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Archive 2013 · How to determine 'optimum' sharpening?
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p.1 #1 · p.1 #1 · How to determine 'optimum' sharpening?

I have read about and experimented with many techniques for optimum sharpening ( if there even is such a thing), however my eyesight isn't fantastic and I'm fairly uncritical when it comes to colour fidelity and sharpness Nevertheless, I want other, more critical viewers to see my photographs reproduced in the best possible way, be it on screen or in print. While I recognise that that different sizes and different media may require different approaches, is there any way of determining optimal sharpness for any given file that does not reply simply on the viewers eyesight and opinion?

Jul 12, 2013 at 06:39 AM
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p.1 #2 · p.1 #2 · How to determine 'optimum' sharpening?

Nope. It is largely subjective and also depends on viewing distance and so on. There are different combinations of sharpening parameters that can cause pretty much equally effective sharpening, and that makes it so much harder to pick a single optimum sharpening method.

If you can see the sharpening halo effects when viewing the intended output at the intended distance then you have definitely overcooked the sharpening. The idea is to see the sharpening effects without actually noticing them.

- Alan

Jul 12, 2013 at 01:09 PM
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p.1 #3 · p.1 #3 · How to determine 'optimum' sharpening?

In my 10+ years of digital photography, the only consistency I have come across is if sharpening halos became too noticeable. Also there can be a subjective point where an iimage is too sharp and is therfore unpleasant to look at.

Unfortunately it is all subjective just as what is the proper color temperature of an image. This will vary by size of the image. Also, prints can often withstand more sharpening and also require more.

Sharpening is best viewed at 100% or greater. Halos and other artifacts (jaggies) become evident at that point and your viewing software is not altering the image by downrezzing. Also, look at a number of areas in the image. You'll soon learn the areas and artifiacts to look for.

Jul 12, 2013 at 01:12 PM
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p.1 #4 · p.1 #4 · How to determine 'optimum' sharpening?

Color can sometimes be done a bit "by the numbers" without relying fully on your visual perception. Sharpening however, is (best I can tell) is largely predicated upon visual perception.

Here's one of the books that I really like regarding what is happening @ sharpening.

After reading this book, I made some "test strips" and began experimenting with sharpening. After playing with my test strips to see what was happening up close and personal at various controlled files, I came to a better understanding of what happens when I apply a given amount of sharpening and/or types of sharpening.

Depending on my goals for the image and the type of image it is, I have a few "mental presets" at what my sharpening will be. In this regard, the application of pre-established sharpening is about as close as I come to sharpening without relying on my visual perception. These are probably what I would categorize as "safe sharpening" but not necessarily optimal sharpening.

My reading of Jeff's book in combination with Dan Margulis books and my trial & error experimentation provides the baseline for my approach to sharpening. I don't print much, but according to others (i.e. Dan, etc.) "oversharpening" for print can be judiciously applied for some image types. I would imagine that this would be yet another area that largely depends on the ink/paper being printed on as to how much oversharpening is tolerated to yield your finished product goals.

This is not what you are probably wanting to hear, but it's what I've found so far. Doing some controlled tests, so that you can get comfortable with a given "preset" or two is about the best I can offer regarding less dependency of visual acuity. You may wind up with a few multiple "presets" for various cameras/lenses or output sizes after you testing.

I still sharpen each image individually for those that I want to "optimize", but for some batch work, I do have my "presets" at my disposal for when I don't want to use critical vision to evaluate (i.e. pre-evaluated @ tests) an image.

Jul 12, 2013 at 03:29 PM
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p.1 #5 · p.1 #5 · How to determine 'optimum' sharpening?

I understand the question in the leading post as "can you in some way know when sharpening is 'just right'?"

The answer to that is that - yes you can. But only in a theoretical sense, never in a practical sense... -Which makes it a non-answer, really.

"Perfect" sharpening could/should be defined as "perfect reproduction" of the target. This means that every last bit of information in the image corresponds to a contrast in the target you shot in a reasonably linear way.

This does not apply to "visual" sharpening in any way though. "Perfect" sharpening is just the "perfect starting point" for visual sharpening...

We do have a very solid knowledge of how human vision contrast vs resolution behaves, so if you know the exact replication ratio of the image (how large the viewer is going to see it, measured in angle of vision), you could get a "perfect" visual sharpening too - but even that will be off target for most applications. Most of the time, the viewer wants a slight oversharpening - just as most viewers want a slight oversaturation.

IMO the greatest fallacy of most sharpening methods used in modern imaging software is that they oversharpen darker contrast, while undersharpening brighter contrast.

If you have two identical knitted sweaters in an image (but one is dark gray and the other is almost white) the darker sweater will reach oversharpening (haloing artefacts) way earlier than the bright sweater.
So, the knitted surface structure in the dark sweater will be much more "contrasty" in the image than the other, brighter, one - even though their surface structure contrast is perfectly identical in reality.

As Rusty (and others) said, finding a good balance between the "sharpness" of different areas in the image is almost purely an opinion of the viewer.

Jul 12, 2013 at 04:11 PM
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p.1 #6 · p.1 #6 · How to determine 'optimum' sharpening?

Diggin' the sweater analogy ... variegated thread of one sweater would likewise present itself with different levels at which crunchies and halos reveal.

Essentially sharpening is ratcheting up the contrast between adjacent areas of a given range. Those areas that begin with high contrast tolerate different levels of change to their contrast than areas that begin with more moderate or lesser levels of contrast. In that regard, it is always evaluative/subjective if your goal is truly "optimal".

For that reason, global sharpening is not something that I lean on very heavily. Instead, employing various techniques with masks and/or layer opacity. I think the single largest issue that lends itself toward optimal sharpening is to realize that global sharpening (alone) of an image will likely not be optimal, but rather a compromise of various areas within the image.

Taking the two sweater scenario and you may see that if you apply global sharpening to the optimal level of one, the other has not been optimized (too much or too little). As point out by others, the crunchies and halos are typically the "warning zone" that you've gone too far. But, it might be that the majority of the image as a whole can tolerate that level of sharpening, just a given area of higher contrast may not.

The choices then become:

1) Live with a few crunchies/halos
2) Pull back on sharpening to reduce crunchies/halos, live with less optimal in other areas of image

These are of course, both compromises that are practical toward global sharpening.

3) Do selective sharpening at various areas of image (requires more effort).

So, when we say "optimal" are we talking about optimal for a global sharpening approach, or "optimal" for a selective sharpening approach. Thinking we can achieve the latter by only applying the former can leave one feeling as though they are doing something wrong with their sharpening without realizing that they are simply approaching it from a compromise situation rather than an optimize one ... if that makes any sense.

Jul 12, 2013 at 05:57 PM

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p.1 #7 · p.1 #7 · How to determine 'optimum' sharpening?

Consider doing a trial of . Here's a review.NIK Sharpener Pro 3.

I have found it useful and it is easy to get good sharpening for prints.

Jul 12, 2013 at 06:53 PM
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p.1 #8 · p.1 #8 · How to determine 'optimum' sharpening?

First it helps to understand how USM works psychologically and technically. Psychologically sharpening is an optical illusion. Technically it is created by detecting borders between contrasting tones/colors and increasing the contrast at the border. By way of analogy it's like creating a stone wall along a property line. The three controls in the basic USM function control the characteristic of the "wall":

Amount = Amplitude (how much contrast is changed)
Radius = Width (how far on each side the contrast is applied)
Threshold = When to apply it (How much difference there needs to be between tones).

This is a screen shot of a tone border enlarged 800% in Photoshop created by hand (not in a photo):


Here it is with (500, .2, 0) USM applied:


As you can see USM alters the contrast at the borders. If you get up and walk across the room the eyes will not resolve the differences in contrast but it should seem sharper.

Some degree of USM improves nearly any photo because the capture process uses an RGB mosiac and anti-alias filters over the sensor are used to prevent that matrix from creating a "stair-step" pattern along diagonals. There's also the variable of lens resolution between lenses and different degrees of resolution at various f/stops that make USM desireable but the amount needed something which can only be determined visually. Even when a resolution chart is shot in a test it the eyes and brain which tell us which looks identical to how the eyes resolve the same chart directly.

Since images used on screens vary in size in a limited range and are viewed at reading distance it's pretty easy to arrive at USM settings you personally find work best by trail and error. Printing adds additonal variables: mechanical variables of the printing process, how the image is resized, and how far away it is seen. The same photo viewed on screen as a thumbnail, part of a web page, as screen wall paper often will benefit from three different levels of USM to look sharpest psycholgically when seen.

Back in '94 when I bought my first .8MP digital camera, an AppleQuicktake100 the problem was compensating for the loss of resolution when up sampling the small camera files for both screen and print. Printed at 300 ppi the out of camera file produced an acceptably sharp print, but it was the size of a postage stamp.

Today there is the opposite problem. The camera files are so large (pixel dimensions) reducing the pixel dimensions of the camera to screen size requires averaging them together. The greater the reduction in pixel dimensions from camera > screen display the more the downsizing will affect resolution of file details and the more USM is needed to enhance the contrast at the increasingly blurred tonal boundaries. If you take the same file of a forest scene and resize it three ways for screen -- thumbnail, 600 x 900 and full monitor size you'll likely find you need the most USM on the thumbnail because the downsizing turns the trees into green blobs. Viewed on the "pixel peeping" level the image will look bad, even to the point of clipping small highlights, but at normal viewing distance is will better create a sense of 3D texture.

USM is not a new trick created for digital. I first used it on camera back color separations in the 70's where a unsharp continuous tone image was created and sandwiched between the RGB images and screen when making the halftone color separation. Drum scanners created the same effect of adding contrast at borders electronically. But the concept was first used by painters hundreds of years ago.

One of my favorite portrait painters is John Singer Sargent and several of his larger than life works hang in the National Portrait Gallery in DC where I live. From across the room the detail and sharpness is outstanding. But viewed from 18" you see he quite literally trowelled on the paint with a palette knife rather than a brush. He created the illusion of sharp 3D edges by adding strategically placed white "specular" highlights on objects creating the illusion of sharpness from a distance. We sense 3D shape in 2D art media from contrasting highlight on the top of objects and the darker shadows the directional key lighting, if used, casts.

The reason art forms like line drawings and silk screens work is because beyond reading distance where fine detail can discerned the brain forms it's clues about 3D shape from the contrast on different planes of objects. The brain sees the contrast pattern and matches it to patterns previously seen by eye and the object is recognized from those "primitive" geometric 3D shape clues. That's why in 3D computer art even when a face rendered in wireframe of broad planes of contrasting tones (e.g. Ironman) is recognizable as a human face.

How does that apply to USM choices? How the content is lit creates different tones on the planes and curves of 3D objects. A cross-lit scene will stand up better to enlargement for a huge print than a flat iit one becuase cross lighting creates contrasting planes on objects.

For example if you were to make a print to hang as a mural on a wall a sailboat on the ocean with it's large geometric shapes would be a better choice perceptually and a forest scene because even at low resolution the viewers would recognize the content in the first from the shape of the hull and sail but the forest would look like a green blob.

Reproduction of fur and feathers is also an interesting study in lighting and USM. Both have tiny mirror-like facets which catch light and create sparkle in direct collimated light. It's the sparkle off the individual hard hair shafts that create the illusion of soft 3D texture of the overall body. But the combination of RGB senor processing and AA filter turn those specular shape clues into mush at capture and the illusion of 3D is lost. Using diffuse lighting (overcast day) makes matters worse because diffuse light doesn't create sharp specular reflections. As a result in a photo a dog whose coat sparkles when seen by eye will look dull and matted in the photo due to the lack of specular clues.

Taking a page from Sargent's playbook what I do with animal photos is selectively over sharped the fur to the point where the halos create on borders when over sharpened create the same illusion of 3D texture seen by eye from the real specular highlights. This visitor to my front porch on an overcast day is an example of that technique:

(500, .2, 0) USM:
Second (500, .5. 0) USM layer selectively applied with masking:

I'll also use it for scenics where there is sunlight glinting off water to bring back the sparkle the camera sensor and AA filter kill. So USM decision are best make on a case-by-case basis by comparison based on how the file is output. It only takes a few tests on typical subjects to get a general idea of what works best for you.

Portraits have different USM goals for skin (make it smooth) and eyes and other details (keep them sharp). The solution to that apparent dilemma is solved by using layers in Photoshop. I use a DIY action that copies the base image layer 3x and applies surface blur to one copy, high pass filtering to the second and 500,.2,0 USM to the third. Then using masks to apply and the sliders to adjust I blur the skin, bring back some of the texture with the high pass layer and enhance the sharpness in the eye and mouth area all selectively. Sounds complicated but in practice it only takes a couple minutes per photo.

I'm "old school" and for general USM to overcome the loss of capture resolution I still use the basic USM controls, but with a "twist" suggested in a old Dan Margulis article in a printing trade magazine I read back before digital photography for using USM to sharpen RGB drum scans. Margulis later wrote several good Photoshop books. For screen images I first apply USM = (500,.2,0) in Photoshop, then follow it by Edit > Fade (luminosity). The second Fade step allows before / after comparision with the 0- 100% slider. Changing mode from (normal) to USM produces results similar to sharpeing just the L channel in an Lab file while still in RGB mode.

If you try it you'll likely find most subjects are over-sharpened at (500, .2, 0) but it will vary with how much the screen image has been reduced in pixel size and how much detail the content has. I just move the Fade slider back and forth from 0% (no USM) to 100% (Max USM) and find the point in the middle that looks best by eye.

For prints because the mechnics or printing, size and viewing distance are all variables the best approach for critical work is to take a section of a photo you plan hang on the will, scale it up then cut out a section and save it as a separate file. Then take that section and apply 8 different levels of USM to it and combine those files on a single test print. Put the test print on the wall, step back to different viewing distances and you'll see how USM amount affect perception of sharpness on the borders. As with the Fade slider on screen images it's the side-by-side comparison which will make it easier to determine what works best for that subject / size/ distance on that printer.

Using the Wall analogy as Amt. is increased the "wall" is made higher. Radius, which originally referred to the width of hard printing dots in photo lithography, controls the width of the "wall". I find large amt. 500 and small .2 radius works best for screen images, but with the fade adjustment the amt. actually winds up in the 300 range for most images. For prints I will increase the radius to 1 - 1.5 to account for the difference in printing vs. screen output and then adjust Amt. based on content with a printed comparison test.

Since the mechanical aspects of printing affect resolution you may find different levels of sharpening is needed for photo prints vs. ink jets. You can't judge USM for prints on screen precisely until you have a baseline of experince with your printer and for critical work you plan to print large the time and expense of a comparison test print hung on the wall and seen by someone else more objectively is worth the investment. When in doubt I'll put a test print on the wall and ask the wife which version she likes best and why.

Jul 14, 2013 at 12:15 PM
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p.1 #9 · p.1 #9 · How to determine 'optimum' sharpening?

+1 @ Chuck's gray panels. This is similar to what Jeff presents in his book that I referenced and the varying "test strips" that I suggested. I found that I like working with a .7 and .3 radius with different Amount and Mask depending on the subject/lighting/goals.

Chuck makes a few good points that I agree with that extend beyond the "depends on output size" and "screen vs. print"

1) The variation of lighting to the scene/areas impacts the inherent amount of contrast in a scene or areas of a scene from one image to another.

This variation in baseline contrast means that there is no "one size fits all", and for optimal sharpening, each image is "case-by-case", first with a read @ the lighting. For studio or other consistent lighting, this doesn't seem to present itself as pertinent as it does for ambient shooters that might range from low light interior to overcast to Sunny 16.

2) Not always do the areas of an image receive the same quality of light (i.e. contrast). Neither are all subjects of the same content/texture for reflecting/absorbing that light. Thus, the "need to read" your light/subject and the goals for your image often require selective sharpening and variable sharpening throughout an image. Masks and layers, opacity and fade tools are indispensable here.

In addition to Chuck's points, I'll add that imo color correction is key to eliminating casts before you begin any sharpening efforts. Most folks realize that heavy casts will "dull" an image. Inversely, removal of color casts will "sharpen" (i.e. afford greatest color contrast). Trying to sharpen an image that still has a "dull" color contrast/cast to it means that you wind up trying to apply too much USM to compensate for the dull(er) than optimal color contrasts ... which makes the higher contrast areas even more prone to halos/crunchies occurring from oversharpening.

Some folks are certainly thinking that the things that Chuck, I and the others have mentioned are overkill, but for optimal sharpening I find them pertinent. In addition to Jeff (sharpening) and Dan (color cast removal), there are numerous other advocates regarding the selective approach to processing through layers and masks in quest of "optimal".

Not everyone wants to take a snapshot through such paces to bring it to optimal. But even without doing that, the use of two stages of sharpening @ HIRALOAM sharpening followed by detail sharpening can be a reasonably quick approach that is global (vs. selective), but still allows you to dial in some sharpening that is image dependent.

The key is simply to not apply too much sharpening to the areas that don't need the contrast boost, while boosting those that need a little help. For that, you've got to have a strategy/approach (subjective/variable) that somehow allows to you separate/distinguish one from the other ... if that make any sense. After that, it is just degrees of refinement and subjectivity, along with a dose of patience.

Jul 14, 2013 at 02:45 PM
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p.1 #10 · p.1 #10 · How to determine 'optimum' sharpening?

Wow, thanks for all the detailed information, incredibly useful and interesting. Time for me to go and play with some software I think....

Jul 15, 2013 at 08:26 AM
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p.1 #11 · p.1 #11 · How to determine 'optimum' sharpening?

An additional point worth noting for USM is that as resizing algorithms have evolved in Photoshop they have incorporated sharpening into the resizing.

When resizing a SOOC image to 900px x 600px using "Bi-cubic Sharper" mode in CS6 I often find little or no additional USM is needed compared to the older version I started using. You can see the difference in an image with fine detail by resizing the same file with the different modes.

I also created an action which does (500,.2, 0) USM then the Fade(Luminosity) step to 70% which I apply when batch converting RAW files to screen size JPG from Bridge, which allows actions to be applied during the conversion. That allows me to apply USM to a batch of "snapshots" globally with no additional effort.

Jul 15, 2013 at 12:23 PM

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