Wanderer2 Loc: Colorado Rocky Mountains

I'm fairly new to post-processing, using Lightroom 4.4 occasionally for a year or two and only on landscapes. I've been wondering what the sharpening tool actually does, at the basic level, to make an image appear sharper. I haven't been impressed that the effect is very great on landscapes but perhaps that's because of my inexperience. Thanks.

Rongnongno Loc: FL

Use 'clarity' instead.

MtnMan Loc: ID

It increases edge contrast. If you shoot JPEG you probably don't need it because your camera does it when creating the JPEG. As noted you'll see more effect from clarity without increasing noise.

Serious sharpening is better done in Photoshop where there is much more control.

Apaflo Loc: Anchorage, Alaska

There are generally two basic types of "sharpen" tools available with image editors. (And two or three others that are not as commonly included.) One method, commonly called "Sharpen" is a high pass filter algorithm. The other is called Unsharp Mask. Knowing the difference in what each does is important for "advanced" image editing.

You need to understand the concept of spatial frequency domain. Consider a picket fence with alternating white slats and a dark background. If the slats are thin and close together they produce a "high frequency" in the spatial domain. If they are wider and farther apart the frequency is lower. If an area of an image has no variation in tones, it has an extremely low frequency.

If there is a sequence of consecutive tonal variations that exactly match the pixel spacing on the camera's sensor, that would produce the maximum resolution possible. To see it however there has to be at least some contrast between the tones. Increasing that contrast, which is what a sharpen tool does, makes the image look sharper. It does not actually increase the resolution, just the contrast.

The way that "Sharpen" accomplishes that is to detect sequences of tonal variations that are at high frequencies by using a high pass filter. The high frequency part of the signal is amplified to make the bright area more distinct from the adjacent dark area. That is, the contrast is boosted, but only at the edges in the high frequency sequences.

The exact opposite can be done, by reducing the amplitude of the high frequency sequences, to give lower contrast. That will blur an image! In fact blur and sharpen tools use exactly the same algorithm, with different parameters!

The UnSharp Mask (USM) tool does essentially the same thing, but in a very different way that has a very distinct effect. USM works only on single edge transitions. A single edge causes a large spike of high frequency in the spatial domain. USM averages high frequencies (by using a blurred mask). It uses the difference between the image and the blurred mask to apply higher contrast. A sequence that Sharpen would detect is seen as a solid tone due to the average, but a single edge is sharpened.

So Sharpen increases the contrast for fine detail sequences while USM does the same for single edge transitions. Lets say we have that picket fence, with an open gate. Sharpen adds contrast to the pickets, but the transition from the fence to the open area of the gate is not affected. USM will affect the edge of the gate area, but not between the pickets on the fence.

Unlike Sharpen, USM cannot be reversed with a blur tool.

Generally, images produced by cameras using a Bayer Color Filter Array to encode colors will benefit from being sharpened. If the original image is resampled to a smaller size (for the web perhaps) the effects of USM are usually more significant than Sharpen. If the image is resampled to a larger size for printing, Sharpen will usually be more effective than USM. (It very much depends on the type of detail in the image, and usually both methods have an effect.)

When an image is resampled to a smaller size the process is a very effective low pass filter in the spatial domain. Hence any sharpening done before resampling is virtually eliminated by resizing! Therefore it is always best to sharpen as the last step, after a resample operation to set the size.

Wanderer2 Loc: Colorado Rocky Mountains

Thanks to all three of you for your replies. Apaflo, your detailed and in depth knowledge of this subject is amazing and I've copied and saved it for future reference. Do you have any suggestions for sources I can read to learn this sort of info about the various aspects of PP? I would like to better understand what I'm doing when I do various things with this software. The only reading on it I've done so far is Scott Kelby's book on LR which of course has a different purpose.

Thanks again.

MWojton Loc: Yardley, PA

Wow- what an explanation! Thanks.

Uuglypher Loc: South Dakota (East River)

Hi, Teton Viewer,
To address , specifically, your interest particularly in the "Detail" menu of Lightroom/Adobe Cameta Raw, it is important to realize that its algorithms of sharpening and noisetducion have undrgone remarkable improvement pver the past three or four iterations to the point that the five controls provide a great deal of refined, fine control over the edge-sharpening process.

"Amount"(the over-all strength of sharpening effects of the other controls),
"Radius"(the number of pixels on either side of the sharpened edge involved in providing perceived increased sharpness/contrast of that edge),
"Detail" ( the brightness of the "halo" contributing to the perceived sharpness produced), and
"Masking" ( the degree to which sharpening is allowed throughout the image or is limited (masked) to only the major, strong edges, thus protecting the broad, gradual changes in tone wih few details from sharpening effects.
.and...
"luminous noise" (reduction of which is usually strongly contributory to effctive sharpening.

"Masking", Luminous Noise reduction", and "Detail" can be used cooperatively to remarkably minimize the effects of noise...and can also permit use of some degree of noise to actually, under some circumstances, enhance the effect of sharpening.

An older, but still unexcelled description of how these controls are used to accomplish the three commonly used stages of sharpening ( capture sharpening, creative sharpening, and output sharpening) is:
"Real World Image Sharpening with Adobe Photoshop, Camera Raw, and Lightroom" by Bruce Fraser and Jeff Schewe, Peachpit Press, 2nd Edition, 2010.

This is out-of-print but easily found on the web. Well worth the investmnt.

Dave in SD

Uuglypher Loc: South Dakota (East River)

Those who may actually get and study the book by Fraser and Schewe mention in my previous post, here is a "test Edge" of the sort used by those authors to demonstrate the effects of each of the sharpening controls found in the "Detail" menu of ACR/Lightroom.
Make a copy of this test edge and use it as an image file on which yo test any and all of the specific sharpening controls. The experience can really iMovie your insight into what you are actually doing in the sharpening process.

http://static.uglyhedgehog.com/upload/2015/3/4/1425511926091-image.jpg

After learning the basics, then apply what you've learned to the two extremes of images characterized, respectively, as High Detail Density (hDD)/ "High Frequency" images and Low Density Detail / "Low Frequency" images that are well described in the Fraser and Schewe book.

Best regards,
Dave in SD

forjava Loc: Half Moon Bay, CA

Agreed; super!