how to sharpen an image.

Why Sharpen? Virtually every digital image needs to be sharpened at some point. It makes no difference which camera (or scanner) is used or how good the lens. This is because of the way an image is translated from the camera's sensor into a set of pixels, a process that is inherently soft.

This is why nearly all digital cameras provide some form of in-camera sharpening. If you plan on editing your digital images at a later date however, it is strongly recommended that you avoid in-camera sharpening for two reasons: in-camera sharpening is less accurate than what can be accomplished in Photoshop and two, the process of sharpening actually damages image data in a way that cannot be reversed. Unless there is a reason for doing otherwise*, it's best to turn off in-camera sharpening altogether.

*Whether to leave in-camera sharpening on or off depends on how the images will be used, and whether or not you'll be editing images at a later date in Photoshop. I'd leave sharpening 'on' if the only use of the files were to make a set of 4x6 prints, email or posting to the web. I'd leave it off if I were planning on editing and making enlargements. The bottom line is that an un-sharpened image can always be sharpened, but a sharpened image cannot be un-sharpened. If in doubt, leave in-camera sharpening off.

How much should an image be sharpened? Dan Margulis, widely considered the world's pre-eminent expert in digital image correction, and author of several authoritative books on Photoshop, answers it this way, "As much as you can get away with."

Knowing just how much that is, and how to achieve it, is the subject of this article. Before we attempt to sharpen an image, we first need to understand what sharpening is and how it works.

Sharpening is an illusion. The process of sharpening an image does not magically create detail where there is none to begin with. What sharpening does create is the 'illusion' of sharpness by increasing contrast between adjoining parts of an image. Our eyes perceive this added contrast as sharpness. Sharpening does have limitations... an out-of-focus image can't be fully sharpened back to perfection. 

For as simple as this sounds, entire books have been written on the subject of sharpening and there are literally dozens of various Photoshop techniques that we use to sharpen an image with precision and pinpoint accuracy... or as Dan would say, "Sharpening with a stiletto."

The goal of this article is to shed light on how to sharpen images for photographic output using a basic technique that employs Photoshop's Unsharp Mask (USM).

It may seem like an oxymoron that something called 'Unsharp' Mask is used to 'Sharpen' an image. The term has its roots in the days of film, when sharpening an image required the use of film-masks that were made slightly out of focus... thus 'Unsharp Mask'.

How Sharpening Works
Figures-1 and -2 show the difference between an unsharpened and sharpened image. The extra 'detail' we see in Figure-2 is created by adding contrast to 'tonal boundaries' or 'edges' within the image.

Dakota

Figure 1: Unsharpened

Dakota

Figure 2: Sharpened



The effect of sharpening is clearly visible in Figure-4 below, which has been enlarged and exaggerated to show the effect. Additional contrast is achieved by lightening one side of a tonal boundary (the sky) and darkening the other (the grass and fur). This increase in contrast is perceived as detail when viewed at normal size.

In order to reap the visual benefit from sharpening, the image is actually damaged in the process, which is one reason why we always work on a duplicate of our original file or at least a duplicate layer.

Dakota

Figure 3: Unsharpened

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Figure 4: Sharpened



Sharpening an image "as much as you can get away with," means walking a fine line between over- and under-sharpening. Sharpen too much, and telltale evidence will appear in your prints, like the visible halo's we see in figure 4. To little, and you've missed an opportunity to make an image as sharp as it can be. For this reason, it's best to error on the side of too little sharpening rather than too much.

Photoshop's Unsharp Mask filter, which I'll refer to as USM from here on out, allows you to precisely control where sharpening is applied and by how much. It boils down to just three settings:

Amount
Determines the 'strength' of the edge-contrast  added to the image.... that is, how dark and how light the two adjoining edges become. See figures 5 and 6.

Figure 5: Low Amount

Figure 6: High Amount



Radius
Determines how wide the edge contrast (or halo) is, measured in pixels. The higher the radius, the larger the halo that is created around the tonal boundary. See figures 7 & 8.

Figure 7: Low Radius

Figure 8: High Radius



Threshold
Determines how much difference is required between tonal boundaries before sharpening is applied. See figures 9 & 10. Notice that a low threshold introduces noise into the sky, because very slight tonal variations in the sky trigger sharpening. Threshold is used to control where sharpening takes place, a very useful tool for avoiding sharpening where we don't want it... like the sky in this example, or a facial portrait where we don't want to add unwanted detail to blemishes or wrinkles.

Figure 9: Low Threshold

Figure 10: High Threshold



When to apply Sharpening
Sharpening is almost always applied as the last step in the image-editing process. This means that the image has already been adjusted for color, contrast, density and whatever other adjustments are desired.

Prior to sharpening, the image needs to be cropped and sized to the paper size it will be printed on, and re-sampled to the correct output resolution. In the case of photographic output, resolution is 300 pixels per inch (ppi). If an 8x10" print is being made, the image is cropped/sized to 8x10" at 300ppi.

If multiple print sizes are made from a single image, each print size needs an individual file that has been sharpened independently of the others. This is because optimum sharpening parameters (Amount, Radius, Threshold) can change for each size .

After the image is sized, adjust the zoom-level until the on-screen image is roughly the size of the print to be made. If making an 8x10, adjust until the image is approximately 8x10" on screen. This will provide a fairly accurate preview of how the effects of sharpening will appear in print. If the image is too large on screen, you'll be inclined to under-sharpen. If it's too small you may over-sharpen. With experience you'll find the viewing size that works best for you.

I might note that many other tutorials on sharpening recommend viewing the image at 100%. I don't find this as accurate or useful, but you may. As with all things Photoshop... experiment.

How to Apply Sharpening in Photoshop
Go to Filter/Sharpen/Unsharp Mask.


Start by setting: Amount to 500%, Radius to .01, Threshold to 5.

Begin raising Radius .01 at a time. This is best done by highlighting the radius field, then tapping the up-arrow key. Observe the change to the on-screen image with each tap. Most images at 300ppi respond well to a radius that is between .05 and 1.5 pixels. Tap away, both up and down, until you find a level of sharpening that is satisfactory to you.

If you find that there is large/objectionable jump in sharpness between a single tap, say between .05 and .06, reduce Amount to a lower level, say 250% or 300%, return to Radius and fine tune.

Use threshold to include/exclude sharpening from certain areas based on the strength of contrast between tonal boundaries (remember the sky in figures 9 and 10). In a landscape image, we typically start with a threshold level around 5. This usually eliminates sharpening in smooth areas like sky, but includes most everything else. In a portrait of a person's face, threshold may be set much higher (10-20) in order to avoid unwanted sharpening of wrinkles or blemishes, while still sharpening key details like eyes, hair, teeth and jewelry. You have the right threshold set when areas that you want sharpened... are, and those you don't... aren't.

This basic procedure runs contrary to some other tutorials on the subject. Some suggest a different entry order such as Radius/Amount/Threshold rather than Amount/Radius/Threshold. Others suggest a starting Amount of 100% rather than 500% and a much higher Radius, up to 5 or 6 pixels or more. The bottom line is that many different approaches render acceptable results, and with a little experience you'll develop a set of procedures that work for you.

Knowing if it's Done 'Right'
It's easy to tell if an image has been over-sharpened, because you'll see telltale signs like halo's and/or objectionable artifacts, or pixelization in the on-screen image. The bottom line is this: if you can tell that the image has been sharpened... it's sharpened too much.

Landscapes that contain a mountain range against a blue sky are particularly sensitive to over-sharpening. This is because the tonal boundary between the mountains and sky provide an obvious give-away, especially if the Amount and Radius are set so high that a white halo appears in the sky adjacent to the mountains. Care must be exercised to take note of special areas like this. If you've over-sharpened, try it again using less aggressive settings, or apply USM at different levels to different areas of the image.

Under-sharpening is a little harder to detect, but easier to forgive. If you've been as aggressive as possible in applying USM, without crossing the line that renders an image over-sharpened... you're good. Celebrate with a Bailey's and coffee.

Print the File
There's nothing left to do but print the file and inspect the result. This article is built around the assumption that the output is onto sensitized photographic material like used at DigiGraphics. Different types of output, like inkjet (Giclee), laser print, etc... will respond differently to various levels of USM. So print and experiment.

If you are ordering photographic output, it's not a bad idea to run a few 8x10 test prints in advance of a larger order, especially while learning USM. Even if you are ordering a large print, it's possible to take an 8x10" section out of it for the purpose of printing an inexpensive sectional-proof before the enlargement is made.

Lastly, we can perform a Basic File Check* on incoming files(s) to insure that USM and other image edits will result in quality sufficient to make your mother say "Wow!"

*Basic file check: Inspection of an incoming Print-Ready order.
First file check: $7.50, additional files: $2.00.
 

Every Image is Different
Two completely different images, sized the same and at the same resolution, will require different USM settings. What works for one will not necessarily work for the other. This is because each image has different tonal boundaries, contrast, color, noise, and initial sharpness.  

Taking it to the Next Level
The information in this article is meant to provide a basic understanding of how sharpening works, and presents one method with which to experiment and hone your skills. This method will produce satisfactory results on a wide range of images, and certainly better than if no sharpening is applied at all.

As with nearly all things in digital image editing, there are numerous ways to skin a cat (do people really do this?). In my Photoshop tutoring classes, I tell students to learn and apply basic techniques -like this one for sharpening- until it fails them in some way. If and when that occurs, it's time to introduce another technique that both builds on the first and addresses a particular limitation. When this happens playing golf, it's time for better clubs.

One such technique is referred to as HIRALOAM, meaning Hi Radius-Low Amount. In Hiraloam, the radius might be 40-60 pixels or more, with the Amount set somewhere between 15% and 30%. This technique is useful for images that don't' respond well to traditional USM, like those that contain a lot of noise or, if scanned from film; grain. Some images benefit from the application of both standard USM and Hiraloam techniques, done in two separate steps. 

If you would like to learn more about sharpening, or solve a particular sharpening problem, try one of these resources:

Do a Google Search on: photo image sharpening
You'll find a wealth of free articles and not-so-free books, tutorials, and software.

You are likely to come across contradictory advice on the Web. I know that's hard to believe. This is testament to the fact that there are many ways to approach sharpening, and when you get down to the nitty-gritty technical aspects, there is both discussion and disagreement even in the professional community. Read with an open mind, don't get bogged down in technical minutiae, and experiment with procedures that make the most sense to you.

There is a lot of gray area in digital image editing, and a lot of different ways to end up with similar results. I would be wary only of those who suggest that their way is the best and only way.... few things in Photoshop are ever that clear cut. I recommend against purchasing sharpening software unless you have mastered the tools and techniques available in Photoshop and find that they are limiting you in some way.  

Purchase Professional Photoshop Fifth Edition by Dan Margulis
DigiGraphics' founder, Les De Moss, contributed to, and participated in, the editing of this book, as well as "Canyon Conundrum" by the same author.

PP5E is highly recommended for anyone wanting to take their Photoshop skills up a level... or ten. It's not a beginner's how-to book, but an in-depth look at the techniques used by professionals to extract every bit of goodness out of an image, including images of dubious quality. An entire chapter is devoted to sharpening.

Both books can be found at www.Amazon.com.

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