Sharpening in Photoshop

Two issues have come to the forefront in this magazine, both at the instigation of Terry Hansen,
our Chairman of Qualifications. As well as the incorrect use of fill flash, Terry has also become
concerned about the misuse (or lack of ) sharpening and asked for a revision article for you
all!

The business of sharpening can get very complex and it would be easy to drift off into the realms of mathematics but seeing how this is p4b we will stick mainly to Unsharp Masking. First
of all though let's set out a few rules:

Rule 1

Sharpening is an intrinsic part of digitizing an image and should not be regarded as a sticking plaster to cover a flaw. It is not a flaw; all digital images have to be sharpened at some stage of their creation.
Rule 2

Do not look to sharpening to recover poor craftsmanship in your image shooting. If your images are fuzzy, look to your technique, your eyesight and your optics! All the rules of conventional photography apply, tripods are sharper than hand holding, flash is sharper than ambient, prime lenses are
sharper than zoom lenses, Nikkors are sharper than Chinese copies - it's the laws of physics, you can't beat them!

Rule 3
Do it once and do it last i.e. sharpen a montage at the end not on individual images. Resharpening a JPEG image after the camera has also had a go at it can really degrade some images.

Rule 4 Do not be seduced by claims for miracle sharpening methods such as single channel, Lightness channel, High Pass Filter sharpening. They are good, they are better but they are refinements not rescue methods! Unsharp Masking Unsharp masking in the digital environment is very similar
to the use of dilute developers in monochrome silver photography. In this conventional image making, the use of very dilute developer causes localised chemical exhaustion in the vicinity of boundaries between very light and very dark parts of the negative. This causes an enhancement of the boundary delineation by means of the Mackie Line. The effect is also known as Acutance and has led to the formulation of a number of high definition developers, which actively exploit the phenomenon. In digital imaging the computer seeks out boundaries with tonal differences and alters the pixels in the boundary area to enhance the apparent sharpness of the final image. The sophisticated abilities of modern computing allow far more control than that which is available with silver emulsions. In Photoshop there are a number of alternatives in the "Filter>Sharpen" menu; ignore the others at least in your early career!
There are three variables in the Un-sharp masking dialogue box

Amount:
Amount determines the emphasis placed on lightening and darkening lines along either side of detected edges i.e. how much lighter the light side is made and how much darker the dark side is made.

Radius:
Radius determines the width of the sharpening lines

Threshold:
Threshold determines where the sharpening will take place, e.g. at places where there are large or small differences in tonal value. For grainy scans a low threshold will result in sharpening of the individual grains. At very high threshold values (i.e. approaching 255) no sharpening at all will occur.

The following are recommendations for the setting that should be employed in various circumstances. Experience and observation might well dictate other setting for optimum sharpening of an image, but they are prepress rules of thumb and have stood the test of time.

Amount
For normal useable resolutions use 80% to 120%
Radius
0.6 pixels at 150 ppi
1 pixel at 300ppi
1.2 pixels at 350 ppi
1.5 pixels at 400 ppi
for fine textures and digital images use a 1 pixel setting

Threshold
5 x 4 4000 x 5000 pixel set "0"
35mm 4000 x 5000 pixel set at "3"
Digital Cameras set at "0", (no grain)

NOTE:
It is considered by many experienced uses of Photoshop that sharpening should be applied in repeated smaller doses. For example if 100% is determined to be the correct total amount, a superior result will result will be obtained by the application of 25% four times.

Preferred Sequence for Sharpening

There are preferred strategies and sequences for the application of sharpening. They are listed below as guidelines - by all means break the rules but do some from a position of knowledge.
1 When judging your sharpening have your image at 100% or 200%.You get 100% quickly by double clicking the Zoom Tool at the bottom of the Tool Bar.
2 When the USM dialogue box is open you change which bit of the image is previewed by clicking on the main image. Use this to check over your image in different parts. The correct value for the eye lashes may be too much for a model's complexion.
3 Do not apply sharpening before changing the size of an image.
4 Leave sharpening as the very last step before output of the file.
5 Sharpening will be needed after operations which recompute pixels, e.g. rotation, scaling, sizing.
6 Beware of over-sharpening subjects such as landscapes and portraits in an unsympathetic way.
7 Do not sharpen files that are going to be subsequently scanned. Sharpening will be carried out by the vendor performing the second scans e.g. if you image a file as a transparency and send this for scanning at the printers.
8 In order to prevent blotchiness of human skin it can be very effective to individually sharpen the R G and B channels. The amount can be tailored for each channel. The total amount should be low (80% to 120%), the radius should be large (1.8 to 2.5) for resolutions between 300 and 400ppi.The threshold should be set according to the graininess of the image.
9 For shots of machinery, the amount of sharpening can be high, as much as 220% to enhance the apparent sharpness of the equipment. Over sharpening will however still result in loss of detail.
10 After sharpening, if you go straight to Edit>Fade Unsharp Mask, you can reduce the effect (Shft-Ctrl F is the short cut)

Diagram This is a technique which has many devotees. You take your layer, duplicate it, run the high pass filter with a radius of 10, set this duplicated layer to hard light and adjust the layer opacity to get the level of sharpening required (usually between 20% and 70%).This gives a real time ability to back off or mask off the effect. A radius of 10 is like using a radius of 10 in unsharp mask - it may not be appropriate for all images. Try other radius settings more like 1, 2 or 3.You can also use Overlay or even Soft Light instead of Hard Light for more subtle sharpening. You can use a layer mask
to apply the sharpening to a local area or paint directly into the layer with 50% grey to control the application of the sharpening. When you have a somewhat larger radius you can run the median filter on your "sharpen" layer to kill any grain enhancement introduced by the High Pass filter. Dodge and burn on the High Pass to "contrast up" local areas of sharpen.Use curves on the sharpen layer - peg the midpoint and ramp up the white point independently of the rest of the curve to adjust the intensity of just the lighten halos; same thing with the black point. A variation is to duplicate the file, convert to grayscale and run the "Find Edges" filter. Invert, soften with Gaussian Blur and adjust
levels to control the contrast.Then load that into the main image as a selection and sharpen. This has a reputation for protecting all the shadows, severe highlights and smooth areas and only sharpens the edges. There is a ton of advice such as that
above on the web. When we Googled for Photoshop Sharpening we were still on relevant stuff after 130 hits.

SOFTWARE SOLUTIONS
For the really enthusiastic sharpener there are a number of products on the market, which bundle all the available technology into one a single place. Foremost amongst these is nik Sharpener, reviewed by Andrew Williams a couple of issues ago. With the benefit of hindsight (i.e. the published magazine) we can see that Nik Sharpener actually did a marginally better job even though the test will have been compromised by resizing in the page layout. It would seem that, providing the image is sharpened for the job in hand, the progra Sharp example 2 m does indeed do a better job. The software company, nik Multimedia also make Dfine, which is aimed at producing a better job of removing noise and JPEG artefacts. The literature with the program is very persuasive and the case well argued. However these are refined methods and we hark back the what we said earlier - the image has to be first class to start with otherwise these refinements will be quite lost in the general fuzz of the image. It is also noteworthy that Photoshop now has additional sharpening expertise in its RAW file plug in. The image on the left has been strongly sharpened and magenta fringing is visible around the highlight. With Luminance Smoothing and Color Noise reduction, the same level of sharpening has much less sharpening artefacts.

Sharp example 3

Sharp example 4

Photo Quote: My own eyes are no more than scouts on a preliminary search, for the camera's eye may entirely change my idea. -Edward Weston