastically increase the amount of perceived noise too. Of course the actual amount of noise isn't increased, but the sharpening can do a pretty good job in bringing the noise out really well in your pictures.

So let me go into a bit more depth with regards to a sharpening method that can help you sharpen noisy images without touching too much on the noise itself.

Colour Modes

For this, let's take a look at image colour modes first. The mode most familiar to us is the RGB mode, where the image is made up of three colour channels, the Red, the Green and the Blue channel.

But there are more modes in which an image can be displayed and modified, for example the CMYK mode, which uses four colour channels, Cyan, Magenta, Yellow and Black.

But the one that's most interesting for our purpose is the so called Lab Mode.

Now before you get any ideas, Lab does not stand for Laboratory, but simply for the three channels that is usses, two colour channels named a and b, and a Lightness channel named L. Thus the Lab.

In RGB mode, we only have colour channels, so the noise information has to be embedded in the colour channels, and usually most of the noise can be found in the blue channel, followed by green, and red has the least amount of noise in it.

In Lab mode, things work a bit different, as the colour channels are separated from the lightness information. And you can convert your image to Lab mode and back to RGB without loosing any image information. In fact, Photoshop uses the Lab mode internally to convert images between different colour modes.

Lab Sharpening

Now here comes the trick that will help us with sharpening our noisy images. We use the Lab mode for that, and instead of sharpening the whole image, we only sharpen the lightness channel. This way most of the noise information that is embedded in the colour channels won't be touched by the sharpening.

Step 1: Select Image / Mode / Lab Color from the menu

Step 2: Display the Channels palette which sits right next to the Layers palette.

Step 3: De-select the a and b channels, so that only the Lightness channel is selected.

Step 4: Now apply your favourite flavour of sharpening to the Lightness channel only, I suggest going with Unsharp Mask. For more information on how to use it, make sure you listen back to Photoshop Corner #4.

Step 5: Enable all channels again by clicking on the eye icon next to the Lab channel

Step 6: Convert the image back to the RGB mode by selecting Image / Mode / RGB from the menu

And there you have it, sharpening your noisy images this way can really make a difference. It's not a cure for all noise problems, but on some images it will work remarkably well.