Do You Really Understand Sharpening?

Sharpening is a discipline everyone uses in photo post-processing, yet it is incredibly complex. Lab sharpening, high pass, multi-step sharpening, edge masking... the various methods are mind-boggling — there's even a 300-plus-page book, 'Real World Image Sharpening', devoted to sharpening alone.

Today, Thomas simplifies it all: starting from the basic principle of sharpening, I'll show you how to use the simplest yet sufficiently powerful Detail panel in ACR/Lightroom to sharpen your images.

Topics: the principle of sharpening; the four elements — Amount, Radius, Detail, Masking

Reading time: about 8 minutes

1

The principle of sharpening

Thomas has seen many 'sharpening secrets' online that directly hand out 'golden parameters' like 'Amount 100, Radius 1.2'. But how much sharpening to apply depends on the image's size, content, output medium and many other factors — there is no such thing as a best parameter. Only by understanding the principles and judging each image with our own eyes can we sharpen best.

The principle of sharpening is actually simple: by enhancing the contrast and separation along object edges, edges appear clearer and sharper. For example, in the edge below, one side is a dark gray object at 75% gray and the other a light gray object at 25%. (The gray values are approximate; only the relative sizes matter.)

I sharpened the image above in PS. You can see 4 extra lines appear along the edge — the image that had only 75% and 25% gray now has 6 tones, from left to right:

(originally 75% gray) object — (60% gray) separation band — (90% gray) edge line — (10% gray) edge line — (40% gray) separation band — (originally 25% gray) object

First look at the very edge of the object: two nearly black and white lines appear, so the original 75% vs 25% brightness contrast jumps to 90% vs 10% — greater edge contrast, looking sharper.

Next, why is there an extra (60% gray) band — lighter than both sides — between the (90% gray) edge line and the (75% gray) object? Because sharpening not only strengthens edge contrast, it also separates the edge from the object itself. This new separation band makes the edge more distinct from the object's interior.

So the principle of sharpening is twofold: first, greater edge contrast; second, more distinct edges (separated from the object's interior).

2

The ACR/LR sharpening tool

Understanding the principle isn't enough — how to use it is the key.

In Adobe Camera Raw, the sharpening tool lives in the third panel — the Detail panel.

In Lightroom (LR), the Detail section sits below Split Toning.

In either application, the sharpening tool in the Detail panel has four sliders: Amount, Radius, Detail and Masking. They correspond exactly to the three major problems in sharpening.

Take the over-sharpened photo above as an example — the three problems are:

1) The leaf edges are sharpened too strongly and look fake. Sharpening strength is decided by the 'Amount' slider.

2) Obvious white and black fringes appear along the leaf edges — the sharpened region is too wide, turning what should be a subtle edge contrast into big white and black blobs. The sharpening region is controlled by the 'Radius' slider.

3) Areas inside the water that aren't edges got sharpened too, suddenly adding lots of noise. Deciding what is and isn't an edge is controlled by the 'Detail' and 'Masking' sliders.

2.1

Sharpening Amount

Amount decides the sharpening strength — i.e., the edge contrast and the degree of separation between edge and object. Below is the edge at ACR's default Amount of 25.

Once Amount is raised to 150, not only is the contrast between the two edge tones greater, but multiple 'separation bands' appear, making the edge even more prominent.

On real photos, we decide Amount based on the subject: portraits may only need 20-30, while architecture and sculptures can exceed 100 without looking overdone. How to judge? Step one: zoom the photo to 100% or more, so you can clearly see the sharpening effect.

Step two: drag the Amount slider until the object's edges and details stand out. (While dragging, hold Alt to turn the photo black-and-white for easier observation.)

2.2

Sharpening Radius

Radius decides the region the sharpening acts on — the larger the radius, the more area around an edge becomes a contrast band. In the photo below, with the default Radius of 1.0, only 3 pixels along the edge are sharpened.

When Radius increases to 3.0, the sharpened region along the edge grows to 8-9 pixels.

Too large a radius creates black and white fringes around object edges — looking rather scary.

Remember: in the Detail panel, the Alt key is extremely useful. While adjusting Radius, hold Alt to see exactly where sharpening acts along the edges.

Holding Alt, an oversized radius shows obvious black and white lines. For most photos, keep Radius at the default 1 or lower — it rarely exceeds 1.5.

2.3

Sharpening Detail

As the word 'Detail' suggests, this slider decides how big an edge must be to get sharpened. A small Detail value means we want little detail — only large, obvious edges get sharpened.

Likewise, hold Alt while adjusting Detail: a mid-gray image appears, where 50% gray areas are detail unaffected by sharpening, while darker and lighter parts are detail that will be sharpened. In this architecture photo, holding Alt with Detail at 0, only the crisply outlined building edges show.

If I raise Detail to 26, some smaller edges get sharpened too. For example, the patterns on the building's wall indicated by the arrow were ignored at Detail 0, but get sharpened at 26.

If I push Detail further to 100, edges big and small all get sharpened — even noise in the pure-color sky gets sharpened, which is obviously not what we want. In general, the default 25 suits most photos.

2.4

Sharpening Masking

Detail is additive — the larger its value, the finer the edges that get sharpened. Masking is subtractive: it identifies large solid-color areas like sky, water and skin, and keeps sharpening away from them. Masking is the most important slider in the sharpening tool — it gives us local sharpening.

With Masking at 0, the whole sky gets sharpened, and what should be pure blue sky shows many grainy artifacts.

Likewise, hold Alt while moving the Masking slider: black means completely protected from sharpening, white means sharpened. At Masking 25, the sky still shows many white lines — those areas would be sharpened. Not good!

Push Masking straight to 100: the sky turns fully black — it won't be sharpened at all. But now the building interior is also largely black — wall details that should be sharpened aren't. Masking has gone too far!

For this image, a Masking value around 60 is balanced. The sky is almost fully black (unsharpened), while the building textures remain white (sharpened).

Now compare the sky's 'grain noise' at Masking 0 vs 60. Left is 0, right is 60 — the protection of solid-color areas is instantly obvious.

Summary

1. The principle of sharpening: first, greater edge contrast; second, more distinct edges (separated from the object's interior). 2. The Detail panel in Lightroom/ACR has four sharpening sliders: 'Amount' controls strength; 'Radius' controls the region affected along edges; 'Detail' controls how big an edge must be to get sharpened; and 'Masking' removes sharpening from solid-color areas. 3. There are no golden parameters — judge with your eyes to avoid the 3 problems: use 'Amount' to avoid excessive strength, 'Radius' to avoid bright/dark fringes, and 'Detail' + 'Masking' so edges that need it get sharpened while areas that don't are protected.

I'm @Thomas Chu Photo — every Sunday evening I share an original piece of photography know-how.