Image sharpening is so fundamental yet it’s one of the most poorly executed areas in digital image editing.
This is how a typical sharpening workflow looks like of the average photographer:
- 1Apply adjustments to the image
- 2Sharpen the image at the end of workflow
- 3Export for web or print
Even though sharpening once is fine, but you're not maximizing the potential of the image to let it shine!
In this post, you’re going to learn why you should your sharpen images more than once and how to do it.
Table of Content
1. Sharpen Image, Not Salvage Image
How to make a blurry picture clear?
A common misconception about image sharpening is that it can magically turn any blurry image back into focus.
Unfortunately, that's not possible because a blurry image is a technical fault. It can be caused by focusing error or camera shake during shutter release.
Essentially, the image is ruined and there isn't a lot you can do about it.
You can apply sharpening to the extreme end but it will only make the image appear less blurry but it can never be “salvaged”.
2. How Does Image Sharpening Work?
In the simplest explanation…
Sharpening increases the contrast between edges to make lines appear more prominent. The mind then perceive this as being sharper.
To be more technical, sharpening increases acutance.
2.1 What Is Acutance?
It’s the transition between image information.
What does that mean?
The image below is magnified to 11:1. The top half is a wall and the bottom half are some red tiles on the wall. Both are different image information. Therefore, we have two image information here.
Acutance is the transition between the wall and the red tiles, i.e. between image information.saaa
A high acutance means the transition is more abrupt, i.e. sharp.
Low acutance means the transition is more gradual, i.e. across a gradient.
When you sharpen images in editing, you increase the acutance to make the image appear sharper.
2.2 How Does Increasing Acutance Sharpens Image?
When you sharpen images, you increase acutance by darkening the pixels along the darker side of the edge and brightening the pixels along the lighter side of the edge.
See the before and the after of the image below. The arrows point to the darken and lightened edge after sharpening.
A darker and lighter edges increase the edge contrast, which then creates the Mach Band effect that tricks our visual system to perceive the image as being sharper.
2.3 Other Ways To Optimize Sharpness
Sharpness is mainly dependent on two factors: acutance and resolution.
Resolution is another big word that confuses people...I've found a video to tell you everything you need to know in 3 minutes!
Essentially, resolution is dependent on the image sensor. It’s fixed and you can’t change it unless you get a new camera with an image sensor that has a higher resolution.
Acutance, on the other hand, is affected by the quality of the lens but can be improved in post-processing as explained above.
Other factors that affect sharpness include viewing distance and print material.
Viewing distance is not something you can control either but how print material affect image sharpness will be explained below.
3. High Frequency Edge vs Low Frequency Edge
Just when you’re about to get round the concept of acutance...I’m throwing another big word at you!
Don’t panic 🙂
It’s a lot easier than it looks...just hear me out.
High frequency edge refers to images that has texture. The term “high frequency” reflects the sharp change in tonal value within the texture. E.g. buildings, rocks, trees, etc. The change in tonal value is sudden.
Low frequency refers to areas that are smooth. It reflects the gradual change in tonal value. E.g. sky, clouds, water, etc.
Why is this relevant?
Because sharpening is not a one size fits all process. Ideally, both high and low frequency edges should be sharpened separately with different settings.
We will dive deeper into that in a minute.
4. Sharpening Settings In Editing
Although I'm using the sharpening tool in Adobe Lightroom here as an example, these settings are pretty much universal and can be applied in most editing software.
Typically, you can apply these four adjustments in sharpening: Radius, Amount, Detail and Masking.
Once you know how each setting affects the image, you will know how to sharpen images in the best way in almost any editing software.
Remember how increasing acutance darkens the pixels along the darker side of the edge and brightens the pixels along the lighter side of the edge?
Radius controls the number of pixels to darken and lighten along the edges.
As a general approach, set the Radius to 1 for high frequency edge sharpening and higher (2-3) for low frequency edge sharpening.
This controls the strength of sharpening.
Basically how dark or bright you like the pixels set by Radius to be.
You can set it to high for high frequency edge sharpening and low for low frequency edge sharpening.
I set Amount between 50-100% for high frequency edge sharpening and from anything above 0 up to 50% for low frequency edge sharpening.
Affects how much sharpening of the fine and coarse details in an image.
Keep it low to sharpen the coarse details only and high to include both coarse and fine details.
In the example above, setting the Detail to low sharpens only the window frame. When the Detail is set to high, the entire window is sharpened including the glass.
Also known as Threshold in some software.
It controls the minimum brightness change that will be sharpened.
Use this to “mask out” fine areas you don’t want to sharpen, e.g. the sky.
In Lightroom, hold down Opt (mac) or Alt (pc) while dragging the slider for Masking. You will see fine areas starting to turn black, which means it’s being masked out.
I start with the Amount by setting it to 100. Then, I set the Radius depending on what my intention is (high or low frequency edge).
The logic for setting Amount first is just so I can visualize how much Radius I need to sharpen the image. I often have to readjust the Radius to get the effect the way I want it.
Afterwards, I use Detail to control fine sharpening before setting Masking to protect unwanted areas from sharpening.
Tips: Hold down Opt (mac) and Alt (pc) when applying sharpening settings in Lightroom to visualize the effect in black and white.
5. How To Sharpen High and Low Frequency Edge
This is where the fun begins because you can see how your image gets sharpened as you move the sliders.
5.1 High Frequency Edge Sharpening
In case you forget, high frequency edge is when there is a sharp change in tonal value across the edge, e.g. texture.
Here’s a general guide to high frequency edge sharpening:
Set the Radius to low (around 1.0), Amount to high (start with 100) and Detail to high (around 50, depending on the image). Use Masking to mask out finer areas that don't need sharpening.
5.2 Low Frequency Edge Sharpening
This is when tonal change across the edge is a gradual, i.e. smooth, such as the sky.
Set the Radius to high (around 2.0 or more), Amount to low (personally, anything below 100) and Detail to low (high Detail exaggerates image noise). You may not have to set Masking...but if you do, keep it low.
6. How To Sharpen Edges With Triple Sharpening
If you’re like me...you probably think you’ve been doing it wrong since like forever…
...because most people only sharpen image once, normally at the end of the workflow.
But to bring out the best of your image, ideally image sharpening should be done in three stages:
- 1Capture sharpening
- 2Creative sharpening
- 3Output sharpening
6.1 Capture Sharpening
Also known as deconvolution sharpening. I called it the BEFORE sharpening.
This means sharpen images on Raw conversion, before you start messing around with adjustments.
Why is this necessary?
Capture sharpening is to address deficiencies in the optical and capture system, i.e. your lens and image sensor. A degree of blurring is inevitable with the sensor's anti-aliasing filter and demosaicing process in Raw capturing.
All digital images benefit from capture sharpening. Similar to lens profile correction and chromatic aberration removal, capture sharpening should be done globally to the entire image.
How is it done?
Using the frequency based method explained above.
But what if you have an image that has mixed frequency edge? The image on the right illustrates my point where the foreground (high frequency edge) is surrounded by a smooth background (low frequency edge).
This is where creativity comes into play!
One way of doing this is through layer mask and blending.
Blending High and Low Frequency Edge Sharpening
In Lightroom, create two snapshots of the image. One for high frequency edge sharpening and the other for low frequency edge sharpening. Label them so you don’t mix them up later. Click here if you don’t know how to create snapshots in Lightroom.
Open one of the snapshots in Photoshop as a smart object by right clicking the image in Filmstrip > Edit In > Open as Smart Object in Photoshop.
In Photoshop, duplicate the smart object layer by going to the main menu on top. Select Layer > Smart Objects > New Smart Object via Copy.
In the new smart object layer, double click on the image thumbnail to bring up the Adobe Camera Raw filter. Go to the Snapshots tab, which is on the far right and select the other snapshot. Hit OK.
Blend both layers together using layer masks. Now you have an image that is optimally sharpened for high and low frequency edge.
6.2 Creative Sharpening
The DURING sharpening.
This is where you sharpen the image selectively to put more emphasis on areas you think are more important.
There isn’t a magic formula here, which is why it’s called “creative”. What and how you sharpen the image depends on the type of image and your vision.
For example, in a landscape image, you might want different levels of sharpening for the foliage and the mountains. In portrait, you can apply a variety level of sharpening to different parts of the model’s features.
Creative sharpening can be done using any sharpening technique...
In Lightroom, you can use the Brush tool to selectively apply sharpening to parts of the image. Alternatively, use a third party plug-in such as Nik Sharpener Pro. Make sure you check software compatibility before purchasing any plug-in for your editing software.
I frequently use the Smart Sharpen Filter in Photoshop and layer masking for creative sharpening. It's more flexible as there are more options to customize the sharpening level.
If you're not familiar with Smart Sharpen, here's a short video for you.
6.3 Output Sharpening
The AFTER sharpening.
Not the type of sharpening everyone does.
But now that you know, consider it when you export your file next time 🙂 Check out the image below to see the difference between before and after output sharpening.
Why is this necessary?
Each medium displays images differently. Also, resizing an image on export forces some pixels to be discarded (therefore losing its sharpness) in order to fit into the new dimension.
Images for web display requires low output sharpening (depending on the type viewing monitor) but images for print need various levels of high output sharpening depending on the print material and its ink absorption.
The aim of output sharpening is to optimize your image for display.
If you’re a Lightroom user, there’s handy way to apply output sharpening on exporting image file. Go to File > Export or keyboard shortcut Cmd/Ctrl + Shift + E, then scroll down and check the box in Output Sharpening.
7. Common Sharpening Mistakes
The commonest mistake in sharpening is over sharpening.
Not only it makes your image looks fake, it can also exaggerate sensor dust and image noise to degrade image quality.
This is one of the reasons why I do second look on images I've edited recently. Taking this extra step allows me to see the image with a fresh pair of eyes.
I can't tell you how many times I've said to myself "what the heck was I thinking?!".
Here are three common pitfalls to sharpen images and how to avoid them.
Halo is an awkward prominent bright line that runs along the edges.
When you sharpen images, you actually create some degree of halo...because that's how an image is perceived sharper! But normally this is only visible when you zoom in to like, for example, 200%.
Halo becomes an issue when it's prominently visible. The width and brightness of the bright line is dependent on the Radius and Amount you set.
To get rid of halo, fine-tune sharpening by reducing the Radius and the Amount until it disappears or become less visible.
7.2 Image Noise
Image noise can be exaggerated following sharpening...especially in low frequency edge such as the sky.
This is where mixed frequency edge sharpening solves the problem.
Alternatively, use Masking to mask out areas you don't want to be affected. Either that or lower the Amount and Detail of sharpening.
7.3 Excessive Sharpening
There are occasions where the image has no technical fault from sharpening but just doesn't look right.
That's commonly because the amount of sharpening is simply too much for the particular image.
The simplest way to deal with this is by lowering the Amount and Detail. This solves the problem 99% of the time!
8. Take Home Message
The idea of triple sharpening may sound tedious but it’s a proven technique to produce razor sharp images.
Having said that, don't confused sharpen images in editing with getting the focus right in camera!
I don't triple sharpen images all the time...but when I feel the image is right, I'd invest some time to develop it to its full potential.
Sometimes, you can combine the first two stages into one if the composition allows. But output sharpening should always be done.
What do you think about triple sharpening? Share it by leaving a comment below.
For more tutorials on image editing technique, please check out the editing technique resource page!