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Digital Oversharpening

 
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DaveW



Joined: 04 Aug 2006
Posts: 1702
Location: Nottingham, UK

PostPosted: Fri May 23, 2008 10:07 am    Post subject: Digital Oversharpening Reply with quote

How do you know if a digital image is over sharpened if no artifacts or halo's are present. Or are artifacts and halo's the only criteria of over sharpening?

In the days of film you could not over sharpen unless you used an acutance developer or unsharp mask in the darkroom. Generally if the negative or slide came back pin sharp nobody then said the image was over sharp since it reflected what the lens saw and the film took.

With digital however, due to the image being deliberately softened in camera to avoid jaggies and moire patterns and then needing "sharpening" either in camera or post processing, or more correctly pseudo sharpening using unsharp mask or similar functions since you cannot put back resolution that was removed only simulate it by artificial means, how do you then determine the correct "sharpening" for an image with no actual reference to what the sensor saw?

Lets hope the Foveon sensor can be perfected to replace CCD and CMOS sensors so we actually get the resolution the sensor actually sees as with film without any unwanted softening of the image in future which then needs to be "sharpened" again.

At the moment the only way to detect if an image shows greater "sharpness" than the sensor actually saw seems to be to look for halo's etc. So if none are detectable does that mean the image cannot be said to be over sharpened, unless you are softening an image for mood or effect that is?

DaveW
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 19966
Location: Richland, Washington State, USA

PostPosted: Fri May 23, 2008 10:52 pm    Post subject: Reply with quote

My criteria are simple: an image is oversharpened if a) it does not accurately portray the real subject, and b) sharpening is the reason why.

Most real subjects don't have halos, so if the image does, that's a pretty strong indication of oversharpening.

But images can also be oversharpened even if they do not show halos. For example, see images #2 and #3 and the followup discussion at http://www.photomacrography.net/forum/viewtopic.php?t=2113 .

On the other hand, even very strong sharpening in the computer is not oversharpening, if what it does is only to compensate for blurring in the optics. I routinely use strong sharpening in extreme macro work because it makes detail visible that otherwise would be hidden due to lens properties.

Why do you ask the question?

--Rik
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DaveW



Joined: 04 Aug 2006
Posts: 1702
Location: Nottingham, UK

PostPosted: Fri May 23, 2008 11:33 pm    Post subject: Reply with quote

Because opinions on what is an over sharp digital image seem to vary, but nobody questions sharp images directly off film or transparencies because generally that sharpness must have been there in the first place so is not "artificially produced". With digital images it is always a matter of conjecture what the lens actually saw before demosaicing.

DaveW
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acerola



Joined: 13 Dec 2006
Posts: 251
Location: Hungary

PostPosted: Sun May 25, 2008 12:41 pm    Post subject: Reply with quote

It's a tricky problem, because if you see a transparency projected, you can sow all the detail. But on the computer screen you see just the fraction of the information because of size interpolation. You have to compensate for that also. Similar problem with digitalized transparency.
And less problem if you make a transparency from digital. You can get a good result without using sharpening.
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