Tag Archives: Compression

Sharpening can do more harm than good.

When you sharpen a *.jpg picture the application attempts to find lines and edges and make them more distinct. The process can cause damage to the file spoiling the picture. The trick is not to sharpen your picture too much. Here are a few things to consider…

First an explanation. If you want to take out any softness or poor focus then you must sharpen. In fact most images benefit from one sharpen action in post-processing. Lets say, in your image editor application, you apply the sharpen tool. When you sharpen a picture you are trying to define the edges of objects in the picture. Softness due to poor focus, out of focus, or movement blur, can have better defined edges. The softness seems to be removed as the edges give the appearance of good focus when they form sharp lines.

The downside to sharpening is that you cannot rescue an image from being very soft or very out of focus. If you attempt to sharpen a soft picture you will need to repeat the sharpening process many times creating over sharpening. This will eventually damage the picture. You get clear lines, halos and spots which indicate the over sharpening.

Look for white edges – that’s over sharpening starting

A sure sign of over sharpening is the appearance of white lines along the edges of an object. Look at the grass in the picture above. The whitened edges have become distinct and have ruined the sharpness of the grass. In fact the grass has become ‘un-grass-like’ because of over sharpening. I did that to make the point.

When sharpening an image you should look carefully at the lines, like the edges of grass. When you look in detail you will see tiny artefacts from the sharpening process begin to appear. This effect will get worse the more you sharpen the picture. In most editing programs you should not sharpen more than twice. Even though you cannot see it, damage is already there and usually it will begin to be visible after two ‘sharpen’ actions.

Repeat ‘sharpens’ happen without your permission

When you sharpen a picture you are hoping to make it a little more distinct to the eye. So lets say you have a reasonably sharp picture you want to publish online. You give it two sharpens. It looks good. So you upload the picture to the website. Suddenly it looks as if its been through over sharpening. What is going on?

First, when you save a file in *.jpg format the save process sharpens the image. You are in fact applying another sharpen action just by the act of saving. Having already sharpened twice, you are applying a third on the save.

Secondly, when you upload a picture, many websites apply compression to the file before displaying it. The website takes a look at your picture. It decides some of the data which creates the image is unnecessary. So it dumps it. This helps the website use less storage space. However, this process is similar to sharpening. Tthe result is that an uploaded image could well look like it has two ‘sharpens’ applied by the upload action. The final result is four sharpens overall. Two from you and two from uploading. The impact on the file can be substantial.

Lessons to be learned

First, it is important not to over sharpen. In some images a good eye can spot even the second sharpen action.
Second, the save process for *.jpg involves a sharpen action too.
Third, when you upload a file to a website, that site may apply sharpening too, as part of the compression process. Both the compression action and the sharpen action have the overall effect of increasing sharpening once again.

In effect image files get ruined simply by repeated saves and uploads. So if you add sharpening of your own it is no wonder that some pictures look damaged when online.

Unfortunately there are very few things you can do about over sharpening damage. The best action for you to take is concentrate your efforts on getting a better image in-camera while doing the shoot. Certainly, it is best to keep the sharpening to a minimum. That way you can make sure that you don’t see as much damage as I have shown in the image above.

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Damon is managing editor of Photokonnexion.com with professional experience in photography, writing, image libraries, and computing. He is also an experienced, webmaster and a trained teacher. Damon runs regular training for digital photographers who are just starting out.
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