geary_ritter Posted December 19, 2002 Share Posted December 19, 2002 Is there anyway that someone can tell whether a digital image has beenslightly sharpened or levels adjusted. Is there any type ofdetectable signature left in the file by any of these adjustments. Iam talking about minor adjustments. Nothing blatant. Link to comment Share on other sites More sharing options...
carl smith Posted December 20, 2002 Share Posted December 20, 2002 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." Link to comment Share on other sites More sharing options...
jim schwaiger Posted December 20, 2002 Share Posted December 20, 2002 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. Link to comment Share on other sites More sharing options...
jim schwaiger Posted December 20, 2002 Share Posted December 20, 2002 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. Link to comment Share on other sites More sharing options...
drymanphoto Posted December 20, 2002 Share Posted December 20, 2002 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. Link to comment Share on other sites More sharing options...
fotografpeterlundberg.se Posted December 21, 2002 Share Posted December 21, 2002 I normaly use 100-150 % and a radius that is half of the resolution. So for screen res of 96 dpi I usually use radius 0,5. For a pic that I will print with 300 dpi res = 1,5. Treshold approx 3. Link to comment Share on other sites More sharing options...
jim schwaiger Posted December 22, 2002 Share Posted December 22, 2002 The threshold can be very useful when you have something with a lot of detail and a sky. Even a threshold of 3 can often let the CCD noise in the sky be amplified a lot. If you see increased noise in an area that should be smooth, increase the threshold. Link to comment Share on other sites More sharing options...
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