Sharpening and noise

Learning goal

You will be able to strike the balance between sharpening and noise reduction while editing a photo. You will explain that sharpening brings out existing detail by enlarging small tonal differences at edges, but can't rescue a frame that missed focus or blurred from motion; you will set the sharpening amount and the sharpening radius; you will apply sharpening only to edges with the sharpening mask; you will recognize the waxy look that appears when noise reduction is overdone; and you will make all these decisions not in the small image that fills the screen, but in the actual pixels view; you will also check how sharpening and noise reduction behave on faces of different skin tones.

The question

The highland frame's colors have settled; but when you enlarge it on screen, the face of the cyclist in the red jacket is a little soft, and in the sky you darkened there is a fine, crawling grain. You raise the sharpening slider: the face sharpens, but the grain in the sky also hardens and becomes noticeable. When you raise noise reduction, the sky smooths out, but this time the cyclist's face flattens as if made of wax. In the evening, at the break spot, in the portrait of your dark-skinned friend you took by lantern light at a high ISO, things are even harder: there are colored blotches in the shadows.

How do you balance sharpness and noise? Why do the two sliders pull against each other, and is it possible to rescue a blurry frame with sharpening? In this lesson we will talk about sharpening and noise reduction.

Context

Think of a picture drawn with a pencil. If you make the edges of a line a little darker and right beside it a little lighter, the line looks more distinct; but if the line isn't there in the first place, there is nothing to darken the edge of. Sharpening works like this too: it doesn't create new detail, it emphasizes the edges of what is there. Noise reduction, on the other hand, does the opposite: it smooths the small random differences between neighboring pixels; but some of these differences are real detail.

In earlier lessons we saw what grain is, why it increases at high ISO, and the difference between luminance and color grain; some programs call grain noise. This lesson tells how to use these two tools together in an editing program, and how, while raising one, to limit the problem the other creates. Because both work at a small scale, this lesson's most important habit is examining the photo up close, at its actual size.

Simple explanation

What does sharpening do, and what can't it do?

Sharpening is the editing process that brings out detail by making the light side of edges in the image a little lighter and the dark side a little darker. Because RAW files aren't sharpened inside the camera, they look a little soft when first opened; that is why most programs apply a little sharpening by default. The amount slider decides how strong this emphasis will be; if raised too far, the bright halos we saw in Lesson 7.5 appear at edges.

The sharpening radius is the setting that decides how far the emphasis will spread from the edge. A small radius emphasizes fine detail like strands of hair, eyelashes and fabric texture; a large radius strengthens thicker edges but can lead to a coarse look and halos in fine detail. What sharpening can't do, on the other hand, is bring back lost detail: in a face where focus was missed or motion blurred it, there are no edges; sharpening only hardens the edges of the blur and makes the frame look worse. That is why cutting blurry frames in the reject round is more correct than trying to sharpen them.

How is sharpening limited?

Sharpening touches not only detail but also grain; the small specks of grain count as small edges too. A sharpening mask is the setting that applies sharpening only to distinct edges and leaves out flat, smooth areas like the sky, a wall or skin. In many programs, holding down a key while setting the mask turns the image into a black-and-white map: white areas show the edges to be sharpened, black areas the flat regions that won't be touched.

The mask matters especially in portraits. Skin is a flat surface with fine texture; unmasked sharpening hardens pores, small lines and blemishes, and can make a face look tired and coarse. Raising the mask and leaving the skin out of the sharpening keeps the eyes, eyelashes, eyebrows and the edges of the hair sharp while leaving the skin natural. This prevents the surface of dark skin from looking rough and ashy, and the pores and redness in the cheeks of light skin from being exaggerated.

Where does noise reduction stop?

Noise reduction reduces graininess by smoothing the random differences between neighboring pixels. Reducing color noise, that is, removing the red, green and blue blotches in the shadows, usually does the image little harm, and in many programs it is on by default at a small value. Reducing luminance noise, on the other hand, calls for more care, because along with the specks it also erases real fine detail.

The waxy look is faces and textures losing their detail and looking flat and shiny like wax or plastic when noise reduction is overdone. This look appears differently on every skin: on dark skin, detail on the shadow side being erased and turning into a blotchy, flat brown; on light skin, losing its freckles, fine hairs and texture and becoming as smooth as a doll's face. Leaving a little grain often looks more natural than a face with no texture; if needed, applying noise reduction locally only to a flat area like the sky protects the face.

At what size is the decision made?

The actual pixels view is showing the photo on screen at one hundred percent, where each image pixel corresponds to one screen pixel. In a photo shrunk to fill the screen, both grain and the halos sharpening creates are hidden; that is why all the decisions for these two tools should be made at actual size. Choosing an area where you can see a face, an edge and a flat area, for example the sky, at the same time and working there helps.

The order many photographers follow is this: first bring grain down to an acceptable level with noise reduction, then apply sharpening only to edges with the mask. That way sharpening doesn't harden again the grain that has already been smoothed. Output sharpening, done separately to share a photo online or print it, is a different step, applied during export according to the photo's final size.

Visual evidence

You open the highland frame in the actual pixels view; you choose a spot where you can see the cyclist's face, the edge of the jacket and a piece of sky at the same time. First you reduce luminance noise by a small value; the crawling in the sky softens, and the jacket's texture stays. Then you raise the sharpening amount and keep the radius small; holding the key down you open the mask, and raise it until the light-skinned cyclist's cheeks and the sky turn black. Their eyes, eyebrows and the jacket's zipper sharpen; the texture of their cheeks and their freckles stay natural.

Then you move to the lantern-light portrait. On the shadow side of your dark-skinned friend there are red and green blotches; when you raise color noise reduction a little, the blotches disappear. When you push luminance noise reduction to a high value, their face flattens like wax, and the detail of their eyelashes and beard is erased; when you cut the value in half, a fine grain comes back, but so does the face's texture. You apply sharpening with the mask only to the eyes and the edges of the hair. The frame isn't completely clean; but the warm, grainy atmosphere of the lantern light and the deep brown of your friend's skin are in place.

Common mistakes

Summary

Sharpening brings out existing detail by enlarging tonal differences at edges, but can't rescue a frame that missed focus or blurred from motion; the amount decides the strength of the emphasis and the sharpening radius its spread, while the sharpening mask limits it to edges and leaves flat areas like skin and sky out. Noise reduction smooths grain but also erases fine detail, and when overdone flattens faces like wax; reducing color noise is usually safe, reducing luminance noise is a step that calls for care. Reducing noise first, then sharpening with a mask, and making all decisions in the actual pixels view strike the balance while protecting the face's texture on every skin.

Mini assignment

In this assignment you will sharpen two different frames at actual size and reduce their noise.

1. Choose a frame shot in good light and a frame shot at a high ISO; open both in the actual pixels view.
2. In the high-ISO frame, reduce first color noise, then luminance noise, in small steps; stop at the point where the waxy look starts on the face and take it back a little.
3. In both frames, set the sharpening amount and radius, then open the mask and raise it until the flat areas turn black.
4. In a portrait, compare unmasked and masked sharpening side by side; look at the skin's texture and the sharpness of the eyes.

Check yourself

• Were you able to see at actual size where you needed to stop noise reduction?
• Was the mask able to keep the skin out of the sharpening while keeping the eyes sharp?
• In the high-ISO portrait, does the person's skin stand with its own texture, without turning blotchy or plastic?

Glossary

Mini quiz

1. What does sharpening do?

• A) It brings lost detail back to a frame that missed focus
• B) It brings out existing detail by enlarging tonal differences at edges
• C) It saturates colors
• D) It reduces grain

2. What is the sharpening mask for?

• A) It limits sharpening to edges and leaves flat areas like skin and sky out
• B) It sharpens the whole image equally
• C) It turns the image black and white
• D) It increases grain

3. Why should sharpening and noise reduction decisions be made in the actual pixels view?

• A) The program works only at that size
• B) The sliders work more precisely
• C) Colors look more accurate
• D) Halos and the waxy look are hidden in the shrunk image

4. In a portrait shot at a high ISO, when you pushed noise reduction all the way, the dark-skinned person's face turned into a flat, blotchy brown. What should you do?

• A) Raise sharpening to the maximum
• B) Pull noise reduction back and leave a little grain, applying it locally only to flat areas if needed
• C) Raise saturation
• D) You have to delete the frame

Answers

1. B — Sharpening doesn't create new detail; it emphasizes the edges of what is there.
2. A — The mask keeps sharpening from hardening grain and skin.
3. D — Small-scale effects only show at one hundred percent.
4. B — A little grain is more natural than a face with no texture; local application protects the face.

Summary card

• Sharpening emphasizes existing detail; it doesn't rescue blur.
• Amount sets the strength, radius the spread; too much makes halos.
• The mask limits sharpening to edges; skin and sky stay out.
• Noise reduction erases detail too; too much gives the waxy look.
• Reduce noise first, then sharpen with a mask; decide at actual size.

Bridge to the next lesson

The highland frame is now sharp and clean; but the corners of this frame, shot with a wide angle, are a little darker than its middle, and the horizon line looks slightly bent toward the edges of the frame. There is also a thin purple line along the edges of the jacket. Fixing these one by one by hand would take hours. Can lens flaws be fixed with one click? The next lesson talks about lens corrections, and how a lens profile removes corner darkening and distortion.