Correcting sharpness

First of all: you cannot correct sharpness. If a picture is out of focus, it is out of focus. There are, however, a number of algorithms that allow to enhance the sharpness of a picture. Photoshop has some of them all reacting differently and more or less effectively.

The most common sharpness filter for Photoshop is "Unsharp Mask", quite a strange name for a sharpen filter. You can read more about the "Unsharp Mask" here: Unsharp masking - Wikipedia .

Shake reduction is quite an interesting filter who tries to reduce the artifacts of shaking, when taking a picture. Please note that also here, we should not count too much on miracles. The visual impact of shaking may be diminished, but there is no replacement for correctly taking the picture.

A very good set of sharpener filters can be found with Google's Nik collection. Unfortunately, the program will no more developed, but is still available on the Google web site for free (I did pay for my version at the time, because the results were so persuasive). They can act as stand alone programs or as plug-ins to Photoshop, Lightroom or other programs.

Any sharpen filter tries to enhance the contrast on hard edges in a picture. This makes that noise also is enhanced. Intelligent masking techniques try to avoid that.

Also, excessive sharpening may introduce artifacts. See here this excellent article: https://www.digitalphotopro.com/revolution/sharpening-artifacts/

I am not I understand what you mean by correct sharpness?  Are your images too sharp, or perhaps do not appear sharp?  A little more information would be great.

I frequently use the Smart Sharpen Filter.   A word of caution, always, always make a copy of your layer with Ctrl+J before applying your Filter.   If image looks too sharp, adjust layer opacity downward.

Improved Smart Sharpen | Adobe Photoshop CC tutorials

Nancy

The following is part of an Information Sheet devoted to the history-and-how of sharpening that I wrote for my students. You may find it helpful. One method in particular, Lab Color for sharpening, is anathema to some Photoshop experts. I am not promoting nor defending it but presenting it as an option.

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Unsharp Masking

Of the many options available in Filter > Sharpen, the one that seems to be out of place is Unsharp Mask. Yet, it sharpens the image.

The term “Unsharp Mask” in Photoshop is derived from a graphic arts photographic color separation procedure. In essence, Photoshop’s Unsharp Mask imitates the sharpening function of a dual-purpose film mask.

The following is a description of the photographic (film) method that was used in commercial printing followed by suggestions when Filter > Sharpen > Unsharp Mask is applied in Photoshop.

Commercial Printing from transparencies (Prior to the use of computers):

One of the first objectives when color separating a color transparency (a “chrome”) using a graphic arts camera and photographic film, is to reduce the density range of the chrome so that it fits more easily within the tone gamut of halftone reproduction, and neither highlight nor shadow detail is compromised.

To achieve this, a low contrast negative is made -- using black & white film -- by contact exposure to the transparency (which is a positive image). Next, the two films – the positive transparency and the low contrast negative black & white film image -- are sandwiched together for use as camera copy. The negative film reduces the density difference between highlight and shadow extremes of the image to be photographed during subsequent color separation. So much for contrast reduction alone.

If, instead of placing the black & white film in direct contact with the chrome during the making of the mask, a sheet of diffusion material (a special frosted plastic) is placed between chrome and film, the resulting negative lacks knife-edge image sharpness. The film – the negative mask -- takes on a soft-edged, out-of-focus look. When this mask is then placed in contact with the chrome for color separation, it performs two functions.

First, since a low contrast negative (the mask) is sandwiched with the positive (the chrome), overall range is reduced. Second, at the points of sharp edge contrast in the original transparency, contrast is increased, as shown in the illustration, giving the impression of a sharper image.

Obviously, if the transparency is not sharp to begin with, there will be no clearly defined edge-contrast to accentuate. An unsharp film mask cannot rescue a blurred, soft-focus image.  Image sharpening is an enhancement technique, not a salvage technique.

As mentioned at the outset, Photoshop has adopted the term Unsharp Mask and its controlled use because it mimics the image sharpening portion of photographic masking procedures. Where, during the making of photographic masks, variations in the amount of sharpening are created by altering film exposure time, its development time and/or the number and thickness of the diffusion sheets, Photoshop offers similar controls in the Unsharp Mask dialog box.

When Filter > Sharpen > Unsharp Mask is chosen, three controls are offered.

Threshold level determines at what point the sharpening will kick in. The higher the number, the greater must be the difference in density of adjacent tones in the original image before the unsharp mask is activated.

Radius denotes the width of the sharpening effect.

Amount governs the degree of change; the amount that the light edge is made lighter and the adjacent dark edge made darker (see the illustration above).

Often, with the Threshold set at 0 for intricately detailed images (or a higher number for flesh tones) and, for starters, the Radius set at about 1/200th of the image resolution, variation in the Amount will produce the desired result. On the other hand, subjects  such as ocean water and similar textures, may benefit from a high Radius and small Amount setting.

At times, sharpening all channels produces a heavy-handed, distracting sharpening. Consider sharpening only the weakest channel in the RGB set or the K plus the next weaker channel in the CMYK set.

Another approach, one which may allow greater sharpening without harsh color artifacts, is to switch to Mode > LAB Color and sharpen only the Lightness channel. Then, return to Mode > CMYK or RGB.

A similar effect may be obtained without changing color modes: Create a duplicate RGB layer and do the Unsharp Masking on this duplicate layer with its Blending Mode in the Layers panel set to Luminosity. This method offers three advantages:

  1. It does not require a switch from RGB-to-Lab-to-RGB

  2. Resulting sharpening may be reduced, if necessary, by using the Layer’s

           Opacity slider

  3. A mask may be added to the Luminosity layer and used to eliminate or

           reduce sharpening locally.

One final reminder: Sharpening, one of the last correction operations, should be done at 100% of image size.

Hi

When you say "correct sharpness" are you trying to overcome a particular issue such as camera shake / poor focus or are you just trying add some general sharpening.

For the former - you will find specific filters in the Filter -Sharpen menu - such as shake reduction

For general sharpening, there is smart sharpen and unsharp mask - but I like the sharpen tools in Camera Raw.

When printing you can often add a bit more sharpening than looks good on screen - but do that as a very last step before printing

A couple of general points
1. For general sharpening do it at the end after all other adjustments. For print sharpening do it after any resizing.

2. Always , always (did I say always ) view at 100% zoom when sharpening.

Dave