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Is sharpening detectable?


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Generally, if it's done only as much as necessarily with the proper settings you might not notice unless it's printed/displayed at the wrong size for the sharpening level applied. This isn't guaranteed, as sometimes there are telltale signs, but for the most part very subtle sharpening won't be noticed except by someone on the lookout for it. You can even tell traditional images sharpened using an unsharp mask technique in the darkroom.

 

The most obvious, is if you get close enough along contrast lines there appears to be a slight inversion of the contrast. The darker side will have a lighter line, and the lighter side will have a darker band. As you may know, that's how the appearance of enhanced sharpness works.

 

If the image is of a sufficiently low level of data (8 bit) and a lot of levels adjustments are applied you can definately see a loss of subtle gradations in areas as the histogram, and consequently the image, is "stretched."

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If sharpening wasn't detectable, it would serve no purpose at all. Most images here are sharpened some after being resampled, and I can usually tell when someone neglects to perform that final sharpening stage.

 

There are also lots of sharpening errors that are noticable. For example, a low threshold will often amplify the noise in skies and other smooth areas to an unrealistic looking level. Overshapening can also produce halo effects that look unnatural.

 

So, in answer to you question, there are lots of people who can detect whether or not the sharpening you applied is appropriate for the image you are displaying. Also note that there is no universal sharpening settings that will be optimal for every picture. If you sharpen a picture of soft clouds the same as a picture of detailed architecture, then you aren't getting the most out of the sharpening tools.

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I forgot to mention the Levels/Curves adjustment. The only way this would be obvious is if the adjustment was very dramatic. If your histogram starts out with 1/3rd of it unused and you set the black & white points, then there will probably be some noticeable posterization or banding in the areas that should have smooth gradations. If you want to see this at the extreme, save a picture with smooth tones/colors in GIF format where only 256 colors are allowed. Or, if your monitor allows it, set it to 256 color mode while viewing the image.

 

To be clear, there is not a history in the saved file that tells what operations were performed. If someone says your sharpening is wrong or the levels are incorrect, they are basing it off of what they see in the image on their monitor (which is one of the biggest variables). If your monitor is nice and bright and his is dark and out of calibration, then your image might look awful to him. But, if several people agree that the image looks too dark, it may be your settings that are off.

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If I am sharpening an image to print, I am very careful with the amount I sharpen it. I might only set the radius to .5 or 1 and the amount to 100. I rarely use threshold and read somewhere that there is little need to adjust it. Does anyone else know how to best use the threshold?

 

For images that I intend on saving as Jpegs and displaying on my monitor, I go much higher with the amount, anywhere from 100 to 200 percent, or more, if needed. But the radius I keep below 1 and usually closer to .3.

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