PSC04: Sharpening

Why do we need sharpening?

There are a few reasons why sharpening is a very important step in today's digital workflow. For example 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 those sensors are equipped with a filter, that slightly softens the image. Another occasion where image sharpness suffers is any form of resizing, for example if you prepare images for the web.

What we call sharpening cannot exactly create more detail in a picture, but it can increase the perceived sharpness of the image. The way the human eye detects sharpness is by contrasts. Wherever there's a clear boundary between a bright and a dark area in a picture, your eye will perceive this as sharpness.

Now in order to increase the perceived sharpness of an image, all you'll have to do is to increase the contrast of those boundaries. Or in short: make the dark side of a contrast boundary a little darker, and the bright side a little brighter.

Different levels of sharpening (zoomed in 500%)

Most digital cameras do that automatically, and in many cases it cannot even be switched off. If you want full control over the sharpness during post processing, you should find out if your camera allows you to switch off in-camera sharpening.

When to sharpen? 

As sharpening changes the contrast balance of your image, and other steps such as resizing reduce image sharpness, it is very important that the sharpening step is the very last step in your workflow.

Unsharp Mask

For most photo professionals, Unsharp Mask is the tool of choice to do their sharpening. The name might be a bit misleading, and it comes from the good old days of analog film photography where photographers had to create and use a special mask during development in order increase the perceived sharpness. Now that we are in the digital domain, we have much finer control over this whole process.

In Photoshop you find the Unsharp Mask tool under Filter -> Sharpen -> Unsharp Mask

There are three settings with the Unsharp Mask tool: Amount, Radius and Threshold.

The radius determines how many pixels the tool should include in its contrast enhancement, the threshold detemines how much of a contrast difference there needs to be in an image in order for the tool to actually do something, and last but not least there's the amount setting that tells Unsharp Mask how pronounced you want the effect to be.

Getting these three settings right is the key to good sharp images that don't look too dull or too over-sharpened.

The Methodical Approach

Let's look at a very methodical approach that should help you get started with the Unsharp Mask tool. But be aware that this can only be a starting point and it can't replace your judgment.

Step 1 - the radius: For illustration purposes zoom in really far into a picture so that you can clearly see the individual pixels. Find a vertical boundary with high contrast, such as the edge of a building against the sky. If you look at that boundary at this zoom level, you'll find that it is not sharp but actually spans a few pixels. That's the picture's "unsharpness", and it is determined by many factors, such as the actual focus of the image or the quality of the lens. Let's say it is 3 pixels wide. This determines your settings for the unsharp mask tool. Set the radius to half of the "unsharpness", in this case to 1.5 pixels.

Step 2 - the amount: Zoom your image to 100% by double-clicking the zoom tool in the tools palette.

Now slowly increase the amount of the sharpening while watching the edges in the image. Make sure the Preview box is checked, so that you can see the changes in the image. At this point you can also uncheck and re-check the Preview box in order to compare the image before and after the change.

Step 3 - the threshold: If you cranked the amount setting up quite high, you might have noticed that the sharpening algorithm does not only sharpen edges, but it will also increase the level of noise in the flat areas of your picture. In order to avoid that, slowly increase the threshold setting until you have a good balance between edge sharpness and noise level.

Avoiding Halos

As the unsharp mask tool increases the brightness on the bright side and the darkness on the darker side of contrast boundaries, it is very easy to get the settings too high and create so called halos. There are dark and bright halos, but usually the bright ones are the ones that are most irritating and give an image an oversharpened look.

Halos usually happen if you get the radius and/or the amount setting too high. But even if you get the settings spot on, you might still find some halos in your picture, and this is where we get into the topic of halo control.

Halo Control - Advanced Unsharp Mask using Layers

Let me present you with a technique that I learned from Russell Brown. It might seem quite blunt, but it works remarkably well and gives you full control over halos, and it's my most frequently used sharpening method.

Once you're finished edting and resizing your image, create a new flattened layer by pressing Ctrl-Alt-Shift-E (PC) or Command-Option-Shift-E (Mac).

Zoom in to 100% and apply an unsharp mask to that new layer with a setting of 500%, 0.4 pixels and a threshold of 0. And yes, this will give you a very oversharpened look, but we're not done yet. Change the name for this layer to "darken" by double-clicking its name, and then set its blending mode to "Darken" too. You can find the blending modes pulldown at the top of the layers palette. With the "Darken" blending mode selected, the new layer will only help pronounce the dark sharpening halos without touching on the bright ones.

The next step is to duplicate the sharpened "darken" layer by dragging it to the "new layer" icon on the bottom of the layers palette, rename it to "lighten" and set its blending mode to "Lighten".

Reduce the opacity of the "darken" and "lighten" layers to 0 and you're done with the preparations.

Now start to gradually increase the opacity of the darken layer until you find that you have enough sharpness. Do the same with the lighten layer, slowly increase its opacity until you see the first halos appear, then reduce the setting a bit.

You will have to play with those settings, but in general the darker halos are much less detectable for the human eye than the bright ones, so in many cases you'll find that you can dial in much more of the darken portion than you can of the lighter one.

But the really cool thing about this method is that you can now create layer masks on those two new layers in order to fully control halos throughout the picture. Imagine you have the sharpening just perfect for the whole image, but there is one tiny area where a bright visible halo appears. Just mask that portion of the lighten layer away and you're all set.

Other Sharpening Methods

Even though Unsharp Mask is the most frequently used way of sharpening your images, there are numerous other methods that can be used. Let's have a brief look at a few other ways of sharpening your images.

High Pass Sharpening

Once you're finished edting and resizing your image, create a new flattened layer by pressing Ctrl-Alt-Shift-E (PC) or Command-Option-Shift-E (Mac).

With this new layer selected, go to Filters -> Other -> High Pass and in the High Pass dialog select a small radius that will just start to show you the edges of your image. Now set the blending mode of this new layer to Overlay, Hard Light, Soft Light or Vivid Light and you'll have a nice sharpening effect. If you find that the sharpening is generally alright but just a bit too much in some areas of the image, simply use a layer mask on that layer in order to reduce the sharpening in the areas where you don't want it.

Luminance Sharpening

This is a sharpening method that works particularly well for images with a higher noise level. An image is typically made up of three colour channels: red, green and blue. If you go to the channels palette by selecting Window -> Channels and then look at the individual colour channels of your image, you'll find that the different channels usually have a different amount of noise in them. Very often the blue channel is the noisiest one.

Luminance sharpening allows you to only sharpen the luminance part of your image without touching the individual colour channels.

Enable all colour channels again by clicking the RGB channel, then select Image -> Mode -> Lab Color and you'll see that all of a sudden the image is made up of a lightness channel and two coulour channels labeled a and b. Click on the eye symbol next to the a and b channels and select the lightness channel only. Now apply your usual Unsharp Mask to the lightness channel only and after you are finished, re-enable all channels by clicking the Lab channel.

Convert your image back to RGB mode by selecting Image -> Mode -> RGB and you're done.

Selective Sharpening

Last but not least, here's a piece of trickery that will allow you to increase the overall perceived sharpness of an image without actually sharpening all of it. Our eyes are easily tricked into believing that an image is sharp by simply very selectively sharpening important areas of an image.

In Photoshop press the R key to bring up the Blur/Sharpen/Smudge tools. Select the Sharpen tool by clicking and holding the mouse on the according icon in the tools palette and then in the tool's option bar on the top of the screen, set it to a strength of 50%.

Now identify the critical areas of your picture. In a portrait you'll want to look for things like highlights in the eyes, the edge of a person's teeth, some strands of hair that contrast with the background, a piece of jewellery that has a high contrast, buttons on a shirt, ... basically anything that shows some contrast edges and detail. By simply painting once or twice over those areas with the Sharpen tool, you can achieve some stunning results. You can even set the blending mode for the tool to Darken or Brighten in order to control halos!

And if you select the "Sample All Layers" option and do your edits on a new layer, you can even change your edits afterwards!

Give this method a try, it can really help you to get the most out of that one special picture without compromising noise levels or sharpening areas that don't need it.

Conclusion

In today's digital photography world, sharpening is an indispensable part of the digital workflow. Many digital cameras take the decisions for the photographers, but especially in the area of sharpening you'll want to try and retain as much control as you can. You might not want to go that extra mile for all your family snaps, but paying attention to sharpening will allow you to get those extra 10% out of your images when you need it.

And always keep in mind that the sharpening stage should be the very last thing you do with your images in your workflow.